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DATA - Audio Levels
Updated 24 December 2004
Audio levels and the various standards seem to be of a constant problem for everyone and
digital audio has done little to make things easier. Standards definitions and trends all seem to
be at odds with each other. Normally in electronics we would speak in volts and amps and
even if we are in tune with the AC world it is RMS volts that we would refer to. Music is
rarely a neat sine wave for a single RMS voltage to be of use. However for testing and
comparing that is all we have and even though a simple single numeral would be nice it often
takes more information to tell us what is going on or to compare one unit to another.
I'm sure we are all familiar with V = I R but so many do forget when delving into audio.
Some thought may be given to Power Amplifiers and loading when multiple Speakers are
added, more current will be needed and perhaps the use of a thicker speaker cable. Tube
Power Amps often have impedance selectors to choose 16, 8 and 4 ohm loads. It seems
though once we get into the smaller Line and Microphone level signal the current and loading
is often forgotten.
For the most part we use dB's to describe audio level, dBm, dBu, dBV and even dB SPL
when talking about acoustic level of sound itself. To often it just gets dropped back to dB and
we just hope everyone understands what we might be referring to at the time.
This page is not meant to be a definitive bible on all things dB but a simple walk through a bit
of theory and perhaps a bit of history to leave you with some knowledge to put things into
perspective as it relates to equipment and styles of recording that are popular today.
Definitions
The bel (named after Alexander Graham Bell) is the basic unit of measure. The decibel being
one-tenth of a bel is the more common unit of measure that was probably first use to measure
the telephone systems of the Bell telephone company around the late 1920's. It was all to do
with the loss of signal over the length of a cable and is a logarithmic function. It gets even
more complicated as it is supposed to be presented as a ratio of different audio levels.
I could clumsily give examples here and quote from books and give formulas like 20 log x/y,
where x and y are the different signal levels ...
blah blah ... but it probably won't help in the long run and only confuse you more. The
important fact to try to remember here, is that as it is a ratio and relative by nature, the decibel
is not really a unit of measure like a Volt or an Amp.
So as a ratio it must be related to typical or common point of reference, lets say, 0dB.
Even though we have just established that all things are relative and that it is likely we are
relative to 0 ... but 0 what ?
A suffix letter is added as follows to point us towards the measurement method used.
dBV - dB referenced to RMS voltage
Probably the easiest one to cope with as the V indicates Volts and all you have to remember is
that it is RMS Volts
0 dBV = 1.000 Vrms
-10 dBV = 0.3162 Vrms
quite simply dBV is referenced to 1 Vrms and this is really all this about
note : the voltages are assumed to be clean sinewave - perhaps more on that latter
dBu - dB un-terminated
People have told me that the u is for un-terminated but it is still a voltage measurement
... and I thought it was dBv that somehow got changed to dBu
0 dBu = 0.7746 Vrms
+4 dBu = 1.228 Vrms
ultimately this one comes from being referenced to the 1 mW into 600 ohms and so the 0.7746 Vrms see below
dBm - dB referenced to RMS power
A little trickier this one as it is a power specification that must have a load for current to flow
in order to get a power measurement.
Typically though in practice a voltage measurement would be taken and the load assumed.
0 dBm = 1 mW (milliwatt)
but into what load?
most often in professional audio line level is the 600 ohm termination so ...
0 dBm into 600 ohm = 0.7746 V = 0 dBu
change the termination load and the numbers change
0 dBm into 50 ohms = 0.224 V = -10.79 dBu
A very different result and this concept does cause an amount of confusion. Much of the new
gear of today is what is known as bridging input with a termination much higher than 600
ohms and so the 0 dBu, or -10 dBV seem to be the popular specification.
All very well until you try to interface with old 600 ohm equipment that is so popular,
particularly DIY'ers.
Power transfer through a transformer may be more important than simple voltage
measurements at the input and the output. Microphones have a variety of source and load
impedance and tend to use transformers so this measurement standard is worth your time to
understand.
even though much of our modern gear is bridging input and therefore much higher than 600
ohms I think this one tells us the most about the capabilities of an output driver.
dBr - dB referenced to an arbitrary reference level
An interesting one here that you may not come across often. Signal to noise equipment and
it's presentation may be calibrated in this way and sometime an old mixing desks may have
some specified levels referred to a reference that could be an internal operating level or output
level.
I'm not going to say any more about this one for now as it can be a little abstract at times and
probably doesn't help the overall understanding of the above.
dBFS - dB Full Scale
I do like this one as it is quite absolute. Full Scale refers to the maximum signal an audio data
converter can handle. When all bits are on , it is as loud as it can get and there is no eleven.
Try not to think analog here where all things are relative. The digital specification gives a
word that is as loud as it gets and from there other words and the bit depth lets a signal get
quieter. The voltage level that various AD's and DA's may have on their inputs and outputs
for the given Full Scale is likely to vary from chip to chip but this should not concern us as
the associated analog circuits should then, without clipping, follow the above standards.
0 dBFS = Full Scale
There is going to be some experts out there that won't like my very short hand description of
the above but I hope it helps the beginners fill in a few gaps and aid in the understanding of
the following charts.
If you have some thoughts on the above , please do contact me.
Putting some perspective into this
A very helpful calculator for levels can be found at the Analog Devices Interactive Design
Tools: Utilities : VRMS / dBm / dBu / dBV calculator
http://www.analog.com/Analog_Root/static/techSupport/designTools/interactiveTools/Analog
Devices Interactive Design Tools Utilities VsubRMS-sub - dBm - dBu - dBV calculator.htm
The purpose of the above chart is show the required current and therefore power required to
apply a given voltage across a load. Starting with the 0dBu (in red above) and terminated into
600 ohms we get 0.7746 volts equaling 1mW which the standard for 0dBm. Moving up and
down the list you can find the equivalent data for every 3 dB increment.
I have included a couple of other specific standards (in blue above) to help detail things. The
obvious ones of +4dB and -10dB are probably self explanatory but the other two may not at
first glance. It is quite simple ... the 15volts and 18volts peak relate to the typical balanced
power op-amp circuit as use with the common NE5534 and most other monolithic audio opamps. Yes there other op-amps that can handle as mush as 24/25volts and we can look at them
later.
We will focus on the NE5534 and 5532 only because they are long serving standards in proaudio and are the basis to which many of the newer high class op-amps own homage too.
Using our favorite op-amp
Powered by the typical +/-15 volts balanced supply an op-amp has the potential to deliver 15
volts peak to peak on it's output.
but can it ?
This would equate to 22.73dBu. Into a 600 ohm load (and therefore 22.73dBu) the opamp
will be required to deliver 17.68mA and 187.5mW.
Looking at the specifications for the Fairchild NE5532 we find, in the Electrical
Characteristics Table, that at
VCC=15V, VEE= - 15V, TA = 25°C
Output Voltage Swing VO(P-P) with a load RL greater than or equal to 600 ohms
+/-12V minimum and +/-13V typical.
Short Circuit Current ISC 38 mA
The Texas Instrument spec for an Texas NE5532 at the the same power supply voltages give
similar results,
however the spec is given slightly differently as 24V minimum and 26V typical.
(24V = +/-12V ) ...be careful when comparing specs as direct comparisons are not always
obvious.
At at the higher VCC± = +18 V, TA = 25°C
Maximum peak-to-peak output voltage swing VOPP with load RL greater than or equal to 600
ohms
30V min and 32V typical
Output short-circuit current IOS 38 mA
In both cases the spec for maximum current when the output is shorted is 38mA. We can
probably assume that the very maximum current these and similar op-amps can deliver even
at the best of time is around 40mA.
Headphones
The above situation gets even more crytical with headphones. The wide variety of headphone
impedances and monitoring levels people work with can mean what is great for one, is not
even close for another. Generally you have to pay closer attention to power than just voltage
level.
Some typical specs for some popular headphones may read like ...
AKG - K 240 Beyer - DT 770
Fostex Sony - MDR
Studio
PRO
T40RP
7509
impedance : 55 impedance : 250
impedance : 50
impedance : 24
ohms
ohm
ohm
ohm
power :
power :
power :
power : 3000mW
unknown
unknown
unknown
sensitivity : 107
sensitivity :
sensitivity :
sensitivity :
dB
unknown
unknown
93dB/m
more to come