Download Car Audio - My Sport Trac

1 Definitions
This section contains background information which defines some of the acronyms and
terminology commonly used in the car audio world. Understanding these definitions is
important in order to understand the other sections of this document.
1.1 What do all of those acronyms mean? [JSC]
`A' is for amperes, which is a measurement of current equal to one coulomb of charge
per second. You usually speak of positive current - current which flows from the more
positive potential to the more negative potential, with respect to some reference point
(usually ground, which is designated as zero potential). The electrons in a circuit flow in
the opposite direction as the current itself. Ampere is commonly abbreviated as "amp",
not to be confused with amplifiers, of course, which are also commonly abbreviated
"amp". In computation, the abbreviation for amps is commonly "I".
`V' is for volts, which is a measurement of electric potential. Voltages don't "go" or
"move", they simply exist as a measurement (like saying that there is one mile between
you and some other point).
`DC' is for direct current, which is a type of circuit. In a DC circuit, all of the current
always flows in one direction, and so it is important to understand which points are at a
high potential and which points are at a low potential. For example, cars are typically
12VDC (twelve volts direct current) systems, and it is important to keep track of which
wires in a circuit are attached to the +12V (positive twelve volts) lead of the battery, and
which wires are attached to the ground (or "negative") lead of the battery. In reality, car
batteries tend to have a potential difference of slightly higher than 12V, and the charging
system can produce upwards of 14.5V when the engine is running.
`AC' is for alternating current, which is a type of circuit in which the voltage potential
fluctuates so that current can flow in either direction through the circuit. In an AC circuit,
it is typically not as important to keep track of which lead is which, which is why you can
plug household appliances into an outlet the "wrong way" and still have a functioning
device. The speaker portions of an audio system comprise an AC circuit. In certain
situations, it is indeed important to understand which lead is "positive" and which lead is
"negative" (although these are just reference terms and not technically correct). See
below for examples. The voltage of an AC circuit is usually given as the RMS (root mean
square) voltage, which, for sinusoidal waves, is simply the peak voltage divided by the
square root of two.
`W' is for watts, a measurement of electrical power. One watt is equal to one volt times
one amp, or one joule of energy per second. In a DC circuit, the power is calculated as
the voltage times the current (P=V x I). In an AC circuit, the RMS power is calculated as
the RMS voltage times the RMS current (Prms=Vrms x Irms).
`Hz' is for hertz, a measurement of frequency. One hertz is equal to one inverse second
(1/s); that is, one cycle per second, where a cycle is the duration between similar portions
of a wave (between two peaks, for instance). Frequency can describe both electrical
circuits and sound waves, and sometimes both. For example, if an electrical signal in a
speaker circuit is going through one thousand cycles per second (1000Hz, or 1kHz), the
speaker will resonate at 1kHz, producing a 1kHz sound wave. The standard range of
human hearing is "twenty to twenty", or 20Hz-20kHz, which is three decades (three
tenfold changes in frequency) or a little under ten octaves (ten twofold changes in
frequency).
`dB' is for decibel, and is a measurement for power ratios. To measure dB, you must
always measure with respect to something else. The formula for determining these ratios
is P=10^(dB/10), which can be rewritten as dB=10log(P). For example, to gain 3dB of
output compared to your current output, you must change your current power by a factor
of 10^(3/10) = 10^0.3 = 2.00 (that is, double your power). The other way around, if you
triple your power (say, from 20W to 60W) and want to know the corresponding change in
dB, it is dB=10log(60/20)=4.77 (that is, an increase of 4.77dB). If you know your
logarithms, you know that a negative number simply inverts your answer, so that 3dB
corresponding to double power is the same as -3dB corresponding to half power. There
are several other dB formulas; for instance, the voltage measurement is dB=20log(V). For
example, a doubling of voltage produces 20log2 = 6.0dB more output, which makes
sense since power is proportional to the square of voltage, so a doubling in voltage
produces a quadrupling in power.
`SPL' is for sound pressure level and is similar to dB. SPL measurements are also
ratios, but are always measured relative to a constant. This constant is 0dB which is
defined as the smallest level of sound pressure that the human ear can detect. 0dB is
equal to 10^-12 (ten to the negative twelfth power) W/m^2 (watts per square meter). As
such, when a speaker is rated to produce 92dB at 1m when given 1W (92dB/Wm), you
know that they mean that it is 92dB louder than 10^-12W/m^2. You also know than if
you double the power (from 1W to 2W), you add 3dB, so it will produce 95dB at 1m with
2W, 98dB at 1m with 4W, 101dB at 1m with 8W, etc.
`THD' is for total harmonic distortion, and is a measure of the how much a certain
device may distort a signal. These figures are usually given as percentages. It is believed
that THD figures below approximately 0.1% are inaudible. However, it should be
realized that distortion adds, so that if a head unit, equalizer, signal processor, crossover,
amplifier and speaker are all rated at "no greater than 0.1%THD", together, they could
produce 0.6%THD, which could be noticeable in the output.
An Ohm is a measure of resistance and impedance, which tells you how much a device
will resist the flow of current in a circuit. For example, if the same signal at the same
voltage is sent into two speakers - one of which is nominally rated at 4 ohms of
impedance, the other at 8 ohms impedance - twice as much current will flow through the
4 ohm speaker as the 8 ohm speaker, which requires twice as much power, since power is
proportional to current.
1.2 What is meant by "frequency response?" [JSC]
The frequency response of a device is the range of frequencies over which that device can
perform in some fashion. The action is specific to the device in question. For example,
the frequency response of the human ear is around 20Hz-20kHz, which is the range of
frequencies which can be resolved by the eardrum. The frequency response of an
amplifier may be 50Hz-40kHz, and that of a certain speaker may be 120Hz-17kHz. In the
car audio world, frequency responses should usually be given with a power ratio range as
well, such as (in the case of the speaker) 120Hz-17kHz +/-3dB. What this means is that
given an input signal anywhere from 120Hz to 17kHz, the output signal is guaranteed to
be within an "envelope" that is 6dB tall. Typically the extreme ends of the frequency
range are the hardest to reproduce, so in this example, the 120Hz and 17kHz points may
be referred to as the "-3dB points" of the amplifier. When no dB range is given with a
frequency response specification, it can sometimes be assumed to be +/-3dB.
1.3 What is a "soundstage?" What is an "image?" [CD]
The soundstage is the position (front/back and high/low) that the music appears to be
coming from, as well as the depth of the stage. A car with speakers only in the front will
likely have a forward soundstage, but may not have enough rear fill to make the music
seem live. A car with both front and rear speakers may have anything from a forward to a
rear soundstage, with an accompanying fill from the softer drivers depending on the
relative power levels and the frequencies reproduced. The high/low position of the
soundstage is generally only obvious in a car with a forward soundstage. The music may
seem to be originating in the footwells, the dash, or out on the hood, depending on how
the drivers interact with the environment.
The stereo image is the width and definition of the soundstage. Instruments should appear
to be coming from their correct positions, relative to the recording. The position of the
instruments should be solid and easily identifiable, not changing with varying
frequencies. A car can image perfectly with only a center-mounted mono speaker, but the
stereo placement of the music will be absent.
1.4 What is meant by "anechoic?" [JSC]
Anechoic means not echoing. It usually refers to a style of measuring a speaker's output
which attempts to eliminate echoes (or "reflections") of the speaker's output back to the
measurement area, which could alter the measurement (positively or negatively).
2 Electrical
This section describes various problems and concepts which are closely related to
electronics.
2.1 My speakers make this high-pitched whine which
matches the engine's RPMs. What is it, and how can I
get rid of it? [IDB]
The answer to this section was generously provided by David Navone of Autosound
2000. The material in these instructions was adapted from the Autosound 2000
Troubleshooting Flow Chart by Ian Bjorhovde with the permission of Autosound 2000.
For more information about Autosound 2000, See section 7 Literature.
This is a set of instructions to debug a stereo installation if there is any noise present after
it is completed. Follow each step carefully! If you have more than one amplifier, repeat
level one for each amp to be sure that none of them are responsible for the noise.
2.1.1 Level 1: Check out the Amplifier(s)
After you have determined that there is noise in the system, determine if the amplifier is
causing the noise. To do this, mute the signal at the inputs to the amp by using shorting
plugs. If there is no noise, then the amp is fine, and you can proceed to level 2. However,
if there is noise, then use a test speaker at the amp's output. If this stops the noise, then
the problem is originating in the speaker wiring, or the passive crossovers. Check to make
sure that none of these are shorting with the body of the car, and start again at level 1. If
noise is still present when using the test speaker, then there may be a problem with the
power supply on the amp. Try connecting an isolated power supply - if this does not get
rid of the noise, then there is something seriously wrong with the amp, and it should be
replaced. If the noise goes away, then there may be a problem with power supply filtering
or isolation. This can be fixed by changing the amp's ground point or b adding external
supply filtering.
2.1.2 Level 2: Reduce the System
The amps have been determined to be noise free. If you have any processors between the
head unit and the amps, disconnect them and connect the head unit directly to the amp. If
this gets rid of the noise, then one (or more) of the processors must be at fault, so proceed
to level 5. Otherwise, try running the signal cables over a number of different routes. If
you are able to find one that does not produce any noise, permanently route the cables in
the same manner, and proceed to level 5. If not, then you must isolate the head unit from
the car's chassis (except for its ground!) -- don't forget to disconnect the antenna, since it
is also grounded to the car. If isolating the head unit does not solve the problem, the
move the grounding point of the head unit. Hopefully the noise will be gone, and you can
install the head unit with a quiet ground and proceed to level 5, otherwise go on to level
3.
2.1.3 Level 3: Move the Head Unit
The amplifiers are fine, but moving both the ground for the head unit and the signal
cables does not solve the noise problem. Take the unit completely out of the dash, and put
it on either the seat or carpet, and run new signal cables to the input of the amp. If this
solves the problem, re-install the head unit, one step at a time and skip to level 5. But if
the noise persists, then move the head unit as close to the amp as possible and use the
shortest possible signal cables. This will verify that the original signal cables are not
causing the problem -- assuming the noise is gone, reinstall the head unit one step at a
time and go to level 5. Otherwise, there may be a problem with the power filtering for the
head unit. As with the amps, power the head unit with an isolated power supply (again
making sure that the head unit isn't touching the car's chassis at all). If the noise goes
away, you can add power supply filtering or an isolated power supply; go to level 2. But
if the isolated power supply does not solve the problem, then you can either replace the
head unit and go to level 2, or check the car's electrical system in level 4.
2.1.4 Level 4: Testing the Car
There does not seem to be a problem with either the head unit or the amplifier, and the
car's charging system is suspect. To see if this is the case, we can use a system in a car
that is already known to be "quiet." Bring both cars together as if you were going to jump
one, and use jumper cables to connect the two batteries. Start the engine of the car with
the noise problem, and listen to the "quiet" car's system. If the noise does not go away,
there is a SERIOUS problem with your car's electrical system (possibly a bad alternator).
Have a qualified mechanic check the charging system out. If there is no noise in the
"quiet" car, then the "noisy" car's charging system is definitely quiet, so continue with
level 5.
2.1.5 Level 5: Adding Signal Processors
We have proven that the amplifiers are good, the head unit is good, and the car's electrical
system is good. Now we need to reconnect each signal processor. Repeat this level for
each signal processor used in your system; if you have added all of your signal
processors, and there is no longer any noise, CONGRATULATIONS! You've removed
the noise from your system! Connect the signal processor. If there isn't any noise, then go
on to the next signal processor. Otherwise, try re-routing the signal cables. If this cures
the problem, the route them permanently over the quiet path, and install the next
processor. If not, then isolate the processor from the car's chassis except for a single
grounding point. If this works, then permanently isolate the processor, and move on to
the next processor. If isolation does not help, then advance to level 6.
2.1.6 Level 6: Processor Isolation Tests
Now, noise enters the system when one particular processor is installed, but regrounding
it does not help. Move the processor very close to the amp, and check for noise again. If
there isn't any, then re-install the processor, carefully routing the cables to ensure no
noise, and continue at level 5 with the next processor. Otherwise, use an isolated power
supply to power the processor, making sure that no part of the processor is touching the
car's chassis. If this solves the problem, the consider permanently installing an isolated
power supply or possibly a 1:1 transformer, and go to level 5 with the next processor.
Otherwise, separate the processor and isolated power supply from the car by many feet
and re- test. If there is still noise, then there is a serious problem with the processor's
design. Get a different processor, and continue at level 5 with it. If separating the power
supply and processor from the car does solve the noise problem, then either the processor
is damaged, or your tests were inaccurate. Repeat level 5.
2.2 My system "pops" when I turn it off. What is
happening and how can I get rid of it? [JD]
This kind of problem is often caused by transients in the signal processor as it powers
down finding their way into the signal path, which the amplifier then transmits to the
speakers.
Usually this can be solved by adding a little turn-off delay to the processor. This allows
the processor to stay powered on for a short time after the amplifiers have powered down,
thus preventing the pop.
Many components sold today (such as crossovers, equalizers, etc) have delays built-in.
Read your manual to see if it is possible to set this delay on your piece of equipment or
be sure to look for this feature during your next car audio purchase.
If your processor does not have this feature, you can build your own delay circuit with a
diode and a capacitor. Add a 1N4004 diode in series with the processor's turn-on lead,
striped side towards the EQ. Then add a capacitor in parallel, the (+) side of the cap
connects to the striped (processor) side of the diode, the (-) side of the cap goes to ground
(not the radio or EQ chassis - connect to the car chassis).
Experimenting with the cap value will give you the right amount of delay before the EQ
shuts off. You don't want it too long, just long enough to make sure the amp is off before
the EQ powers down. 220 - 1000 uF is about right, and make sure the cap is a polarized
electrolytic, 16V or higher. Also keep in mind that the diode will introduce a 0.7V drop
on the remote wire, which can cause the processor to power down before the rest of the
system.
2.3 What is the best power wire to use? [JSC]
There is much debate over the benefit of certain wiring schemes (oxygen-free,
multistranded, braided, twisted, air core, you name it). However, most people do agree
that the most important factor in selecting power wire is to use the proper size. Wire is
generally rated in size by American Wire Gauge, abbreviated AWG, or commonly just
gauge. To determine the correct wire size for your application, you should first determine
the maximum current flow through the cable (looking at the amplifier's fuse is a
relatively simple and conservative way to do this). Then determine the length of the cable
that your will use, and consult the following chart, taken from the IASCA handbook See
section 6.1 What is IASCA, and how do I get involved? [JSC, HK, IDB],
Current
0-20A
20-35A
35-50A
50-65A
65-85A
85-105A
105-125A
125-150A
0-4
14
12
10
8
6
6
4
2
4-7 7-10
12
10
8
8
6
6
4
2
12
8
8
6
4
4
4
2
Length of run (in feet)
10-13 13-16 16-19 19-22
10
8
6
4
4
2
2
2
10
6
6
4
2
2
2
0
8
6
4
4
2
2
0
0
8
6
4
4
2
2
0
0
22-28
8
4
4
2
0
0
0
00
If aluminum wire is used instead of copper wire, the next larger size (smaller number)
should be used. You should also consider the installation demands: will you need to run
the wire around corners or through doors or into the engine compartment? These sorts of
problems in the car audio application require some special care in cable selection. You
will want to have cable that is flexible; it should have thick insulation as well, and not
melt at low temperatures. You don't want to install wire that is rigid and prone to cracks
and cuts, or else the results could literally be explosive.
2.4 What is the best speaker wire to use? [JSC, JW]
Again, there is much debate over the benefit of the various schemes that are being used
by different manufacturers. In general, however, you will probably want to upgrade your
speaker wire from the factory ~20 gauge to something bigger when you upgrade your
amplifiers and speakers. In most cases, 16 or 18 gauge should be sufficient, with the
possible exception of high-power subwoofers. According to an example by Jerry
Williamson, using 18 gauge instead of 12 gauge would only result in a power loss of
0.1dB, which is essentially undetectable by humans. Thus, other factors play more
important roles in the selection of speaker wire. One issue is that different wires will have
different line capacitances, which could cause the wire to act as a low pass filter.
Generally, however, the capacitances involved are so small that this is not a significant
problem. Be sure to heed the warnings above regarding cable flexibility and insulation,
especially when running wire into doors and other areas with an abundance of sharp
metal.
2.5 I heard that I should run my power wire directly to
my car's battery. Why should I bother, and how do I do
it? [JSC]
For some components, like head units and equalizers, it's acceptable to use the stock
wiring for power. However, amplifiers generally require large amounts of power, and
accordingly will draw large amounts of current. The factory wiring in most cars is not
designed to handle large amounts of current, and most wires have 10-20A fuses on them.
Thus, you will almost always want to run the power line for your amplifier directly to the
positive terminal of the battery. This could require drilling a hole through the car's
firewall, or at least spending time hunting for an existing hole (the steering column is a
good place to start looking). Always remember to place a fuse on your wire as near to the
battery as possible! For various reasons, such as an accident or simple wear and tear, your
wire's insulation may eventually crack, which could allow the conducting wire to make
contact with the chassis of the car and short the battery through this wire, which could
lead to a serious fire. The closer you place a fuse to the battery, the more protected you
are. Also, when running wire through areas with sharp metal corners, it is a good idea to
use rubber grommets to provide extra protection against tearing through your wire's
insulation.
2.6 Should I do the same thing with my ground wire,
then? [JSC, IDB]
No. In almost every case, the best thing to do is to ground your amplifier to a point that is
attached to the chassis of the car and is as close to the amplifier as possible. The ground
wire should not need to be more than about eighteen inches long, and should be at least as
large as the power wire. The point to which you make your ground connection should be
an unpainted piece of bare metal.
Some cars (Audi, Porsche) have galvanized bodies, and in these cars, you must find one
of the manufacturers' grounding points or else some noise can result.
2.7 Sometimes when I step out of my car, I get a really
bad shock. What is wrong with my system? [IDB]
Nothing. This is caused by static buildup by rubbing against the seats, floor mats, etc.,
just like walking across a carpet in a home. You can avoid this shock by touching
something metal on your car before you put your foot on the ground.
2.8 When my car is running and I have the music
turned up loud, my headlights dim with the music. Do I
need a new battery or a new alternator? [CD, MO]
The headlights will dim because of a momentary drop in the voltage level that is
available to power the vehicle's accessories, including the headlights, amplifiers, the
engine, etc. This voltage drop can be caused by a very large current demand by an
accessory, such as an amplifier trying to reproduce a loud bass note.
The first thing to do is to get your battery and alternator checked for proper functioning.
A failing battery can place undesirable loads on the alternator, leaving less power for
your system.
If the power system appears to be working correctly, an improved alternator may be
required for the large current demands of the audio system. When upgrading an
alternator, be careful in your purchase, for there are some potential problems. An
alternator which advertises a certain output level may only achieve that output at very
high engine RPM ranges, for instance. Also, the new alternator must be adjusted to
provide an output voltage within a reasonable range in terms of the voltage regulator.
If you find your car will not start after playing the stereo for long periods of time with the
engine off, and the present battery is in good working order, then another, paralleled
battery could prevent this embarrassing problem.
2.9 What is a "stiffening capacitor", and how does it
work? [JSC]
Stiffening Capacitor (note capitals) is a trademark of Autosound 2000. However,
"stiffening capacitor" (note lowercase), as a generic term, refers to a large capacitor
(several thousand microfarads or greater) placed in parallel with an amplifier. The
purpose of doing so is to provide a sort of reserve power source from which the amplifier
can rapidly draw power when it needs it (such as during a deep bass note). The electrical
theory is that when the amplifier attempts to draw a large amount of current, not only will
the battery be relatively slow to respond, but the voltage at the amplifier will be a little
lower than the voltage at the battery itself (this is called line drop). A capacitor at the
amplifier which is charged to the battery voltage will try to stabilize the voltage level at
the amplifier, dumping current into the amplifier. Another way to think about it is that a
capacitor in parallel with a load acts as a low pass filter See section 3.10 What is a
crossover? Why would I need one? [JSC], and the voltage level dropping at the amplifier
will appear as an AC waveform superimposed upon a DC "wave". The capacitor, then,
will try to filter out this AC wave, leaving the pure DC which the amplifier requires.
2.10 Should I install one in my car? If so, how big
should it be, and where do I get one? [JSC]
If you have a problem with dimming headlights when you have your music turned up and
the bass starts to hit and the engine is running and you don't want to upgrade your
alternator, or if the transient response of your amplifier is unacceptable to you, a
stiffening capacitor could help you out. The commonly accepted "formula" for
determining the proper size capacitor to use is 1F/kW (one farad per kilowatt). For
example, a system running at 300W would need a 0.3F (or 300,000uF) capacitor.
To install the capacitor, you should not simply attach it to your power and ground wires
near your amplifier, as it will draw very large amounts of current from your battery and
could blow fuses (or over charge). Instead, you should insert a small-value power resistor
(25 ohm, 1/2 watt) or a 12VDC test lamp in between the power lead and the capacitor,
and then charge it. If you use a lamp in series with the cap, when the lamp goes out, the
capacitor is done charging. When it is done charging, carefully remove the capacitor's
leads from the charging circuit, being certain not to touch the two leads together. You
may then permanently install the capacitor by wiring it in parallel with your amplifier (be
careful not to short the leads!). Large caps are currently available from some audio
dealers, such as Phoenix Gold. You could also try electronics shops or mail-order houses.
2.11 What about adding a new battery? Or upgrading
the amplifier? [IDB]
Generally, adding a second battery is great if you want to listen to your stereo with the
car turned off (and be able to start it again later!). As far as simply upgrading your current
battery to a larger model, you may find that this will help solve the problem because
batteries like the Optima 800 offer a larger number of cold cranking amps. However, the
"response time" between when a battery kicks in a large amount of extra current and how
long it takes a capacitor is vastly different.
Although a battery may be able to respond in tenths of a second, bass notes are often
much shorter and need current immediately -- which capacitors can supply. The
difference between the two is that while the capacitor can supply a large amount of
current immediately, their voltage quickly drops making them ineffective -- but by this
time, usually the bass note has passed, and the capacitor has done its job of "stiffening"
the supply voltage.
Upgrading your alternator becomes a concern when you need a large amount of current
from it frequently. If you are always listening to your stereo at a fairly high volume
(assume your amp is drawing 20A), and then you use the A/C and a few other misc.
accessories in your car, you can get to the point when the alternator can no longer provide
enough current to effectively supply the car and charge the main battery. So, the main
battery is always supplementing the alternator's current supply, and slowly (but surely)
dies a relatively quick death.
3 Components
This section describes various components that you can have in a car audio system, along
with common specifications, desirable features, some of the best and worst brands, and so
on.
Be aware that there is no standardized testing mechanism in place for rating car audio
products. As such, manufacturers are open to exaggerating, "fudging", or just plain lying
when it comes to rating their own products.
3.1 What do all of those specifications on speakers
mean? [JSC, CD]
Input sensitivity is the SPL the driver will produce given one watt of power as measured
from one meter away given some input frequency (usually 1kHz unless otherwise noted
on the speaker). Typical sensitivities for car audio speakers are around 90dB/Wm. Some
subwoofers and piezo horns claim over 100dB/Wm. However, some manufacturers do
not use true 1W tests, especially on low impedance subwoofers. Rather, they use a
constant voltage test which produces more impressive sensitivity ratings.
Frequency response in a speaker refers to the range of frequencies which the speaker can
reproduce within a certain power range, usually +/-3dB.
Impedance is the impedance of the driver See section 1.1 What do all of those acronyms
mean? [JSC], typically 4 ohms, although some subwoofers are 8 ohms, some stock Delco
speakers are 10 ohms, and some stock Japanese imports are 6 ohms.
Nominal power handling is the continuous power handling of the driver. This figure tells
you how much power you can put into the driver for very long periods of time without
having to worry about breaking the suspension, overheating the voice coil, or other nasty
things.
Peak power handling is the maximum power handling of the driver. This figure tells you
how much power you can put into the driver for very brief periods of time without having
to worry about destroying it.
3.2 Are component/separates any better than fullrange
or coaxials? [JSC, DK]
Usually, yes. Using separates allows you to position the drivers independently and more
carefully, which will give you greater control over your imaging. For best results, try to
keep the mid and tweeter as close together as possible -- this will make the two drivers
act more like a single point source (which is ideal).
For rear fill applications, however, coaxial speakers will perform fine, as imaging is not a
primary concern. However, it is very common to use a low pass crossover with the rear
speakers (at 2500 Hz) since rear-fill is intended to produce "ambiance," and high
frequencies (> 2500 Hz) can confuse the soundstage, making it appear that music is
originating behind you.
3.3 What are some good (and bad) brands of speakers?
[JSC]
People will emotionally defend their particular brand of speakers, so asking what the
"best" is is not a good idea. Besides, the best speaker is the one which suits the
application the best. In general, however, various people have claimed excellent
experiences with such brands as Boston Acoustics, MB Quart, a/d/s/, and Polk. Also,
most people agree that you should avoid brands like Sparkomatic and Kraco at all costs.
3.4 What do all of those specifications on amplifiers
mean? [JSC, BG]
Frequency response refers to the range of frequencies which the amplifier can reproduce
within a certain power range, usually +/-3dB.
Continuous power output is the power output of the amplifier into one channel into a
certain load (usually four ohms) below a certain distortion level (usually at most
1%THD) at a certain frequency (usually 1kHz). A complete power specification should
include all of this information, e.g. "20W/ch into 4 ohms at < 0.03%THD at 1kHz"
although this can also be stated as (and be assumed equivalent to) "20W/ch at <
0.03%THD". The amplifier should also be able to sustain this power level for long
periods of time without difficulties such as overheating.
Peak power output is the power output of the amplifier into one channel into a certain
load (usually four ohms) below a certain distortion level (usually much higher than the
continuous rating level) at a certain frequency (usually 1kHz). A complete power
specification should include all of this information, e.g. "35W/ch into 4 ohms at <
10.0%THD at 1kHz" although this can also be stated as (and be assumed equivalent to)
"35Wch at < 10.0%THD". Consumer warning: some manufacturers will state the "peak
power output" rating by including the amount of power which can be drawn from
"headroom", which means power supply capacitors. They usually will not tell you this in
the specification, however; indeed, they tend to prominently display the figure in big,
bold letters on the front of the box, such as "MAXIMUM 200W PER CHANNEL!!!"
when the continuous rating is 15W/ch and the unit has a 5A fuse.
Damping factor represents the ratio of the load being driven (that is, the speaker - usually
four ohms) to the output impedance of the amplifier (that is, the output impedance of the
transistors which drive the speakers). The lower the output impedance, the higher the
damping factor. Higher damping factors indicate a greater ability to help control the
motion of the cone of the speaker which is being driven. When this motion is tightly
controlled, a greater transient response is evident in the system, which most people refer
to as a "tight" or "crisp" sound. Damping factors above 100 are generally regarded as
good.
Signal to Noise or S/N is the ratio, usually expressed in decibels, of the amount of true
amplified output of the amplifier to the amount of extraneous noise injected into the
signal. S/N ratios above 90 to 95dB are generally regarded as good.
3.5 What does "bridging an amp" mean? [MHa]
Bridging refers to taking two channels of an amplifier and combining them to turn the
amplifier into a one channel amplifier.
3.5.1 Why should I bridge my amp?
For increased power. If your amp can handle the load, it will put out more power through
a bridged channel than it would into through a non-bridged channel. Theoretically, a
"perfect" amplifier that puts out X watts into Y impedance into each of two channels will
put out 4X watts into Y impedance into one bridged channel. Be aware that some amps
more closely approximate that perfect amp than others, and some manufacturers build
current limiters into their amps to allow them to remain stable into difficult loads at the
expense of power gains.
3.5.2 Why shouldn't I bridge my amp?
There are several reasons: you might need those extra channels; your amp might not be
stable into the load your speakers present if the amp is bridged; you might be a hyperperfectionist that can't stand the thought of an small increase in distortion; or perhaps you
just don't need more power. Car audio power is relatively cheap, and if you are not trying
to make a mega-gonzo system, you may not need to double your power.
3.5.3 What happens when an amp is bridged?
Basically, one channel's signal is inverted, and then the two channels are combined to
form one channel with twice the voltage of either of the original channels.
Ohm's Law for Alternating Current states that I = V/Z where I is current, V is voltage,
and Z is impedance. We also know that P = IV, where P is power. If we use Ohm's Law
and substitute into the power equation, we get P = V(V/Z), which can be rewritten as P =
(V^2)/Z. Therefore, power is the square of voltage divided by impedance.
Now, why do we care about all that? Because it explains precisely what happens when an
amp is bridged. I'll give a practical example and explain the theoretical basis of that
example.
Imagine you have a two-channel amp that puts out 50 watts into each channel when
driven into a load of 4 ohms per channel. Since we know P and Z, we can plug these
numbers back into our power equation and find V. 50 = V^2/4 -> V = sqrt(200). So,
we're seeing a voltage of 14.1 volts across each channel.
Now, imagine we bridge this amp, and use it to push just one of those 4 ohms loads.
When the amp is bridged, the voltage is doubled. Since we know the voltage (2*14.1
volts), and the impedance (4 ohms), we can calculate power. Remember that P = V*V/Z.
That means P = (28.2)^2/4, which is 198.1 watts. It should be clear by now that the new
power is approximately 200 watts - quadruple the power of a single, unbridged channel!
You can probably see that should be the case, especially if you look back at the power
equation. Since P = V*V/Z, if you double V, you quadruple power, since V is squared in
the power equation.
Now, all this assumes the amp is stable into 4 ohms mono. The mono channel is putting
out four times as much power as a single unbridged channel, so it must be putting out
twice as much as the two single channels combined. Since the voltage on the supply side
of the amp is dependent on the car's electrical system, it doesn't change (OK, the
increased current might cause a voltage *drop*, but let's not worry about that now).
Looking at the first power equation, at the supply side of the amp, we see P = IV. Now,
when we bridged the amp, we doubled the power, but the input voltage stayed the same.
So, if we hold V constant, the only way to double the power is to double the current.
That means the amp is now drawing twice as much current when it's running at a given
impedance mono than it would be running two stereo channels at the same impedance.
There are only two ways the amp can do that - it can simply draw more through it's
circuits, and dissipate the extra heat, or it can utilize a current limiter, to prevent the
increase in current. Of course, using the current limiter means you don't get the power
gains, either! So, if the amp can't handle the extra current, and it doesn't limit the current
in some way, kiss it goodbye. For that reason, an amp is typically considered mono stable
into twice the impedance it is considered stereo stable.
3.5.4 Does bridging an amp would halve the impedance of the speakers?
Impedance is a characteristic of the speakers. The speakers don't give a flip how the amp
is configured: they have a given impedance curve, and that's that. It should be clear that
when you bridge an amp, you are changing *the amp*. The speaker's impedance is *not*
a function of the amp, but the amp's tolerance to a given impedance depends completely
on the way the amp is configured. If you'll remember from section 4, an amp bridged into
a given impedance draws twice as much current as it would if it were driving two
separate channels, each at that impedance. So, a four ohm speaker stays a four ohm
speaker, if it's hooked to one channel, a bridged channel, a toaster, or the wall socket.
But, it is more stressful for the amp to drive any impedance bridged than unbridged.
So, why do people talk about the impedance halving? Well, it's a simple model that isn't
correct but is easy to explain to people who don't know what's really going on. It goes
like this: When you bridge the amp, each channel is "seeing" half the load presented to
the amp. So, if you bridge an amp to 4 ohms, each channel "sees" 2 ohms. Therefore,
each channel puts out twice as much power, and the combined output is quadruple a
single channel at 4 ohms.
Why is that still wrong? Because each channel isn't really used as a single channel.
You've used part of one channel, and an inverted part of another channel to create a
totally new channel, the bridged channel. Also, there's no way for a channel to "see" only
part of a circuit. If it's "seeing" half the speaker, it's "seeing" it all.
Second, it makes it awkward if people believe that the impedance is really, literally,
changing. If you use that model, is it safe to run a 4 ohm mono stable amp into a 4 ohm
speaker? It should be, but we just said the impedance halves, so that's now a 2 ohm
speaker, and you can't use it. That's wrong, and confusing, and it makes people think they
can't do things they really can.
3.5.5 Can I bridge my 4 channel head unit?
Generally, NO. Unless the manuals that came with your head unit specifically state that
your head unit can be bridged, then do NOT attempt it -- this could destroy the head
unit's internal amplifier, and possibly void your warranty.
3.6 What is "mixed-mono?" Can my amp do it? [JSC,
IDB]
Some amplifiers which are both bridgeable and able to drive low impedance loads also
allow you to use mixed-mono mode. This involves driving a pair of speakers in stereo
mode as well as simultaneously driving a single speaker in bridged mono mode off of
ONE pair of the amp's channels.
To do this, you connect the mono speaker (typically a subwoofer) to the amp as you
normally would in bridged mode, and then connect the left and right stereo speakers to
the left and right stereo channels, respectively.
However, for this to work, the amplifier must actually use both input channels in bridged
mode. Many amplifiers, when placed in bridged mode, will simply "copy" and invert
either the left or the right channel. This practice ensures high output to the mono speaker,
but eliminates the possibility of mixed mono since you lose one channel.
It is VERY important to use passive crossovers when configuring your amplifier in
mixed-mono mode in order to keep from overloading the amp. The reason almost all new
amplifiers are able to run in mixed-mono mode (even if they are only 2-ohm stable) is
that the impedance seen by each channel of the amplifier is the same across the entire
frequency spectrum when using passive crossovers. Here's how it works: Take a typical
2-channel amplifier that is stable to 2 ohms (stereo) or 4 ohms (mono). When the
subwoofer is connected with a low-pass crossover (at 100Hz, for example) then the
amplifier "sees" a 2 ohm load on each of its channels (see 3.5) from 100Hz and down.
When the full range speakers are connected with a high-pass crossover (at 125Hz, for
example), the amplifier "sees" a 4 ohm load on each of its channels from 125Hz and up.
The passive crossovers prevent the amplifier from seeing more than one speaker on either
channel at any given frequency. Of course, between the two crossover points the amp
DOES see more than one speaker (and therefore the load on the amp dips to 1.33 ohms
when using 4 ohms speakers).
A graph of impedance vs frequency for ONE channel of an amplifier would look similar
to this when using 3 4-ohm speakers and crossover points at 100Hz(LP) and 200Hz (HP):
+-----------------------------------------------------------+
|
****************************************|
|
*
|
|
*
|
|*************
*
|
|
* *
|
|
*
|
|
|
|
|
|
|
+^-----^-----^-----^-----^-----^-----^-----^-----^-----^---^+
25
50
100
200
400
800 1.6K 3.15K 6.3K 12.5K 20K
4
2
1
0
3.7 What does "two ohm stable" mean? What is a
"high-current" amplifier? [JSC]
An X-ohm stable amplifier is an amp which is able to continuously power loads of X
ohms per channel without encountering difficulties such as overheating. Almost all car
amplifiers are at least four ohm stable. Some are two ohm stable, which means that you
could run a pair of four ohm speakers in parallel on each channel of the amplifier, and
each channel of the amp would "see" two ohms. Some amps are referred to as highcurrent, which is a buzzword which indicates that the amp is able to deliver very large
(relatively) amounts of current, which usually means that it is stable at very low load
impedances, such as 1/4 or 1/2 of an ohm. Note that the minimum load rating (such as
"two ohm stable") is a stereo (per channel) rating. In bridged mode, the total stability is
the sum of the individual channels' stability See section 3.5 What does "bridging an amp"
mean? [MHa].
3.8 Should I buy a two or four (or more) channel
amplifier? [JSC]
If you only have one line-level set of outputs available, and wish to power two sets of
speakers from a single amplifier, you may be able to save money by purchasing a two
channel amplifier which is stable to two ohms rather than spending the extra money for a
four channel amp. If you do this, however, you will be unable to fade between the two
sets of speakers (without additional hardware), and the damping factor of the amplifier
will effectively be cut in half. Also, the amp may run hot and require fans to prevent
overheating. If you have the money, a four channel amp would be a better choice. You
would need to add a dual-amp balancer in order to maintain fader capability, however,
but it is more efficient than building a fader for a two channel amp. If you wish to power
a subwoofer or additional speakers as well, you may want to purchase a five or six
channel amp.
3.9 What are some good (and bad) brands of
amplifiers? [JSC, IDB]
As with speakers, people emotionally defend their amplifier, so choosing the best is
difficult. However, some brands stand out as being consistently good while others are
consistently bad. Among the good are HiFonics, Phoenix Gold, a/d/s/, and Precision
Power.
Generally, "good" amplifiers tend to cost more (in money/watt) than "bad" amplifiers. So
when you see an amp advertising 300W for only $100, and are comparing an amp with
50W for $300, you will usually find that the 50W/$300 amp will be of much higher
quality than the 300W/$100 amp.
3.10 What is a crossover? Why would I need one? [JSC]
A crossover is a device which filters signals based on frequency. A high pass crossover is
a filter which allows frequencies above a certain point to pass unfiltered; those below that
same point still get through, but are attenuated according to the crossover slope. A low
pass crossover is just the opposite: the lows pass through, but the highs are attenuated. A
band pass crossover is a filter that allows a certain range of frequencies to pass through
while attenuating those above and below that range.
There are passive crossovers, which are collections of purely passive (non-powered)
devices - mainly capacitors and inductors and sometimes resistors. There are also active
crossovers which are powered electrical devices. Passive crossovers are typically placed
between the amplifier and the speakers, while active crossovers are typically placed
between the head unit and the amplifier. There are a few passive crossovers on the market
which are intended for pre-amp use (between the head unit and the amplifier), but the
cutoff frequencies (also known as the "crossover point", defined below) of these devices
are not typically well-defined since they depend on the input impedance of the amplifier,
which varies from amplifier to amplifier.
There are many reasons for using crossovers. One is to filter out deep bass from relatively
small drivers. Another is to split the signal in a multi-driver speaker so that the woofer
gets the bass, the midrange gets the mids, and the tweeter gets the highs.
Crossovers are categorized by their order and their crossover point. The order of the
crossover indicates how steep the attenuation slope is. A first order crossover "rolls off"
the signal at -6dB/octave (that is, quarter power per doubling or halving in frequency). A
second order crossover has a slope of -12dB/octave; third order is -18dB/octave; etc. The
crossover point is generally the frequency at which the -3dB point of the attenuation
slope occurs. Thus, a first order high pass crossover at 200Hz is -3dB down at 200Hz, 9dB down at 100Hz, -15dB down at 50Hz, etc.
It should be noted that the slope (rolloff) of a crossover, as defined above, is only an
approximation. This issue will be clarified in future revisions of this document.
The expected impedance of a passive crossover is important as well. A crossover which
is designed as -6dB/octave at 200Hz high pass with a 4 ohm driver will not have the same
crossover frequency with a driver which is not 4 ohms. With crossovers of order higher
than one, using the wrong impedance driver will wreak havoc with the frequency
response. Don't do it.
3.11 Should I get an active or a passive crossover? [JSC,
JR]
Active crossovers are more efficient than passive crossovers. A typical insertion loss
(power loss due to use) of a passive crossover is around 0.5dB. Active crossovers have
much lower insertion losses, if they have any loss at all, since the losses can effectively
be negated by adjusting the amplifier gain. Also, with some active crossovers, you can
continuously vary not only the crossover point, but also the slope. Thus, if you wanted to,
with some active crossovers you could create a high pass filter at 112.3Hz at 18dB/octave, or other such things.
However, active crossovers have their disadvantages as well. An active crossover may
very well cost more than an equivalent number of passive crossovers. Also, since the
active crossover has separate outputs for each frequency band that you desire, you will
need to have separate amplifiers for each frequency range. Furthermore, since an active
crossover is by definition a powered device, the use of one will raise a system's noise
floor, while passive crossovers do not insert any additional noise into a system.
Many people find it advantageous to use both active and passive crossovers. Often, a
separate amp is dedicated to the subwoofers, to give them as much power as possible.
The other amplifier is used to power the mids and tweeters. In this scheme, an active
crossover is used to send only the sub-bass frequencies to the sub amp, and the other
frequencies to the other amp. The passive crossovers are used to send the correct
frequencies to the individual speakers (e.g., mids and tweeters).
Thus, if you have extra money to spend on an active crossover and separate amplifiers,
and are willing to deal with the slightly more complex installation and possible noise
problems, an active crossover is probably the way to go. However, if you are on a budget
and can find a passive crossover with the characteristics you desire, go with a passive.
3.12 Should I buy an equalizer? [JSC]
Equalizers are normally used to fine-tune a system, and should be treated as such.
Equalizers should not be purchased to boost one band 12dB and to cut another band 12dB
and so on - excessive equalization is indicative of more serious system problems that
should not simply be masked with an EQ. However, if you need to do some minor
tweaking, an EQ can be a valuable tool. Additionally, some EQs have spectrum analyzers
built in, which makes for some extra flash in a system. There are two main kinds of EQs
available today: dash and trunk. Dash EQs are designed to be installed in the passenger
compartment of a car, near the head unit. They typically have the adjustments for
anywhere from five to eleven (sometimes more) bands on the front panel. Trunk EQs are
designed to be adjusted once and then stashed away. These types of EQs usually have
many bands (sometimes as many as thirty). Both types sometimes also have crossovers
built in.
3.13 What are some good (and bad) brands of
equalizers? [HK]
Generally, companies that produce 1/3 octave (30 band) and 2/3 octave (15 band)
equalizers are good. These include AudioControl, USD, Rane, Phoenix Gold. Most
people try to stay away from equalizers that contain a "booster;" these are made by
Kraco, Urban Audio Works and others.
3.14 What do all of those specifications on tape deck
head units mean?
3.15 What are features to look for in a tape deck?
3.16 What are some good (and bad) brands of tape
decks?
3.17 What are features to look for in a CD head unit?
3.18 Should I buy a detachable faceplate or pullout CD
player? [IDB]
It is getting difficult to find pullout CD players any more, since detachable faceplates are
much more convenient to carry around. However, there is the obvious trade off -- it is
still possible to steal the chassis for the detachable face unit, when that is not possible
with a pullout. Although some companies will advertise that it is very difficult to get
replacement faces without the original receipt, thieves can still get the faceplates.
Some companies, such as Eclipse, are starting to offer alternative methods for preventing
theft. Some Eclipse decks now offer ESN (Eclipse Security Network), where the owner
chooses a "key" CD that must be inserted to "revive" the deck should it lose power. The
entire deck stays in the dash, with nothing to carry around; this expands on the trend
towards convenience while offering the owner peace of mind.
3.19 What are some good (and bad) brands of CD head
units? [HK]
Generally, Alpine, Clarion, Eclipse, McIntosh, Phillips and Pioneer are considered to
produce good quality CD head units. They all have their problems, but these brands seem
to be common and relatively problem-free.
Bad brands include Kraco, Radio Shack, Rockwood and other "bargain" brands.
3.20 Can I use my portable CD player in my car? Won't
it skip a lot? [JSC]
You can use any portable CD player in a car provided that you have either an amplifier
with line level inputs (preferred) or a tape deck. If you have the former, you can simply
buy a 1/8" headphone jack to RCA jack adapter and plug your CD player directly into
your amplifier. If you have the latter, you can purchase a 1/8" headphone jack to cassette
adapter and play CDs through your tape deck. The cassette adapters tend to be far more
convenient; however, there is a significant tradeoff: by using cassette adapters, you limit
your sound to the frequency response of the tape head, which is sometimes as much as an
entire order of magnitude worse than the raw digital material encoded onto the CD itself.
Portable CD players which were not designed for automotive use will tend to skip
frequently when used in a car (relatively). CD players that are specially designed for
automotive use, such as the Sony Car Discman, tend to include extra dampening to allow
the laser to "float" across the bumps and jolts of a road. Some people have indicated
success with using regular portable CD players in a car when they place the CD player on
a cushion, such as a thick shirt or even on their thighs.
3.21 What's that weird motor noise I get with my
portable CD player? [JSC]
Many people report problems while playing CDs from a portable CD player into their car
audio systems. The problem, stated very simply, has to do with the stepping of the motor
requiring a varying amount of current and non-isolated power and audio signal grounds.
Using a liberal application of capacitors and inductors, this voltage variance can be
restricted to a window of 8.990 to 9.005V for a 9V CD player, yet even the swing
between these two levels is enough to cause annoyingly loud noise on the outputs. It has
been reported that this entire problem can be solved by using a true DC-DC inverter at
the power input to the CD player.
3.22 What are some good (and bad) brands of portable
CD players?
3.23 What's in store for car audio with respect to MD,
DAT and DCC? [HK]
MiniDisc (MD) seems to have a better future than Digital Audio Tape (DAT) or Digital
Compact Cassette (DCC) which don't seem to have appeal to the public. Ease of use
seems to be an important factor and the CD formats allows direct access to musical tracks
at an instant. Although MD doesn't match the sound quality of the standard CDs it will
probably be popular since the players have a buffer to eliminate skipping. DAT will
remain as a media for ProAudio for recording purposes before pressing CDs.
3.24 Are those FM modulator CD changers any good?
What are my other options? [PW, JGr]
Almost all manufacturers offer an FM modulator for their changers. As with all
equipment, some are good and some are not. A person thinking about using an FM
modulator must take into consideration that the sound quality will only be as good as the
tuner in your head unit. Also, FM is limited in its frequency response. There is usually a
noticeable loss of the high frequencies, due to the nature of transmitting via FM.
If you do not want to use an FM Modulated CD changer, some manufacturers make
controllers for changers that feature line-level (RCA) outputs. This allows you to connect
the changer directly to an amplifier, bypassing the stock system altogether. Some models
offer line-level inputs, allowing you to connect the stock system to the changer (so you
can continue to use your radio/tape). Clarion, Sony, and Kenwood make such units.
Your third option is to use the aftermarket changer that corresponds to the stock unit in
your car. Not all cars have this option, but it is becoming easier. See section 3.25 What
kind of changer will work with my factory head unit? [PO].
3.25 What kind of changer will work with my factory
head unit? [PO]
Many factory head units these days have the ability to control a remotely mounted cd
changer. Generally, the head will have a button labelled "CD" to switch sources to the
external changer. In this mode either the radio preset buttons and/or the tuner up/down
buttons will control which CD and/or track is playing. Check your car's manual to make
sure your head can control a changer and how the buttons work.
Once you know your head can control a changer, you wonder "What kind of changer will
work with my factory head unit?" Of course, the one the dealer wants to sell you will
work. However, the dealer makes lots of money selling you a changer, and there are often
other after-market solutions, usually involving an adapter cable and a name-brand
changer. The dealer will tell you that their solution is better and that's why it costs so
much more (often more than twice as much as an aftermarket solution).
The car manufacturers are constantly changing the interfaces between their heads and
changers, in an effort to get you to buy their solution. However, the after-market is
constantly reverse-engineering the interfaces and providing alternative solutions for the
cost-conscious consumer.
Two companies that make such adapters are Precision Interface Electronics (or PIE,
http://www.pie.net) and Peripheral Interface Components (http://www.stingeraamp.com/peripheral/s-ind.htm). Check their web sites to see if there's an adapter for
your car's factory head. They also list which changer(s) will work with their adapters.
For example, many of Honda's late-model heads were made for them by Alpine, so the
OEM changer you'd pay your friendly Honda dealer ~$700 for is essentially the same as
Alpine's changers. The only difference is the interface wiring, where they swapped two
pins, specifically so you'd have to get it from the dealer. (If you're interested in the
details, see http://integra.cyberglobe.net/caraudio/diagrams/DIN.html). The after-market
adapters for this head simply swap the pins back, so you can use the regular Alpine
changer, which can be bought for ~$300.
Once you know which adapter/changer combo will work, you can get it from your local
car audio dealer or favorite mail order place. The advantage of getting it from a local
dealer is that they'll be able to install it for you. However, if you have the time and are at
all mechanically inclined, you should readily be able to install it yourself.
3.26 What are some good (and bad) brands of CD
changers?
You will find that those companies who make high-quality in-dash CD players will also
make good CD changers. See section 3.19 What are some good (and bad) brands of CD
head units? [HK], for a list.
3.27 Why do I need a center channel in my car, and how
do I do it? [HK, JSC]
If a proper center image isn't achievable via a two channel configuration, installation of a
center channel can help. Since the majority of recordings are done in two channel, a two
channel system designed correctly should be able to reproduce a center image which was
captured during recording. A center channel is not simply a summation of the left and
right channels, like bridging an amplifier; rather, it is an extraction of common signals
from the left and right channels. This usually means the lead vocals, and perhaps one or
two instruments. These signals will then be localized to the center of the stage, instead of
perhaps drifting between the left center and right center of the stage. A signal processor is
usually required in order to properly create a center channel image. The image should
then be sent to a driver in the physical center of the front of the car, at an amplification
level somewhat lower than the rest of the speakers. The correct frequency range and
power levels will depend on the particular installation, though a good starting point is
perhaps a pass band of 250-3000Hz at an amplification level of half the power of the
main speakers (3dB down).
3.28 Should I buy a sound field processor? [DK]
Sound field processors (also known as DSPs) are fun toys to play with, and can have
some use, but it is generally good to keep the KISS principle in mind: Keep It Simple,
Stupid.
The fewer signal processors (this includes equalizers, and active crossovers) that are in
your system, the less chance there will be for noise to enter your system. You'll also save
money, have a lower noise floor. Surround sound processors and bass regenerators are
nothing more than bells and whistles and are totally superfluous in a properly designed
system.
3.29 What are some good (and bad) brands of signal
processors? [IDB]
If you do decide to buy a signal processor, try to stick with reputable brands like:
AudioControl, Clark, Crystal-Line, Phoenix Gold, Rane or Clarion. Try to stay away
from brands such as Petras, Urban Audio Works and Kraco.
3.30 I keep hearing that speakers for Company X are
made by Company Y. What's the deal? [IDB, DK]
Many of the speakers you've ever purchased or ever will purchase have been assembled
in plants "along side" speakers from other manufacturers, but that does NOT imply in any
way, shape or form whatsoever that the two brands are even VAGUELY similar. This is
often done in order to reduce costs because purchasing your own gaussing stations and
mass producing your own drivers takes a LOT of money to implement.
3.31 What is a Line Driver? Do I need one? [LC,IDB]
A line driver is a device that amplifies a signal, such as the low-level signal output from a
head unit. Line drivers are made to amplify the line level signal to as much as 10 volts or
higher. This, of course, is useless unless the receiving end can handle 10 volts as input.
To solve this problem, there are line receivers which bring the line level voltage down
from 10 volts or more to about 1 volt. Usually, the line driver and receiver are placed as
close to the sending signal source and destination as possible, to minimize noise pick up.
The automobile is an inherently noisy electrical environment. So RCA cables may pick
up noise as it makes its way to the amplifier. Note that noise here refers to the induced
noise, not ground loop noise such as engine whine. A simple way to fight against this
noise is to make the signal level carried in the RCA cable very high, thus increasing the
signal's resistance to induced noise and resulting in a higher signal to noise ratio at the
destination of the RCA cable. Most head units produce a fairly low output voltage (< 1.5
V), although recently high end head units advertise 4 volt or higher output, and won't
usually need a line driver.
The line driver will increase dynamic range in certain cases where excessive noise is
masking the lower level signals. However, a line driver will not increase the dynamic
range when used in a system with little noise to begin with.
There is some truth to the claim that a line driver will let you play your stereo louder
since there are cases where the amplifier still doesn't play at its full potential even when
its gain is turned all the way up and the volume on the head unit is maxed out. Adding a
line driver here will allow you to turn down the gain on the amp while using a lower
volume setting on the head unit.
But before you jump in with both feet, remember that all electronics has their own
inherent noise. Thus if you don't have a serious case of induced noise, a line driver will
do little good since it might add enough noise to offset what little noise it "takes away."
The line driver is a patch to the noise problem rather than a fix so it is still not the
ultimate solution. My personal experience has shown to ME that a properly installed
system with none-malfunctioning components will have little noise, even if you use low
grade components such as those made by the less desirable manufactures. Also, a lot of
crossovers and EQ units have rather high low-level output signals. Some times as high as
8 volts. So be sure to take this into consideration.
3.32 Can I play MP3 files in my car? [IDB]
Unfortunately, MP3 (MPEG Layer 3) audio files written to a CD-ROM can not be played
by any of the current "mainstream" head units or CD changers.
Many knowledgable individuals have spent hours installing PCs into their cars or
building specialized hardware to play MP3 files in their car. Although many of these
systems have been "hacked" together, there are now a number of commercially available
systems that can be purchased and installed.
One of the best sources for information about playing MP3 files in your car is located at:
http://www.mp3car.com
This site provides links to many of the commercial products as well as the "hobbyist"
projects that you could follow to build your own system.
4 Subwoofers
This section describes some elements necessary for understanding subwoofers - how they
operate, how to build proper enclosures, how to pick the right driver for you, and how to
have a computer do some of the work for you.
4.1 What are "Thiele/Small parameters?" [CD, RDP]
These are a group of parameters outlined by A. N. Thiele, and later R. H. Small, which
can completely describe the electrical and mechanical characteristics of a mid and low
frequency driver operating in its pistonic region. These parameters are crucial for
designing a quality subwoofer enclosure, be it for reference quality reproduction or for
booming.
`Fs'
Driver free air resonance, in Hz. This is the point at which driver impedance is
maximum.
`Fc'
System resonance (usually for sealed box systems), in Hz
`Fb'
Enclosure resonance (usually for reflex systems), in Hz
`F3'
-3 dB cutoff frequency, in Hz
`Vas'
"Equivalent volume of compliance", this is a volume of air whose compliance is
the same as a driver's acoustical compliance Cms (q.v.), in cubic meters
`D'
Effective diameter of driver, in meters
`Sd'
Effective piston radiating area of driver in square meters
`Xmax'
Maximum peak linear excursion of driver, in meters
`Vd'
Maximum linear volume of displacement of the driver (product of Sd times
Xmax), in cubic meters.
`Re'
Driver DC resistance (voice coil, mainly), in ohms
`Rg'
Amplifier source resistance (includes leads, crossover, etc.), in ohms
`Qms'
The driver's Q at resonance (Fs), due to mechanical losses; dimensionless
`Qes'
The driver's Q at resonance (Fs), due to electrical losses; dimensionless
`Qts'
The driver's Q at resonance (Fs), due to all losses; dimensionless
`Qmc'
The system's Q at resonance (Fc), due to mechanical losses; dimensionless
`Qec'
The system's Q at resonance (Fc), due to electrical losses; dimensionless
`Qtc'
The system's Q at resonance (Fc), due to all losses; dimensionless
`Ql'
The system's Q at Fb, due to leakage losses; dimensionless
`Qa'
The system's Q at Fb, due to absorption losses; dimensionless
`Qp'
The system's Q at Fb, due to port losses (turbulence, viscosity, etc.);
dimensionless
`n0'
The reference efficiency of the system (eta sub 0) dimensionless, usually
expressed as a percentage
`Cms'
The driver's mechanical compliance (reciprocal of stiffness), in m/N
`Mms'
The driver's effective mechanical mass (including air load), in kg
`Rms'
The driver's mechanical losses, in kg/s
`Cas'
Acoustical equivalent of Cms
`Mas'
Acoustical equivalent of Mms
`Ras'
Acoustical equivalent of Rms
`Cmes'
The electrical capacitive equivalent of Mms, in farads
`Lces'
The electrical inductive equivalent of Cms, in henries
`Res'
The electrical resistive equivalent of Rms, in ohms
`B'
Magnetic flux density in gap, in Tesla
`l'
Length of wire immersed in magnetic field, in meters
`Bl'
Electro-magnetic force factor, can be expressed in Tesla-meters or, preferably, in
meters/Newton
`Pa'
Acoustical power
`Pe'
Electrical power
`c'
Propagation velocity of sound at STP, approx. 342 m/s
`p'
Density of air at STP 1.18 kg/m^3 (rho)
4.2 How does speaker sensitivity affect real world SPL?
Will a higher sensitivity give me a larger SPL? [MS]
When it comes to mids and highs, efficiency (sensitivity) is a fairly good indicator of
output differences at the same power level. When it comes to subwoofer performance, the
driver's sensitivity is irrelevant unless you are also specifying a box volume.
An efficient sub requires a larger box to achieve equivalent extension to a less efficient
sub. In a small box, the less efficient sub will actually be LOUDER at low frequencies at
the SAME POWER as the more efficient sub.
Linear excursion is a very good indicator of ultimate output capability (given sufficient
power to drive the speaker to that point.) To make sound you must move air; therefore,
the more air you move, the more sound you make. When comparing two speakers of
equal surface area, the one with greater excursion capability will play louder given
sufficient power.
4.3 What are the enclosure types available? [JLD, JG]
Only the order of the enclosure itself is shown here. The addition of a crossover network
increases the order of the system by the order of the crossover. Example: If a First-Order,
6dB/Oct. crossover (single inductor in series with the speaker) is used with a Fourth
Order enclosure, the total system is a fifth order. Note: Air volumes and ratios shown
here may not be to scale. This is designed to provide order information only.
First Order
Infinite-Baffle or Free-Air
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Acoustic- or Air-Suspension
or Sealed
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Fourth Order
Bass-Reflex or
Vented or Ported
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Fourth Order
Single-Reflex Bandpass
Second Order
Isobaric* Acoustic-Suspension
(Compound Loaded)
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Fourth Order
Passive Radiator
Bass-Reflex
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Fourth Order
Isobaric*
Bass-Reflex
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Fourth Order
Isobaric* Single-Reflex Bandpass
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Fourth Order
Three Chamber
Single-Reflex Bandpass
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Fourth Order
Three Chamber Isobaric*
Single-Reflex Bandpass
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Fifth Order = Fourth Order Enclosure + First Order Crossover
= Third Order Enclosure + Second Order Crossover, etc.
Sixth Order
Dual-Reflex Bandpass
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Sixth Order
Three Chamber
Dual-Reflex Bandpass
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Sixth Order
Isobaric* Dual-Reflex Bandpass
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Quasi-Sixth Order
Series-Tuned Bandpass
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Seventh Order = Sixth Order Enclosure + First Order Crossover, etc.
Quasi-Eighth Order
Series-Tuned Dual-Reflex
Bandpass
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Triple-Reflex Bandpass
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* Isobaric or Coupled Pair (Iso-group) Variations:
A variety of configurations may be used in the isobaric loading
of any order enclosure. Physical and acoustic restrictions may
make one loading configuration preferable to another in a
particular enclosure.
Composite or Push-Pull
or Face-to-Face Loading
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Compound or Piggy-Back
or Tunnel Loading
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Back-to-Back Loading
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Planar Loading
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>>> indicates direction of
>>> simultaneous cone movement.
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4.4 Which enclosure type is right for me? [IDB, DK]
This answer is not designed to tell you exactly what kind of enclosure to build, but rather
to give an idea of the advantages and disadvantages to the simple configurations (Infinite
baffle [1st order], Sealed [2nd order], Ported [4th order] and basic bandpass). Building
and designing more complicated systems (order > 4) is not for the light at heart.
4.4.1 Infinite Baffle ("free-air")


Advantages...
o No box necessary!
o This means it's usually cheaper to design and implement in your system
Disadvantages...
o Requires that a good seal be obtained between front and rear of driver. In a
car, this can be quite difficult and may require the installer to remove trim
panels to plug any holes that would let energy "bleed through".
o The responsibility for damping cone motion rests solely on the driver's
suspension. As fatigue sets in, this becomes a critical issue in infinite
baffle set-ups.
o Less efficient in the sub-bass region than above mentioned enclosures.
o Potentially more expensive drivers than good boxable woofer - The
suspension must be extremely hearty and long-lasting to withstand high
power applications.
4.4.2 Sealed Box


Advantages...
o Small enclosure volumes
o Shallow (12 dB/Octave) roll off on low end
o Excellent power handling at extremely low frequencies
o Excellent transient response/ group delay characteristics
o Easy to build and design
o Forgiving of design and construction errors
Disadvantages...
o Not particularly efficient
o Marginal power handling in upper bass frequencies
o Increased distortion in upper bass over ported design
o When using high power and small box, magnet structure is not in an ideal
cooling environment
4.4.3 Ported Box


Advantages...
o 3-4 dB more efficient overall than sealed design
o Handles upper bass frequencies better with less distortion
o Magnet is in good cooling environment
o When properly designed, a ported box will slaughter a sealed in terms of
low frequency extension
Disadvantages...
o Size (not so critical outside the mobile environment)
o Woofer unloads below Fb
o More difficult to design/ can result in boomy, nasty sounding bass if
misaligned
4.4.4 Bandpass Box


Advantages...
o When properly designed and implemented, can provide superior LF
extension and efficiency.
o Cone motion is controlled more and therefore mechanical power handling
is increased.
o Cones are physically protected from contents of trunk flying around.
o Output is easily channeled directly into the interior of sedans.
Disadvantages...
o Difficult to build (not recommended for newbies), and very sensitive to
misalignment due to calculation or construction errors.
o Their characteristic filtering often masks any distortion that occurs as a
result of amplifier clipping or overexcursion and thus will give the user no
warning that the driver is over-stressed and about to fail.
o Need substantial mid-bass reinforcement to make up for narrow
bandwidths in efficient alignments.
o Transient response is largely dependent upon the alignment
chosen....wider bandwidths will result in sloppier performance, narrower
bandwidths (and thus higher effiencies) result in better transient
performance.
o They can oft times be quite large.
4.5 How do I build an enclosure? [AO]
These instructions are for building a first order (sealed) subwoofer enclosure. Building
ported or bandpass boxes is more difficult, and those designs are less forgiving of
mistakes. These instructions apply for all box designs, but be sure of the measurements
before you make your cut. Building your own enclosure can save you a lot of money, but
only if you don't need to buy all of your materials twice because of mistakes!
You will need:
Wood
I only use MDF See section 4.6 MDF for Dummies [IDB], but others have
reported success using other hardwoods like birch and oak. Do not use plywood it's far to flexible and porous. Use a minimum of 3/4" wood -- flexing sub
enclosures lose precious energy!
Screws
For one inch wood use #8 2 inch wood screws. For 3/4 inch wood use #8 1 3/4
inch screws. Double grip Drywall screws also work well.
Adhesive
I use "Liquid Nails" which comes in a caulk tube or a bucket, but any paste type
of adhesive will work. Spray adhesives will not work.
Silicone sealant
White, brown, clear, caulk tube, or squeeze bottle, it doesn't matter. Make sure
you don't get silicone lubricant (which comes in a spray can)!
Terminals/Terminal Cup
To allow easy connections from your amp.
Besides these materials you will need several tools:
Table saw or radial arm saw
I use a radial arm saw, just because it's a little easier and accurate, but a table saw
will work also. You can use a circular saw, but be very careful to make your cuts
straight.
Jig saw
For cutting your speaker hole.
Drill
You will also need a 1/8" drill bit, a screwdriver bit, and a countersinking bit.
Pencil
To mark your cuts, make notes, etc.
Measuring tape
Safety Goggles
Face Mask
Breathing MDF dust has not been proven to cause health-related problems, but
hang around with with a few installers at your local shop for an afternoon and
you'll see why you need a face mask. :-)
Start by marking the cuts you need to make on your wood. Double check your math, and
your measurements.
Use the table or radial arm saw to cut your wood. When you're done you should have six
pieces of wood which fit together tightly to form a box.
At this point you will need to trace the cut out for your subwoofer onto the front of the
box. Remember that if you have a 10 inch subwoofer you do NOT want a 10 inch cutout.
The 10 inch measurement is from the outside of the mounting ring. The actual cutout
diameter should be with your instructions. Transfer the proper sized circle onto the sub
box and cut it out with the jig saw. If you have trouble starting cuts with a jig saw, drill a
1/2" hole in the wood inside the circle. You can drop your blade into the hole and then
cut out to the edge of the circle and around.
After you have cut out your mounting hole you will need to cut out a square on one of the
sides for your terminal cup. Transfer the proper size rectangle onto the wood and cut it
out with the jig saw.
Now you are ready to start assembling the enclosure. Choose one of the ends, and one of
the sides. Apply a bead of adhesive along the edge of the end piece. Affix it to the edge
of the bottom piece. Flip it over (have a friend hold the other end and hold the end in
place,) and screw the edge to the end. Use one screw at each corner and then one more
screw about every 8 inches. Drill a pilot hole with your 1/8" drill bit, then drill a
countersink with your countersinking bit. Finally, drive the screw in. Make sure that you
don't strip the hole.
Repeat the above procedure with the other end. You should now have the two ends
connected to one side. Affix the other three sides the same way.
Finally, you'll want to seal the insides of the box with silicone. Apply a bead of silicone
across all the inside edges and around the terminal strip.
Allow the box to dry over night and then place your speaker into the hole. Screw it down
and you're done!
4.6 MDF for Dummies [IDB]
Since MDF (Medium Density Fiberboard) is used so frequently in building subwoofer
enclosures and other projects, this section provides some more detailed information.
4.6.1 What is MDF? [PS]
MDF (Medium Density Fiberboard) is a hardboard product comprised of hot compressed
wood fibers glued together. The color of this material can vary from light to dark brown
depending on the brand of manufacturer. Particle Board or Flakeboard consist mostly of
recycled wood chips. The size of the wood chips used varies which means that the
coarser the wood chips reduces the structural density of the material.
4.6.2 Where can I get MDF? [PS]
Wholesale distributors of lumber products. Home Improvement Centers, such as Home
Depot or Home Base, and Lumber Yards may or may not stock MDF products, but they
will be able to order it for you. Contact woodworking or cabinet making shops in your
area, ask them where to get it or if it may be possible to buy the material from their
business.
4.6.3 What type of saw blade works best when cutting MDF? [PS]
If you're using a circular saw, a good 40 tooth carbide tipped blade works best. If you're
using a table saw, a 50/60 tooth carbide tipped combination or ripping blade will provide
smooth cuts.
4.6.4 What type of router bits work well with MDF? [PS]
As far as router bits go, use only two flute carbide bits and make several passes as
opposed to one single pass. If possible, trace the outline of what you intend to router and
remove the majority of the material with a jigsaw which will reduce the amount of
material being removed and will put less strain on the router as well, not to mention, this
will all reduce the amount of MDF dust.
4.7 What driver should I use?
4.8 Is there any computer software available to help me
choose an enclosure and a driver? [JSC, MH, DK]
Various enclosure design software is available via ftp from
ftp://ftp.uu.net/usenet/rec.audio.high-end/Software/.
The most popular program there is Perfect Box, which is in the file `perf.uu' (or
`perf.zip').
Note that NO program can tell you what enclosure is best for YOUR car! The program
does not take into consideration your space limitations, the type of car you drive, the type
and number of mid-bass drivers you use, your musical preferences and the goals you
have for your system. Many people follow (blindly) what a computer program says is
"optimal," and end up unhappy with the results. Therefore, it is always a good idea to
discuss a design you think looks good with a qualified installer or (even better) with the
manufacturer.
For an overview of many programs and devices available for enclosure design, obtain the
file `sahfsd01.doc' at the ftp.uu.net archive. The filename stands for "Software and
Hardware for Speaker Design", and was added to the archive in June 1994 by an
anonymous contributor.
4.9 What is an "aperiodic membrane?" [CD, DK]
An aperiodic membrane is one part of a type of subwoofer enclosure. It is an airpermeable sheet which has frequency-dependent acoustical resistance properties. The
original design goes back to Naim, for use in home systems, but has been applied by
several individuals and companies in car audio.
The completed system will be aperiodic, which means it will prove to be over-damped
with a Q well below 0.7. In contrast, the most commonly used sealed enclosures have
Qtc's in the range of 0.8 to 1.1 which are considered, by definition, to be underdamped.
When improperly used, a high-Q system may have poor transient response, nasty peaks
in frequency response, and high rates of roll-off. Aperiodic systems will feature excellent
Aperiodic systems are characterized by better transient response, flatter frequency
response and somewhat extended low frequency response.
Another benefit of the system is that you can pretty much choose whichever driver you'd
like to use, as long as they are big. The Thiele/Small parameters (which would normally
determine what kind of box would be used) are taken into consideration by the membrane
designers so that the response is extended and overdamped, regardless of the
characteristics of the driver.
Physically, the aperiodic membrane isn't for every car. It requires sealing the trunk from
the passenger compartment in an air-tight manner, as well as sealing the trunk from the
outside for best results. The drivers are then mounted into the baffle between the
passenger compartment and the trunk, as would be standard in an infinite-baffle/free-air
set-up. The aperiodic membrane is then placed either in front of the driver or behind the
driver, depending on the type. When mounting behind the driver, the membrane is used
as the rear-wall of a very small box which the driver sits in (as in Richard Clark's
infamous Buick Grand National). So, in short, it's not suitable for trucks, jeeps, R/V's, or
hatchbacks.
You should probably only get an aperiodic membrane if you've got money to burn, lots of
amplifier power, some big subs, a sedan, a desire for trunk space, and no wish to boom. If
your tastes lean towards bass-heavy booming, as opposed to well-recorded acoustic
instruments, you're not going to be pleased with the result.
4.10 Can I use my subs in the winter? [MS]
The following applies to all speakers in extremely cold conditions, but the question most
often occurs in reference to subwoofers.
The suspension of the speakers will stiffen considerably at very cold temperatures (lower
than 30 degrees F). So will certain cone materials which may become more brittle.
If a very cold speaker is played very hard there is a small potential for damage because
more stress is placed on the cone's neck. The likelihood of damage is minimal for wellconstructed and well-designed automotive speakers, however.
Thermally, the danger is minimal because the ambient temperature and the coil
temperature are so low that it is highly unlikely that a coil will overheat and burn, despite
limited movement and ventilation.
At temperatures between +20 degrees F and 0 degrees F, it is a good idea to play the
system at a moderate level until the car's heater has warmed the vehicle interior. As the
speakers warm up, they will play louder and lower signifying that their suspensions are
warming up and returning to nominal compliance.
If the temperature is extremely cold (less than 0 degrees F), you should avoid playing the
system at all until the vehicle interior is warm. This is to avoid stress fractures in the
surround material (especially with rubber surrounds).
4.11 How can I carpet my enclosure? [AO]
What you will need:





Adhesive (3M Super 77 or Super 90 is excellent.)
Carpet.
A good sharp pair of scissors.
A razor sharp utility knife. Buy a BOX of blades, they go dull fast.
Solvent to clean up excess adhesive.
Before you start, find a large, clean, flat surface on which to set the box as you carpet it.
Start by unrolling the carpet onto the surface, smoothing it out so that its flat, and setting
the box on top of it edgewise. Also, make sure that you remove the speaker, any ports and
terminal cups from the box.
The instructions on how to carpet the box are as follows:
1. Place the box such that it is centered on the carpet lengthwise, and one edge of the
box is about one inch from the edge of the carpet.
2. Roll the box back so that the side of the box that was previously done faces
forward, and the carpet beneath it is exposed. Coat both the box and carpet with
adhesive, but do NOT apply the carpet to the box - the adhesive needs a few
minutes to set up (follow the instructions from your adhesive to find out how long
you should wait).
3. After the adhesive has set up, roll the box back into position. Wait a few more
minutes for the adhesive to bond.
4. Now coat the side of the box adjacent to the remaining carpet (the side facing
backwards) and the carpet next to it with adhesive, let the adhesive set up, and roll
the side you just coated onto the coated carpet. Repeat this until three sides of the
box are carpeted.
5. Before carpeting the next side, the 1 inch of carpet sticking over the edge must be
removed. To do this, rotate the box so that the first side that was carpeted is up.
Pull the carpet sticking over the edge down towards the uncarpeted edge and cut it
off with the knife, flush with the uncarpeted side of the box. You will have to run
the knife nearly parallel to the uncarpeted side to get a perfect cut.
6. This done, spray the remaining side and carpet, and roll the box onto it. Shear off
the remaining carpet sticking out from all edges with the scissors leaving a 1 inch
border everywhere.
7. Clean up the ends of the box so that the carpet is flush with the sides of the box as
in step 4.
8. Next cut off the remaining 1 inch flap of carpet (located at the point where you
began carpeting). This is the tricky part, as you don't want to be able to see this
seam. Again, pull the flap down over the edge of the box, but this time cut it at
roughly a 45 degree angle. If you are successful you shouldn't be able to see the
wood under the seam, but will probably see the white of the adhesive and the back
of the carpet.
9. Soak some of the solvent onto a rag and use this to scrub the edge you just cut off.
It should dissolve the adhesive and the carpet backing somewhat, causing the
carpet on the edge to become fuzzy. Keep scrubbing the edge until you can no
longer see the seam.
10. Now carpet the ends of the box. Cut two pieces of carpet slightly larger than the
ends of the box and lay one of them flat on the surface. Spray the carpet and one
end of the box with adhesive, and set the end of the box on the carpet, so the box
stands on end.
11. After the adhesive has dried sufficiently cut off the remaining border of the carpet
as in 7 and 8.
12. Repeat step 9 and 10 for the other end of the box.
Congratulations! You've just carpeted your box!
4.12 Are large magnets always better than small
magnets? [ST]
Magnet size is meaningless!
Every speaker will have an optimal BL (See section 4.1 What are "Thiele/Small
parameters?" [CD, RDP],) product, the field strength in the air gap multiplied by the
length of the voice coil wire in the field.
If the BL product is too low, the speaker is electrically not very well damped (which will
result in a woofer with a high Qts). A bump in frequency response and a level drop in
midband efficiency may be the result. If the BL product is too high, the speaker is
electrically overdamped (Low Qts woofer). A very high midband efficiency, but the
driver starts to roll of early.
An high BL product can be achieved in a number of ways: increase field strength; or
increase wire length in magnetic gap.
The increase in field strength is limited; so some manufacturers use very thin wire for the
voice coil, as such they can achieve a high BL product with a low field strength (cheap
magnet). Or they use an 8 layer voice coil... needless to say that electrical power handling
will decrease enormously.
Long stroke woofers, having only a part of the voice coil in the air gap, need a very high
field strength to achieve a high BL product. Often this means a big magnet as well...
Use magnet size as an indication, but as nothing more than that.
4.13 I know the box volume required for my subwoofer,
but what are the best dimensions for my enclosure?
[IDB]
The specific dimensions of a subwoofer enclosure aren't really important. Once you know
the appropriate volume of the box, and you know where in your car you want to install it,
you will have some idea of the restrictions in the dimensions. For example, if the distance
between the floor of your trunk and the bottom of the rear deck is 16", then you probably
shouldn't make your box any taller than 16". Likewise, if width of the trunk (between the
wheel wells is 38", then you've got that much space to work with.
You can also infer some other information about your box, from the speaker
specifications (for a PPI PC10):
Mounting Depth:
Speaker Displacement:
4.5625"
.032 ft^3
Here is an ASCII drawing of a subwoofer enclosure to help illustrate a few things:
|--- L ---|
__________
___
/
/|
|
/
+
/ |
|
/ x
/ |
D
/_________/
|
|
|
|
|
_|_
|
| /
/
|
| /
W
|_________|/
_/_
+ = Center of speaker
x = center of port
W = Width of box
L = Length of box
D = Depth of box
Because of the speaker's mounting depth, you know that the box MUST be at 5.5" deep
(it's always good to leave at least 1" of space behind the speaker, but leave more if you
can). If you use a straight port (3" diameter PVC) then the box will need to be at least 13"
deep (leaving 2 inches between the end of the port and the back of other side of the
enclosure), assuming that your port will reside completely within the enclosure. If you
use Flex-Port or choose to have a portion of the port extending outside of the enclosure,
you can make the box less deep.
We also know that with a 10" woofer, the length and width should both be a minimum of
12" (leave 1" on either side of the woofer). Obviously both dimensions can't be 12"
otherwise we can't put the port on the same face as the speaker.
With volume, remember a couple of things. Recommended volume is the Net Internal
volume. Both the speaker and the port will displace volume from the box -- this means
that you MUST account for these (i.e., make the interior volume of the box larger than
the recommended value). In this case, you know the following:
Required Volume:
Speaker Displacement:
Port Displacement:
1.25 ft^3
0.032 ft^3
??
3" (interior diameter) PVC will usually have an exterior diameter of 3.5" (0.25" wall
thickness). Volume of a cylinder = pi*r^2*h, where r = 1.75" and h=12.25" (13" port
length - 0.75" for the thickness of the MDF). Therefore the port displacement is: 117.9
in^3 or 0.068 ft^3 (1728 in^3 = 1 ft^3).
Required Volume:
Speaker Displacement:
Port Displacement:
1.25 ft^3
0.032 ft^3
0.068 ft^3
TOTAL INTERIOR VOLUME: 1.35 ft^3 or 2332.8 in^3
Now, to calculate the dimensions of the box: For a rectangular box, L x W x D =
Volume.
If we know that the minimum interior depth is 15" (because the port is 13" and you
should leave 2" between the port and the wall of the enclosure), we can say,
L x W x 15"
L x W
= 2332.8 in^3
= 155.52 in^2
or
Since we know that L must be at least 12" (since the speaker is 10" in diameter), we can
continue:
12" x W
W
= 155.52 in^2
= 12.96"
So, we have determined that the interior dimensions of the box must be 12" x 12.96" x
15". Obviously this can't work, since there is nowhere to put the port! You will either
have to have the port extend outside of the enclosure, or use flex-port. Keep in mind that
if you have part of the port "sticking out" of the enclosure, the Port Displacement will
also change!!.
For simplicity, (in this example) let's use flex-port. This will let you decrease the
thickness of your box to about 6". So,
L x W x 6"
L x W
12" x W
W
=
=
=
=
2332.8 in^3
388.8 in^2
388.8 in^2
32.4"
This is a little more reasonable. This will be a flat, wide box. Again, keep in mind that
these are internal dimensions. If you use 3/4" MDF (and you should) to build the
enclosure, this will add 1.5" to each of these figures for the external dimensions. e.g.:
13.5" x 33.9" x 7.5"
Remember, the 6" depth and 12" length are just the minimum values, you can alter them
if, for example, you don't have enough space between your wheel wells for a 34" box. If
you only have, say, 28", then:
maximum external width = 28"
maximum internal width = 26.5"
L x W x D
L x 26.5" x 6"
L
= 2332.8 in^3
= 2332.8 in^3
= 14.7" (internal)
Got it? It can be a pain, but that's part of the fun in building a speaker enclosure.
5 Installation
This section describes how to do what you want once you know what it is you want to do.
5.1 Where should I buy the components I want? [JSC]
Most of the time, you will either buy from a local dealer, or from a mail-order house.
Buying from a local dealer can be good because you get to deal directly with a person:
you can show them your car, ask specific questions, haggle prices, get quick service
when there are problems, get deals on installation, etc. But there can also be advantages
to buying mail-order: generally cheaper prices, sometimes better service, etc. In either
case, you should always check prices before you buy, inquire about warranty service, and
ask about trial periods.
5.2 What mail-order companies are out there? [JSC,
JM, MM, IDB]
Crutchfield -- 800/955-3000
1 Crutchfield Park, Charlottesville, VA 22906 USA


Advantages:
o Great customer service
o Generally knowledgeable sales and tech support personnel
o Custom mounting kits, wiring harnesses, etc. free of charge.
Disadvantages:
o limited product line
o generally higher prices than local shops
J.C. Whitney -- 312/431-6102
1917-19 Archer Avenue, P.O. Box 8410, Chicago, IL 60680 USA


Advantages:
o Lots of "miscellaneous" items
o 10kW amps for $19.99
Disadvantages:
o 10kW amps that really only put out 1mW and break after first 10 minutes
of use.
Parts Express -- 800/338-0531
340 E. First St., Dayton, OH 45402 USA


Advantages:
o Large selection of electronics supplies at respectable prices.
o Showroom prices said to be better than catalog prices.
Disadvantages:
o Also carries some of the same quality-level components as J.C. Whitney.
Classic Research/Z-Box -- 520/571-0171
5070 E. 22nd St., Tucson, AZ 85711 USA


Advantages:
o Creates custom door panels with car audio in mind.
Disadvantages:
o Only services expensive sports and luxury cars.
MCM Electronics -- 800/543-4330
650 Congress Park Drive, Centerville, OH 45459-4072 USA


Advantages:
o Sells lots of decently priced trinkets (fuses, fuse holders, wire, etc.)
o Has excellent service and available technical support.
Disadvantages:
o ?
There are many other mail-order houses that can be found in the back of magazines, such
as S.B.H. Enterprises, Speed and Sound and Smile Electronics, but people seem to have
mixed feelings about these companies. The prices are very low, often due to the fact that
these companies are not factory authorized -- this means that there could be problems
getting the unit serviced by the manufacturer should it break. To get around this, these
mail order houses will often provide their own service departments, to repair defective
units. Generally, it is advised to be careful when dealing with any mail order companies,
to protect yourself.
There are also a number advertisements for mail order houses (such as Apex Audio &
Electronics or Insider's Audio) that offer special deals or free equipment if you buy their
expensive catalog or pay a membership fee. While these ads may be enticing, keep in
mind that they are often too good to be true! Usually you have to buy a large amount of
equipment before you qualify for the "bonus" or the prices are very high for most
equipment.
5.3 What tools should I have in order to do a good
installation? [JSC]
Electrical tape
Make sure you get some that can withstand extreme temperature ranges.
Wire cutters/strippers and crimpers
Get a big pair with stripper holes precut for individual wire sizes.
Angled screwdrivers
Makes taking dash and rear deck speakers out a lot easier.
Multiple size screwdrivers, both flathead and Phillips.
Magnetic screwdrivers can be a big help when trying to get screws into (or out of)
tight spaces.
Various wrenches, pliers, and socket sets
The specific sizes you need will depend on your vehicle.
Metal drill and saw
You'll need these if you need to modify your vehicle for new speaker cutouts or to
accommodate a new head unit.
Hot glue gun
Good for putting carpeting or door panel trim back in place after modifications.
Razor knife
Helps for detailed modifications of door panels or carpeting, especially when
installing new speakers.
Wire
See section 2.3 What is the best power wire to use? [JSC], and See section 2.4
What is the best speaker wire to use? [JSC, JW].
Soldering Iron
Makes excellent connections, but can be messy if not careful.
Shrink wrap or flex tubing
Good for protecting wire, especially in the engine compartment.
Multimeter
Helps to diagnose installations.
Extra hardware
Screws, nuts, bolts, connectors, etc.
Fuse puller and extra fuses.
In addition to the fuses for your stereo system, check your car's fusebox to find
the various sizes you'll need. Also, you can use needle-nosed pliers to pull fuses.
Wire ties
Helps to tuck wire away in otherwise exposed areas.
Small light source
A flashlight will do - you just want something that you can poke around the
innards of your car with.
Tape measure
5.4 Where should I mount my speakers?
5.5 What is "rear fill", and how do I effectively use it?
[HK, JSC]
Rear fill refers to the presence of depth and ambiance in music. A properly designed
system using two channels will reproduce original rear fill on the source without rear
high frequency drivers. Since recordings are made in two channels, that is all you will
need to reproduce it. What is captured at the recording session (coincident pair mics,
Blumlein mic patterns, etc.) by a two channel mic array will capture the so called rear fill
or ambiance. Many of the winning IASCA vehicles have no rear high frequency drivers.
Also a lot of this has to do with system tuning. If rear high frequency drivers are added,
however, the power level of the rear fill speakers should be lower than that of the front
speakers, or else you will lose your front-primary staging, which is not what you want
(when was the last time you went to a concert and stood backwards?). The proper amount
of amplification for rear fill speakers is the point where you can just barely detect their
presence while sitting in the front seat. Separates are not a requirement for rear fill; in
fact, you may be better of with a pair of coaxial speakers, as separates may throw off
your staging.
5.6 How do I set the gains on my amp? [JSC]
The best way to do this is with a test tone and an oscilloscope (See section 5.18 How can
I use an oscilloscope to set the gains in my system? [AO].) Since most people have
neither item, the following will work approximately as well.
1. Disconnect all signal inputs to the amp
2. Turn all sensitivity adjustments as low as possible
3. Turn head unit on to around 90% volume (not 100% or else you'll have head unit
distortion in there - unless you've got a good head unit) with some music with
which you're familiar, and with EQ controls set to normal listening positions
4. Plug in one channel's input to the amp
5. Slowly turn that channel's gain up until you just start to notice distortion on the
output
6. Turn it down just a wee little bit
7. Disconnect current input
8. Repeat steps 4-7 with each input on your amp
9. Turn off head unit
10. Plug in all amp inputs, and you're done
If by some chance you do have an oscilloscope (and preferably a test disc), you do
essentially the same thing as above, except that you stop turning the gains up when you
see clipping on the outputs of the amplifier.
Note that if you are paralleling multiple speakers on a single amp output, you need to set
the gains with all of the speakers in place, since they will be affecting the power and
distortion characteristics of the channel as a whole.
5.7 How do I select proper crossover points and slopes?
[DK]
Basically, this requires a degree of patience. The subwoofer should be started off at about
100Hz and adjusted until you are happy with the sound. Keep in mind that the higher the
crossover point, the more power the driver on the high-pass will be able to handle but
raising excessively may cause the low-pass driver to sound raspy or unnatural. The idea
here is to first make rough selections to protect the drivers and then to fine tune crossover
point selections to achieve optimum fidelity. It's all a matter of what sounds good to you
after that, but remember that even minute changes in crossover frequency can make
dramatic differences in the way your system sounds and images.
As a rule, subs should be crossed over no higher than 120Hz, a 6 1/2 mid should be able
to handle about 90 Hz, a 5 1/4" should be okay with about 100Hz, a 4" -- about 500Hz,
and tweeters vary from about 3500-5000Hz. These points all assume the use of a
12dB/octave crossover ... if you have a steeper roll-off a lower crossover point may be
chosen. Remember, these are not hard and fast rules but rather a rule of thumb to help
you get started (and so you don't blow up all your speakers when you are setting your
gains!).
5.8 How do I flatten my system's frequency response
curve? [IDB, DK]
First, you'll need a good quality equalizer - either a 2/3 octave (15-band) or 1/3 ocatve
(30 band) equalizer or a quasi- parametric equalizer such as PPI's PAR 224 that allows
you to choose the center frequency and bandwidth (Q) of each knob on the EQ. This will
allow adjustments to very specific frequency ranges. Next, you'll need to get a hold of an
RTA (Real Time Analyzer), which is an expensive piece of equipment that good shops
will usually have. The shops can then equalize the system by making a measurement with
the RTA, and varying the levels on the equalizer in order to make the overall response
curve flat.
Unfortunately, most shops will not do this for free, since proper equalization can take
anywhere from a half hour to many many hours.
Another method involves buying an SPL meter (available from Radio Shack for between
$32 and $60) and a test disc (Autosound 2000 makes one that runs about $25) that plays
discreet frequency ranges - in 1/3 octave groups. Then, moving through the range of
frequencies, SPL measurements can be taken at each range, and you can plot out a
"response" curve. You'll be able to see what frequency ranges need to be boosted and
which need to be cut. This process will be time consuming (more so than an RTA, which
can analyze the entire frequency spectrum simultaneously), but should be much less
expensive than having it professionally done.
One last note: While a smooth curve will get the most points at an auto sound
competition, you must NOT rely only on the RTA to tell you what sounds good. Use the
RTA to get a good start, and then use your (better, use someone experienced in tuning
systems) ears to do the fine-tuning.
5.9 How do I wire speakers "in series" and "in
parallel?" [IDB]
Wiring speakers in series involves connecting at least two speakers so that the first
speaker's positive lead is connected to the amplifier's positive terminal, and the negative
lead is connected to the positive lead of the second speaker. If there is a third speaker, its
positive lead will be connected to the second speaker's negative lead ... and so on. The
last speaker in the chain will have its negative lead connected to the amplifier's negative
terminal.
Speakers that are wired in parallel are all connected to the positive and negative terminals
of the amplifier. So, when two speakers are wired in parallel, you'll connect each
speaker's positive lead to the amplifier's positive terminal, and you'll connect each
speaker's negative lead to the amplifier's negative terminal.
Be careful when wiring multiple speakers in parallel or series so that you do not exceed
your amplifier's rating. To calculate the effective impedance of a number of speakers, use
the following formulas:
Series Connections:
Z(t) = Z(1) + Z(2) + Z(3) + ... + Z(n)
That is, add up all of the impedances for each speaker to
get the total impedance. For example, with 3 4-ohm speaker
in series, the total impedance is 4 + 4 + 4 = 12 ohms.
Parallel Connections:
1/Z(t) = 1/Z(1) + 1/Z(2) + 1/Z(3) + ... + 1/Z(n)
That is, add up the inverse of the impedance of each
speaker and invert the sum to get the total impedance. For
example, with 3 4-ohm speakers in parallel, the total
impedance is 1 / ( 1/4 + 1/4 + 1/4) = 1 / (3/4) = 1.33 ohms.
5.10 Are there any alternatives for Dynamat? It's too
expensive! [MM, PK]
In this question, "Dynamat" refers to all commercial products that are marketed expressly
for reducing ambient noise in the car. Dynamat, Stinger RoadKill, et al. all have similar
pricing, so this question is intended to give non-standard options.
There is a material known as "Ice Guard," which is used by roofing contractors. It is
similar to Dynamat, both in thickness and density. It is self-adhesive on one side, and
seems to work very well. Unfortunately, it is sold only in large quantities (225 ft^2 rolls),
and runs about $70 for this much. Perhaps a few people could get together for a roll, or it
might be possible to get scraps from a roofing contractor.
MCM Electronics See section 5.2 What mail-order companies are out there? [JSC, JM,
MM, IDB], sells a product called "Sound Deadening Pads" (part #60-2010) which cost
$0.90 for each 10" x 10" square.
5.11 How many devices can I attach to my remote turnon lead? [IDB]
The remote turn-on lead that most head units will not provide very much current (usually
250-300mA), so there is a limit to the number of components you can activate with it.
Generally, it is safe to hook up two devices to the lead without having to worry about
problems. However, if you'll be activating more components, then you should probably
use a relay.
5.12 How do I wire a relay in my system? [IDB]
There are two types of relays that are commonly used in 12-volt automotive applications:
Single-pole Double Throw (SPDT) relays, which have 5 pins, and Single Pole, Single
Throw (SPST) relays, which have 4 pins. Depending on the application, you can use
either of these; for remote turn-on leads See section 5.11 How many devices can I attach
to my remote turn-on lead? [IDB], an SPST relay is fine, SPDT relays are often used in
alarm installations. Make sure that you get a 12-volt relay -- this specifies the voltage
required to make the relay "switch."
The connections on the two types of relays look like this:
SPST
=====================
(87)
+---------+
|
-|
|
|
(86) | |
| | (85)
|
|
|
|
|
+---------+
(30)
SPDT
=====================
(87)
+---------+
|
-|
|
|
(86) | | -- | | (87a - center)
|
| (85 - right)
|
|
|
+---------+
(30)
Pins 85 and 86 connect to the coil which causes the relay to switch. On both relays, pins
30 and 87 are normally disconnected. When the relay is activated (switched) pin 30 and
87 are then in contact. The difference with the SPDT relay is that in the "normal" state,
pins 30 and 87a are in contact.
To hook up a relay (either kind) for a remote turn on, make the following connections:
Pin 30
+12 Volts (Battery +)
Pin 87
Amplifiers' remote turn-on terminal
Pin 86
Head unit remote turn-on lead
Pin 85
Ground
Pin 87a
No connection (SPDT only)
5.13 How do I design my own passive crossovers? [JSC,
JR]
A first order high pass crossover is simply a capacitor placed inline with the driver. A
first order low pass crossover is an inductor inline with the driver. These roles can be
reversed under certain circumstances: a capacitor in parallel with a driver will act as a
low pass filter, while an inductor in parallel with a driver will act as a high pass filter.
However, a parallel device should not be the first element in a set; for example, using
only a capacitor in parallel to a driver will cause the amplifier to see a short circuit above
the cutoff frequency. Thus, a series device should always be the first element in a
crossover.
When like combinations are used, the order increases: a capacitor in series followed by
an inductor in parallel is a second order high pass crossover. An inductor in series
followed by a capacitor in parallel is a second order low pass crossover.
To calculate the correct values of capacitors and inductors to use, you need to know the
nominal impedance Z of the circuit in ohms and the desired crossover point f in hertz.
The needed capacitance in farads is then 1/(2 * pi * f * Z). The needed inductance in
henries is Z/(2 * pi * f). For example, if the desired crossover point is 200Hz for a 4 ohm
driver, you need a 198.9 x 10^-6 F (or 199uF) capacitor for a high pass first order filter,
or a 3.18 x 10^-3 H (or 3.18mH) inductor for a low pass first order filter.
To build a second order passive crossover, calculate the same initial values for the
capacitance and inductance, and then decide whether you want a Linkwitz-Riley,
Butterworth, or Bessel filter. An L-R filter matches the attenuation slopes so that both 3dB points are at the same frequency, so that the system response is flat at the crossover
frequency. A Butterworth filter matches the slopes so that there is a peak at the crossover
frequency, and a Bessel filter is in between the two. For an L-R filter, halve the
capacitance and double the inductance. For a Butterworth filter, multiply the capacitance
by 1/sqrt(2) and the inductance by sqrt(2). For a Bessel filter, multiply the capacitance by
1/sqrt(3) and the inductance by sqrt(3).
You should realize, too, that crossovers induce a phase shift in the signal of 90 degrees
per order. In a second order filter, then, this can be corrected by simply reversing the
polarity of one of the drivers, since they would otherwise be 180 degrees out of phase
with respect to each other. In any case with any crossover, though, you should always
experiment with the polarity of the drivers to achieve the best total system response.
One other thing to consider when designing passive crossovers is the fact that most
passive crossovers are designed based on the speakers' nominal impedance. This value is
NOT constant, as it varies with frequency. Therefore, the crossover will not work as it
has been designed. To combat this problem, a Zobel circuit (also known as an Impedance
Stabilization Network) should be used. This consists of a capacitor and resistor in series
with one another, in parallel with the speaker, e.g.,
+
INPUT
-
________
__
o----|
|----o-----o + | | /
| Xover |
R1
| |/
|
|
C1
| |\
o----|________|----o-----o - |__| \
To calculate these values, R1 = Re (in ohms) x 1.25, and C1 = (Lces in henries / Re^2) *
10^6. See 4.1 for definitions of Re and Lces. R1 will be in ohms, and C1 will be in uF
(micro- farads). As an example, an Orion XTR10 single voice coil woofer has Re = 3.67
ohms and Lces = 0.78 mH. So, R1 = 3.67 * 1.25 = 4.6 ohms. C1 = ( 7.8E-4 / 3.67^2 ) *
10^6 = 57.9 uF (be careful with units -- 0.78 mH = 7.8E-4 H)
As with the definition of crossover slopes, the above definition of the phase shift
associated with a crossover is also an approximation. This will be addressed in future
revisions of this document.
5.14 How do I build my own passive crossovers? [JSC]
This section assumes that you have a basic understanding of how to solder, so the actual
assembly of the crossover is not discussed. Rather, tips on choosing the proper types of
capacitors and inductors are given here.
To obtain low insertion losses, the inductors should have very low resistance, perhaps as
low as 0.1 to 0.2 ohms.
Also, be sure to select capacitors with proper voltage ratings. The maximum voltage in
the circuit will be less than the square root of the product of the maximum power in the
circuit and the nominal impedance of the driver. For example, a 4 ohm woofer being
given 100W peak will see a maximum voltage of sqrt(100*4) = sqrt(400) = 20V. Make
sure that the capacitors are bipolar, too, since speaker signals are AC signals. If you
cannot find bipolar capacitors, you can use two polar capacitors in parallel and in
opposite polarity (+ to - and - to +). However, there are some possible problems with this
approach: the forward voltage rating will probably not be equal to the reverse voltage
rating, and there could be a reverse capacitance as well. Both problems could adversely
affect your circuit if you decide to use opposite polarity capacitors in parallel.
5.15 Can I split the single pre-amp output from my
head unit to drive two amplifiers with a Y-cable? [IDB]
[This section was written by someone who wishes to remain anonymous, but I will field
any questions on the subject -IDB]
Yes. When two loads are connected in parallel (such as with a Y-cable) they get the same
voltage as each other. They do NOT get the same voltage as if only one load was
connected because the head-unit has an internal resistance (typically around 600 ohms).
So, given that the amp has a typical input impedance of around 10k ohms then we get
something like this:
-------------------------------------------------------HEAD UNIT
________
|
|
AMP
|
______|
|_________Vamp___________
|
|
| R(head)|
|
|
|
|
_
|
__|__
|________|
|
|
__|___
|__| - _
|
/
\
|
|
|
|
|
-___|__
| Vi |
|
|
|R(amp)|
|
_|
\_____/
|
|
|______|
__| _|
|_______________________________|________|
|
|
|
|
--------------------------------------------------------
for the single amp situation. Please realize that the R(head) and R(amp) are internal to the
head unit and amplifier and in fact are not deliberately added resistors but are
characteristic of the real world circuits (non-ideal) in the head-unit and amplifier (and
eq's, etc.). These numbers are typical, check your specific equipment for its particular
specs. the worst case situation is a high source output impedance and low load input
impedance.
So, assuming a typical head unit and single amp the voltage seen at the amp (Vamp) is
given by (Ohms law/Kirkov's law/1st year EE/high school electronics technology
class/etc.):
Vamp1 =
R(amp)
Vi * -----------------R(amp) + R(head)
Vamp1 =
Vi * 0.94
Now, putting two amps in parallel from the original signal, R(amp) is effectively halved
while R(head) is unchanged. Using the same voltage divider formula we get:
10000/2
Vamp2 = Vi * --------------------10000/2 + 600
Vamp2 = Vi * 0.89
So, for an Alpine 4V preout, Vi in the diagram (the open circuit head unit line level
output) is 4V. Thus Vamp1 = 3.76V and Vamp2 = 3.56V. With two amplifiers' inputs
connected in parallel, the voltage is reduced from 3.76V to 3.56V or approximately 5%,
not a big deal.
If you had a more typical 1V preout you would get Vamp1 = 0.95V and Vamp2 = .89V,
also not a noticeable drop.
This is also why this is slightly more susceptible to noise than a direct one-to-one
connection. If the noise level inserted due to cabling was 0.1V per cable then the noise
level in the signal reaching each of the two amps would be a slightly higher percent of the
signal level but not doubled. (this is also why the 4V head unit is favored over the 1V
unit for noise immunity: 0.1V noise / 3.76V or 3% is much less than 0.1V noise / 0.95V
or 10% even in a one to one connection).
5.16 How do I turn a stereo signal into a mono signal
[BW]
Creating a mono signal is often necessary when you are powering a subwoofer by
bridging the amplifier. Many people do not realize that bridging an amplifier does not
always provide a mono signal -- many amplifiers will simply use only one input channel,
which means that the subwoofer won't be receiving the full signal.
Some amplifiers have a switch that will allow you to combine the left and right channels
into a mono signal. Some signal processors and head units provide a subwoofer-out
channel that can be switched between stereo and mono.
If you don't have this feature on any of your equipment, you will need to provide a mono
signal to the amplifier. The common thought is to use a Y-adapter to "combine" the left
and right channels. However, by using a Y-adapter, you are actually summing the line
voltages and directly shorting the left and right channels at the head unit, which could
cause problems.
The correct way to create a mono signal is to cut off the ends of the RCA cables, combine
the signal grounds (the outer shield), and then use a 1 kOhm (1/4 watt, 5% tolerance)
resistor to each of the center conductors. Solder and insulate the resistors so that you
don't short them prematurely, and then connect the two resistors together. Connect the
summed signal ground to the shield of the new RCA plug, and the summed center
conductor to the center pin of the RCA plug.
5.17 How do I determine a speaker's polarity? [IDB]
If you have a speaker and the terminals are no longer marked, you can do a simple test to
determine which terminal is positive (+) and which is negative (-). This test is useful for
midrange/midbass/subwoofers, but not for tweeters.
Use a 1.5V battery (AA, C, D) and connect the (+) terminal on the battery to one terminal
of the speaker, and connect the (-) terminal to the other terminal of the speaker.
If the cone moves OUT, then the battery is connected "properly," i. e., the (+) terminal of
the battery is connected to the (+) terminal of the speaker, and the (-) terminal of the
battery is connected to the (-) terminal of the speaker.
If, however, the cone moves IN, the battery has been connected "backwards," i. e., the (+)
terminal of the battery is connected to the (-) terminal of the speaker, and the (-) terminal
of the battery is connected to the (+) terminal of the speaker.
5.18 How can I use an oscilloscope to set the gains in my
system? [AO]
This section assumes you are already familiar with your oscilloscope and will not go into
setting it up. If you haven't already, spend a few minutes with your scope's manual.
You'll need a test disc with a variety of test tones. I use the official IASCA test disc, but
there are some of the "Bass Discs" that have test tones as well. You do NOT want to use
sweeps, only pure tones. There is an AutoSound 2000 disc (#101?) which has a signal
which is unclipped for 20 seconds, clipped for 5, and then unclipped for the last 5
seconds. I have never used the AutoSound 2000 discs, but know them to have excellent
recommendations (as well as all the test tones you could ever need). Viewing this track
on your scope's display could be useful if you've never seen clipping on an oscilloscope
display before.
You start by finding the clipping level of your head unit. Many of the better head units
will not clip the pre-amp outputs, even at full volume, but it's always better safe than
sorry. Disconnect the RCA's from your head unit. Pop in your test disc and skip to the
track with a 1 KHz tone. If your CD player has a repeat function, set it to repeat just this
track. That way you won't have to skip back at the end of the tone. Set your bass, treble,
fader, and balance all to center. Turn the volume all the way up. Probe your right and left
front (and rear if you have them) one at a time. Your scope should show a wave, either a
sine wave or a clipped sine wave. If you have a standard pure sine wave then all is good,
and you're ready to proceed. If you have a clipped wave then you need to turn down the
volume, one click at a time until you see a perfect sine wave on your scope's display.
Remember this point, as this is the highest you can EVER turn up your head unit. After
you set the level for one of your outputs the rest should be the same, but check them all
just to be sure. The results will be the same if you leave the RCA's plugged into the head
unit and disconnect them at the other end (from your amp/EQ/processor/whatever) but
unless you have a remote control you'll be running back and forth to change the volume.
If your head unit has subwoofer pre-amp outputs you'll need to test them using a different
tone. I usually test subwoofer outputs at a level midway between the crossover points.
For example, if your subwoofer outputs are crossed over at 80Hz then you would want to
use a 40Hz test tone. Other than that the procedure for testing subwoofer pre-outputs is
the same as testing front or rear outputs.
Once you've found the clipping level of your head unit it's time to proceed down the
signal chain. If you are using an EQ, preamp, DSP, or other processor (NOT including
crossovers) test them next. Leave your EQ set the way you usually use it. Hook up the
processor and probe all the outputs of your processors in the same method as you did
your head unit. You should probe each of the outputs using tones that match the bands of
your EQ. For example, if you have a 9 band EQ with bands at
50/100/200/400/800/1.5k/3k/6k/12kHz you would probe your EQ 9 times, once with a 50
Hz test tone, once with a 100 Hz test tone, and so on. If your EQ also includes a
crossover you'll need to follow the crossover procedure. If any of these processors are
clipping you will probably need to turn down your head unit's volume control or make
any adjustments on that unit that you can. For example, if you are testing an EQ and you
have any bands excessively boosted, try bringing down that band first. That may be
causing your clipping.
To test your crossover you need to probe each output using a test tone that is midway
between the high and low pass. For example, a channel which is crossed over between
100 Hz and 20 KHz (like a front channel) would be tested at 9950 Hz. Since you'll be
hard pressed to find a 9975 Hz test tone on your CD use the 10Khz tone. For a rear
channel crossed over with a lowpass of 3500 Hz you would use a 1750 Hz tone. As you
again would have problems finding a 1750 Hz tone on a CD use a 2 KHz tone. For a
subwoofer channel lowpassed at 70 Hz you would use a 35 Hz tone. This one you may
find on your CD, if not use 30 Hz or 40 Hz.
Assuming your crossover has level settings you will want to turn the level for whatever
channel you're testing all the way up and probe the output. Assuming the output is
clipping, back the level down slowly until you see a perfect wave on your scope.
Now it's time to check your amp's outputs. Hook up your amps and play the same test
tones you were using on the crossover. Disconnect the speakers and then turn the gains
all the way up. Probe the first channel's output. Adjust the gain the same way you did
your crossover. Back the gain down slowly until your wave isn't clipping anymore.
Voila! You are done. You have just effectively eliminated clipping from your system. If
you turn your bass or treble up, or boost up a channel on your EQ you may introduce
some clipping. After major EQ work you may want to redo this procedure, starting at the
EQ.
5.19 Why are kickpanels such a popular location for
mounting speakers? [ES]
There is a lot more to mounting speakers in the kick panels than just equalizing the path
length difference (PLD). Two of which are: on-axis response, and angling for pattern
control. On-axis response refers to the fact that most speakers sound best when listened to
on-axis, or as close to on-axis as possible. Second, after mounting your speakers in the
kick panels you can then angle the speakers to take advantage of their off-axis response
to use output level to overcome any PLD that is still present. The pattern control I am
mentioning is one of the ways a horn loaded compression driver works very well, they
not only use amplitude to overcome any PLD that is still present they minimize early
reflections that can destroy imaging staging and spectral balance.
PLD can be improved more than marginally when you consider the stock locations in a
lot of vehicles, or the locations most installers choose. Measure the PLD between
tweeters when mounted high in the dash or at the front corner at the top of the door and
you will notice its probably on the order of 24". This mounting setup requires a lot of
amplitude adjustment to correct the problems induced by this difference. The nearer
tweeter is out phase from the opposite side and is arriving much sooner and with much
greater amplitude due to the fact is not as far away. When all these factors are added
together, it is very difficult for even the most flexible DSP unit to correct. On top of that,
not many people or installers have access to the necessary tools to properly set up time
delays using a DSP -- TEF, MLSSA or other very expensive time domain measuring
equipment are required to do the job properly.
There will always be trade offs involved and deciding which trade offs to take can be
very hard. A small dropout due to phase cancellation will probably not be noticed by
most people but most people will quickly notice when a vehicle is not imaging properly,
and if you can move the problem to higher frequency where we determine localization
more from amplitude rather than phase differences, it will be much easier to deal with.
Also, if you minimize the time/phase difference it will be much easier to correct with
amplitude.
Some people complain that kickpanel mounting gives a low sound stage. However, keep
in mind that when any stereo system is imaging properly the point sources can no longer
be localized. When our brains can no longer localize the point sources it will then hear
things at eye level.
5.20 How can I build custom kickpanels? [MB]
Building custom kickpanels for your car is a fairly advanced task, and requires
knowledge (and experience) in working with fiberglass. Auto Sound & Security
published an article in the August 1996 issue that covered the basics of working with
fiberglass.
This is kick panels in a nutshell. It takes about 2 days to do this right, although it is
possible to do overnight (a LONG night) in one vehicle.
Step 1: Cover base area with plastic & duct tape.
Step 2: Lay fiberglass over the entire area. Don't worry about getting the pieces cut to the
exact size and shape, you will trim them later.
Step 3: Build the baffles for your speakers.
Step 4: After the fiberglass has cured, set the baffles (with the speakers properly mounted
into the panels and use a backstrap to secure the baffle to the panels. Now, spend some
time listening to the car and aim the baffles until you get the best image and stage in the
car. Note: Take your time aiming the speakers -- once you fix the baffles, you won't be
able to readjust the speakers. You may want to spend a few days listening to different
positions to determine what sounds best.
Step 5: Remove the speakers from the baffle and fill from behind with self-expanding
insulation foam. This will allow you to mold the baffle into the rest of the car.
Step 6: After the foam has hardened, sand it to the shape you want the panel to have.
Step 7: Lay fiberglass over the foam to form the top panel of the kick panel.
Step 8: After the fiberglass has hardened, sand the surface smooth. This may require the
use of a little bondo to get things perfect.
Step 9: Remove the foam by grinding it out from the inside of the kickpanel.
Step 10: Cover the panels with carpet, vinyl, leather, fleckstone or other substance of
your choice.
Step 11: Install the kick panels in the car.
Step 12: Install the speakers in the panels.
Step 13: Make speaker baffles from Plexiglas, and grilles.
Step 14: Enjoy.
5.21 What's worse for a speaker, too much or too little
power? [IDB]
Problems occur (in everyday operation) when distortion is fed to a speaker. This occurs
MUCH more often when you are dealing with an underpowered system -- typically the
owner will turn up the volume too much or set the amplifier gains too high to try and get
more volume from the system. These introduce distortion to the signal -- this will destroy
*any* speaker. (See section 5.22 Why is distortion harmful to my speakers? [RK].)
When a speaker is overpowered, however, it is not nearly as common to have these kind
of problems, so speakers aren't blown as much. Of course, it is certainly possible to
destroy a speaker (thermally) by overpowering it, but you'll have a pretty hard time doing
this on your own, especially with standard car audio amplifiers.
5.22 Why is distortion harmful to my speakers? [RK]
Distortion is hard on speakers for two reasons.
Reason 1: Distortion causes the power spectrum to shift upwards in frequency. A bass
note, when distorted, will have lots of high frequency energy. This will cause mid-ranges
and tweeters to fry, if the amplifier is operating full range. It doesn't harm woofers,
necessarily.
Reason 2: Distortion causes the average power to be much higher. Typically, a music
signal that never clips has an average power level of 1/4 the peak power level for even
the most compressed speed metal or pop. More dynamic music will be 1/8 the peak level
or less on average. When you clip the amp hard, the average output moves up to the fullrated output of the amp or more. The peak to average ratio can be less than 2 to 1, with
the peaks being at double the rated power of the amp, and the average being at the rated
power of the amp or higher.
Thermally, the speaker can handle the average power being 1/4 the rated power of the
amp (little to no clipping), but it will have a much harder time with the average power
being the amp's rated power or more (massive clipping). As you might expect, this is
pretty hard on the amp, too.
For transients, most speakers can handle a ton of power. But for long term signals, the
power handling is much less.
5.23 What tools do I need to cut Plexiglas? [PS]
A good quality Carbide Tipped Triple Chip saw blade is preferred when cutting
Plexiglas. This reduces the amount of chipout on the opposite side of the material and
produces a smooth edge.
A Router can be used to cut Plexiglas as well. The Router should be at least 1.5 HP or
better and with changeable collets for a 0.5" Shank quality 2 flute Carbide bit. This
reduces the vibration and chattering when cutting and will produce a more smooth edge.
Since Plexiglas is an acrylic plastic (polymer), heat generated by the saw blade or router
bit when cutting will melt the material and create a glazed imperfection viewed from the
face if you do not maintain a steady rate of cut. It is best that when cutting you do a rough
cut first, then do the finished size cut to reduce stress on the cutting blade and equipment.
5.24 Are there any other special requirements for
working with Plexiglas? [PS]
Check the protective paper covering for any rips, tears or folding at the time of purchase.
In most cases if visual inspection is not done at the place of purchase, it's very difficult to
prove product defects or damage done by the seller.
Inspect all surfaces that will come into contact to reduce tearing and scratching of the
protective paper covering and the Plexiglas finish. Make sure that the protective paper
covering remains in place until all machining and milling tasks are finished.
6 Competition
This section describes the competition branch of the car audio world - what it is, and how
to get involved.
6.1 What is IASCA, and how do I get involved? [JSC,
HK, IDB]
IASCA is the International Auto Sound Challenge Association, a sanctioning body for
car audio competitions held throughout the world. Competitors earn points at each
competition, and those that perform the best each year can advance to the finals. Prizes
(trophies, ribbons, and sometimes cash) are usually given out to the top competitors in
each class at every competition.
IASCA memberships can be purchased at your local car audio retailer, if they are an
IASCA member. You can call IASCA at 602/437-4678 to get a list of IASCA shops in
your area.
6.2 What is USAC, and how do I get involved? [HK]
USAC is another sanctioning body, similar to IASCA. However, USAC places greater
emphasis on SPL measurements than IASCA.
6.3 What are the competitions like? [HK, CD, IDB]
[HK writes:]
They are much like loud car shows: a lot of cars parked with their hoods/doors/trunks
open showing their audio systems. There are two types of judging styles: 1) drive through
- where competitors drive their own vehicles to judging stations to be judged, and 2)
walk-arounds - where the teams of judges will walk around the event site and judge
vehicles that fit within their judging assignments. Typically SPL is done first with the
mic stand in the driver's seat and the competitor in the passenger side adjusting only the
volume. Hearing protection must be worn. After SPL measurements are completed, RTA
measurements are performed by playing pink noise. When the volume level is within the
specified "window" around 90db-110db, the RTA judge will signal you out, and at that
point you must exit the vehicle for the actual scoring measurements. The next area for
judging should be sound quality where two judges will sit in your car and judge the sound
quality based on IASCA's reference CD/tape. The next area is installation judging where
the competitor has 5 minutes to explain and show the installation of his/her vehicle. It is
very useful to have a picture book/album of photos of the installation that may not be
visible to prove that items not visible do exist. When that is completed, the competitor
can park the vehicle and show spectators the vehicle. These procedures may differ from
show to show, and at the regional/final levels they are very strict in what can and can't be
done, e.g. a judge will make sure no adjustments are made after SPL until after sound
quality judging is over, ear protection, etc.
[CD writes:]
Most involve a lot of waiting around. Thus, they are perfect for meeting other people
interested in car audio, and seeing some installations which may give you some ideas.
They're also perfect for listening to some cars that sound a lot better and a lot worse than
your own. In IASCA competition, the cars are judged on:




Installation Quality (187 pts possible)
o Wiring (40 pts)
o Source Units (15 pts)
o Amplifiers (25 pts)
o Speakers (25 pts)
o Other Devices (25 pts)
o Overall Creativity (30 pts)
o Attention to Show Details (20 pts)
o Security and Convenience Features (7 pts)
Sound Quality (248 pts possible)
o Tonal Accuracy and Spectral Balance (100 pts)
o Soundstage and Ambiance (65 pts)
o Imaging (50 pts)
o Sound Linearity (30 pts)
o Ergonomics (+/- 3 pts)
o Noise Adjustment (up to -20 pts)
Frequency Response - RTA (40 pts maximum)
Sound Pressure Level - SPL (1 point per dB)
6.4 Should I compete? [CD]
You should compete if:
1. You have an okay sounding stereo
2. You have an okay installation (i.e. no amps/changers sliding around in the
trunk)
3. You'd like some pros to comment on your system
4. Your feelings won't get hurt if you don't get first place
5. You've been to a contest and talked to competitors about it
6. You've read the rule book
7. You've listened to a test disc in your car, and understand what the sound
quality judges are listening for
You can compete even if you don't do all of the above, but the recommendations will
help you understand and gain the most from competing.
6.5 What class am I in? [HK, JSC]
This section is mainly geared toward IASCA.
[HK,JSC write:]
There are three classes: novice, amateur, and pro. The novice class is intended to be an
unintimidating level where beginners can start out; however, a competitor may only be in
the novice class for one year, at which time he is automatically moved to the amateur
class. Most competitors stay in the amateur class indefinitely, unless they become
affiliated with a car audio shop or manufacturer, at which point they are moved into the
pro class.
[CD writes:]
Are you or were you employed by a car audio manufacturer or dealer?
Yes:
You compete in pro
No:
Is this your first year of competing?
Yes:
You compete in novice for the first year
No:
You compete in amateur
Note that modifying your amplifiers, buying your equipment below retail, or being
sponsored by a manufacturer or dealer will get you kicked into pro.
Also note that any home built active gear in the signal path (e.g. custom built equalizers,
crossovers, or noise gates) will get you kicked out of novice.
Once you know what group you are, you next need to know what power category you are
in. Add up the 4-ohm non-bridged rating of all your amplifiers, including your head unit
if your head unit is powering speakers (rather than exclusively feeding amplifiers). Then,
find the category you fit into:
Novice
Amateur
Pro
Expert
1-150
1-150
1-150
1-600
151-300
151-300
151-300
301-600
301-600
301-600
601+
601+
601+
601+
Thus, if you had a Rockford Punch 4040 (20Wx4) and a Punch 60ix (30Wx2), with a
head unit that put out 6Wx2 (powering, perhaps, a center channel) you're in the 151-300
class. It does not matter if your amps are bridged down to .002 ohms; it's only the 4ohm
rating that counts. If you no longer used your head unit to power speakers, you would be
in the 1-150 class.
Competition is usually most vicious in the 151-300 and 301-600 categories at typical
contests.
6.6 Where can I find out when these Sound-Offs are?
[IDB]
The best way to get the most current list is to call either of the sanctioning bodies
(IASCA or USAC). They can be reached at 602/437-4678 or 601/939-7828, respectively.
You can also visit the Official IASCA and USAC pages on the web at the following
URLs:
IASCA -- http://www.iasca.com
USAC -- http://www.soundoff.org
6.7 How do I get sponsored by a manufacturer? [CM]
Ah, the green grass on the other side of the competitor's fence; sponsorship. Sponsorship
is not the easiest thing to obtain. And it might cause more problems than you anticipated.
Obtaining sponsorship really depends on how much dedication you show for competing
and finding the right manufacturer sponsorship deal at the right time.
Keep in mind that car audio manufacturers consider sponsorship a form of advertising.
Each manufacturer has a different policy concerning sponsorship. They are looking for a
competitor that has shown they are committed to competing for a long time. They want
someone that has shown that they know how to win, but also, how to handle defeat
gracefully. They want someone that is well known in competition circles, someone that
new competitors can come to and ask questions about the system and the manufacturer's
products. They are basically looking for people that will put their product out there for
people to see and help sell the equipment.
As a competitor, you must show that you are committed by to going to as many shows as
possible. You must open up your their vehicle and show off the system. Manufacturers
want a show system, which often means turning your vehicle into a "trailer queen." That
may require you to buy a trailer and tow the vehicle to the shows you compete in (this
also means that you may need a new daily driver). And of course, if your bought
equipment below retail or were given the equipment, then you will be forced to compete
in the Pro or Expert class.
There are different levels of sponsorship. It usually starts with manufacturer technical
support. The manufacturer will give direct technical assistance to the competitor. At
larger shows, they may reserve a space for you in their "tweek and tune" area.
Next, the manufacturers may let you buy equipment from one of their dealers or the
factory and a discounted price. This will definitely put you into the Pro or Expert class.
Some manufacturers will let you mix and match equipment from different manufactures.
Others want you to use their equipment only.
Then the manufacturer might start giving you equipment and providing some or all of the
installation costs.
The holy grail of sponsorship is when the manufacturer provides the equipment,
installation, and pays some or all of your competition expenses. It takes a lot of money to
travel to shows. By the time someone reaches this level, they are frequently hired as an
employee of the manufacturer. This is a good way of making the car audio business your
career.
7 Literature
This section describes various literature which you can read to brush up on your car audio
skills, or to keep current, or to see other people's installations, or whatever else you'd like.
7.1 What magazines are good for car audio enthusiasts?
[JSC, MI, NML, JR]
Car Audio and Electronics
P.O. Box 50267
Boulder, CO 80323-0267
[email protected]
$19.95/year
(12 issues)
800/243-6400
Car Stereo Review
P.O. Box 57316
Boulder, CO 80323-7316
$17.94/year
(6 issues)
303/447-9330
Auto Sound and Security
P.O. Box 70015
Anaheim, CA 92825-0015
$28.95/year
(12 issues)
714/572-2255
Car Sound Buyer's Guide
939 Port Washington Blvd.
Port Washington, NY 11050
[email protected]
$17.95/year
(Quarterly)
516/944-5940
Car Hi-Fi
Freepost
TK660
Leicester
#23.60/year
(8 issues)
081 943 5943
KE87 4AW
Bilstereo Forlags AB
Box 230 84
104 35 Stockholm
Sverige (Sweden)
[email protected]
Skr ??
(? Issues)
08-34 29 70
7.2 Are there any newsletters I can read? [IDB]
Unfortunately, AutoSound 2000 Tech Briefs is no longer in publication, and as a result,
there are no more newsletters currently in publication.
7.3 What books can I read? [JSC, JW, TT]
Loudspeaker Design Cookbook
by Vance Dickason
Published by Audio Amateur Press
ISBN 0-9624191-7-6
$25-$30
Designing, Building and Testing Your Own Speaker System
by David Weems
Published by McGraw Hill
ISBN 0-8306-3374-X
$16.95
Killer Car Stereo on a Budget
by Daniel L. Ferguson
Published by Audio Amateur Press
ISBN 0-9624191-0-9
$19.95
The Car Stereo Cookbook
by Mark Rumreich
Published by McGraw-Hill
ISBN 0-07-058083-9
$24.95
7.4 Can I contact any manufacturers on-line? [IDB]
Yes, there are a number of manufacturers who have started to use the Internet and can
offer advice for component selection, installation and advice. A list of these
manufacturers is available on the WWW, at the URL:
http://www.mobileaudio.com/intdir/