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Microphones
Basic Principles and Types
A microphone is a transducer. It takes energy in the form of an acoustic pressure wave and
transforms it into an electrical signal. This signal can then be amplifier, stored, or manipulated in
a variety of ways. Not all recorded or amplified musical signals arrive to our ears via
microphones. There are other types of transducers such as contact pickups that are used on
amplified acoustic guitars and similar instruments, and magnetic pickups as typified by electric
guitars and basses. In some cases the instrument generates an electrical signal directly as with
electronic piano, synthesizers, etc. Microphones run the gamut from small modules that cost less
than a dollar to studio devices costing well over $1000. As one might guess, there is wide
variation in performance across this range.
Microphones respond to the applied acoustic pressure. That is, they have some form of diaphragm
that is moved by the pressure across it (i.e., the difference in pressure between the front and the
back of the diaphragm). In some cases the back of the diaphragm is sealed off so that there is no
rearward excitation. These are known as pressure microphones. In other cases, the back of the
diaphragm is left open or accessed via acoustical delay ports, both of which affect the response of
the microphone at different angles (i.e., the polar response). These devices are known as pressure
gradient microphones as there is a differential pressure (i.e., a gradient).
There are five major types of microphones. The first is the carbon microphone. These were
commonly used in older style telephones. While rugged and producing a high output, they suffer
from excessive noise, poor frequency response, and high distortion. They are not used for serious
recording. Crystal microphones rely on the piezo-electric effect. This is an effect produced by
some crystals wherein flexure of the crystal will produce a voltage (and vice versa). Crystal
microphones (and the similar ceramic microphones) were once the mainstay of the hobbyist due
to their low expense. As with the carbon microphone, crystal devices are not normally used for
serious recording. Moving-coil or dynamic microphones range in quality and price from very low
to very high and are used by both pro and hobbyist alike. The diaphragm is attached to a voice
coil that in turn is suspended in a magnetic gap. Movement of the diaphragm causes the voice coil
to move. The fixed magnetic field surrounding the coil then induces a voltage in the coil. The
operation is similar to a dynamic loudspeaker in reverse1. Dynamic microphones can exhibit very
wide frequency response, low distortion, and wide dynamic range. These devices are often used
to record very loud instruments up-close, such as horns and bass drums. These signals can
sometimes overload more sensitive designs. One of the more popular professional dynamic
microphones is the SM57 family manufactured by Shure. These devices have been a mainstay of
popular music artists for decades. The current version is the Beta 57. Ribbon microphones operate
by combining the idea of conductor and diaphragm into one. These devices use a small metallic
ribbon stretched between two magnetic pole pieces. Movement of the ribbon (conductor) induces
a small voltage. Ribbon microphones are generally considered to have good transient response
although they tend to be overloaded by very loud sounds.
Finally, there is the electrostatic or condenser microphone. The basic idea is that a diaphragm
(metalized) serves as one plate of a capacitor. Movement of the diaphragm creates a change in
capacitance that can be turned into a voltage. As the diaphragm can be made very light, condenser
microphones tend to have excellent transient response. Generally, the larger the diaphragm, the
1
In simple single duplex intercom systems, loudspeakers are often used as both microphone and
loudspeaker.
ET163 Audio Technology Lecture Notes: Microphones
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greater the sensitivity, although this may detract from polar response. One down side of
condenser microphones is the need for a polarizing voltage. Unlike the other microphone types,
condensers require a DC voltage. In a studio, this is supplied through a means known as phantom
powering. In this system a 48 volt DC potential is applied to both the in-phase and out-of-phase
leads, with respect to ground2. In this manner, normal dynamic or ribbon microphones are not
affected. Many condenser microphones have built-in transducer/pre-amps circuits and may have
external switches for attenuation or frequency response tailoring. Some examples of large
diaphragm studio condensers include the Neumann U-87 and the AKG C414. A stereo pair of U87’s currently retails for about $6k. At the other extreme are small electret (pre-polarized)
condenser modules that can be purchased for less than one dollar.
Frequency Response
Ideally, any microphone would completely cover the range of human hearing, extending from
below 20 Hz to greater than 20 kHz. The response would be flat, boosting or attenuating no band
of frequencies with respect to any other. Very wide and flat response is available, particularly in
the case of calibrated measurement microphones. These are often small diaphragm condenser
types. By keeping the diaphragm small, the “shadowing” effects of the microphone itself are
minimized. Interestingly, many very popular studio microphones do not exhibit ruler-flat
response and this is well known among recording engineers. Specific microphones may exhibit
low frequency boost or cut, mid frequency peaks, broad high frequency boost, or other
aberrations. The engineers will peak a specific microphone to exploit these characteristics in
specific situations. For example, a modest peak in response in the 3 to 5 kHz range may help to
increase voice intelligibility. Similarly, a modest high frequency boost may help to add a sense of
openness or “air” to certain instruments.
Some microphones, particularly those with very directional response (see next section) may
exhibit pronounced proximity effect. In essence, this is a low frequency boost that occurs when
the microphone is positioned very close to the sound source. This boost can be several decibels in
magnitude in the bass region. This can lead to a vocalist sounding “chesty”3 if the effect is not
compensated for with bass rolloff.
Polar Response (Directivity)
All microphones respond to signals along their primary axis (i.e., frontal), but they vary widely in
their response to off-axis signals. Generally speaking, microphones are classified as either
omnidirectional or directional. There are several sub-categories of directional microphones
including bidirectional (figure-eight), cardioid, super cardioid, and hyper cardiod.
Ideally, omnidirectional microphones respond to all signals around it equally. This is generally
true at lower frequencies although due to shadowing there is some discrimination of high
frequency sounds from the sides and rear. By channeling sounds through to the back of the
diaphragm, microphones can be made directional. Therefore directional microphones are
generally of the pressure gradient type. The simplest case is the bidirectional microphone. As
signals are allowed to directly impinge on the rear of the diaphragm, sounds from directly in front
as well as directly behind are given preference. Sounds from 90 degrees off-axis do not produce a
pressure gradient across the diaphragm and thus are discriminated against. Rear access can also
2
Professional microphones use a 3 lead system: in-phase, out-of-phase, and ground (shield). Thus, all
professional microphone pre-amps use a differential style input. This helps to minimize induced noise.
3
This was a favorite effect of many FM radio DJs of the 1970’s who wished to sound “hip”.
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be made through ports along the microphone case. These ports create acoustical delay lines, thus
altering the phase of the off-axis signals. By controlling the delays, a variety of polar responses
become possible. The most basic of these is the cardioid, named so because its polar response is
uniquely heart-shaped, showing greatest attenuation to rearward signals. The super cardioid
response provides further rejection of off-axis signals, although there is some acceptance of
signals 180 degrees off axis. The hyper cardioid follows this idea further and creates a very
directional response, but again at the cost of increased response to signals directly behind. In this
regard one might think of the super and hyper cardioid responses as being transitional between
the basic cardioid and bidirectional types.
The tags of cardioid, super cardioid, and so on describe the nominal polar response. Generally, all
microphones tend to be more omnidirectional at low frequencies and more directional at the
higher frequencies. Manufacturers publish polar response plots taken at several frequencies and
plotted simultaneously. In this manner the general polar-frequency response characteristic can be
determined.
Specialized Devices and Techniques
Along with the devices discussed previously, there are a few special-purpose microphones. One
such device is the pressure-zone or boundary layer microphone. These devices appear rather odd;
normally a plate a few inches across with a small housing at one end. The basic principle is that
of the pressure zone: At any boundary, the pressure of all directly incident waves (all frequencies)
will be in phase and maximal. This is because the particle velocity must be zero at the boundary.
A small transducer is afixed just millimeters from this boundary. Consequently, all signals
received are in phase and at maximal amplitude.
For very directional usage, either shotgun or parabolic reflector microphones are used. Shotgun
microphones have a very long housing with specialized ports and delays to achieve a very
directional response. These are sometimes used on soundstages to maximize the pickup of the
actors while minimizing undesired off-stage noises. Parabolic reflector microphones use a
parabolic “dish” to reflect on-coming sound into a microphone. To be highly effective, the dish
needs to be on the order of a wavelength or more in size. Consequently, portable dishes do not
improve directional response at low frequencies and only give the greatest increases above 1kHz
or so.
In outdoor situations crosswinds can create an undesired noise signal. This signal tends to be
stronger in the bass range. To alleviate this, microphones are often fitted with foam windscreens.
These devices serve to dissipate the oncoming wind energy, thus reducing the noise signal.
Generally, the larger the windscreen the more effective it is, although care must be taken so that it
remains transparent to even the highest frequencies. Failure to do so may result in unwanted
attenuation of the higher harmonics of the desired signal.
ET163 Audio Technology Lecture Notes: Microphones
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