A microphone is an example of a transducer, a device that changes information
from one form to another. Sound information exists as patterns of air pressure;
the microphone changes this information into patterns of electric current. The
recording engineer is interested in the accuracy of this transformation, a
concept he thinks of as fidelity.
A variety of mechanical techniques can be used in building microphones. The two
most commonly encountered in recording studios are the magneto-dynamic and the
variable condenser designs.
In the magneto-dynamic, commonly called dynamic, microphone, sound waves cause
movement of a thin metallic diaphragm and an attached coil of wire. A magnet
produces a magnetic field which surrounds the coil, and motion of the coil
within this field causes current to flow. The principles are the same as those
that produce electricity at the utility company, realized in a pocket-sized
scale. It is important to remember that current is produced by the motion of the
diaphragm, and that the amount of current is determined by the speed of that
motion. This kind of microphone is known as velocity sensitive.
In a condenser microphone, the diaphragm is mounted close to, but not touching,
a rigid backplate. (The plate may or may not have holes in it.) A battery is
connected to both pieces of metal, which produces an electrical potential, or
charge, between them. The amount of charge is determined by the voltage of the
battery, the area of the diaphragm and backplate, and the distance between the
two. This distance changes as the diaphragm moves in response to sound. When the
distance changes, current flows in the wire as the battery maintains the correct
charge. The amount of current is essentially proportioinal to the displacement
of the diaphragm, and is so small that it must be electrically
amplified before it leaves the microphone.
A common varient of this design uses a material with a permanently imprinted
charge for the diaphragm. Such a material is called an electret
and is usually a kind of plastic. (You often get a piece of plastic
with a permanent charge on it when you unwrap a record. Most plastics conduct
electricity when they are hot but are insulators when they cool.) Plastic is a
pretty good material for making diaphragms since it can be dependably produced
to fairly exact specifications. (Some popular dynamic microphones use plastic
diaphragms.) The major disadvantage of electrets is that they lose their charge
after a few years and cease to work.
There is no inherent advantage in fidelity of one type of microphone over
another. Condenser types require batteries or power from the mixing console to
operate, which is occasionally a hassle, and dynamics require shielding from
stray magnetic fields, which makes them a bit heavy sometmes, but very fine
microphones are available of both styles. The most important factor in choosing
a microphone is how it sounds in the required application. The following issues
must be considered:
This is a measure of how much electrical output is produced by a given
sound. This is a vital specification if you are trying to record very tiny
sounds, such as a turtle snapping its jaw, but should be considered in any
situation. If you put an insensitive mic on a quiet instrument, such as an
acoustic guitar, you will have to increase the gain of the mixing console,
adding noise to the mix. On the other hand, a very sensitive mic on vocals might
overload the input electronics of the mixer or tape deck, producing distortion.
Any microphone will produce distortion when it is overdriven by loud
sounds. This is caused by varous factors. With a dymanic, the coil may be pulled
out of the magnetic field; in a condenser, the internal amplifier might clip.
Sustained overdriving or extremely loud sounds can permanently distort the
diaphragm, degrading performance at ordinary sound levels. Loud sounds are
encountered more often than you might think, especially if you place the mic
very close to instruments. (Would you put your ear in the bell of a trumpet?)
You usually get a choice between high sensitivity and high overload points,
although occasionally there is a switch on the microphone for different
This is the feature that runs up the price of microphones. The distortion
characteristics of a mic are determined mostly by the care with which the
diaphragm is made and mounted. High volume production methods can turn out an
adequate microphone, but the distortion performance will be a matter of luck.
Many manufacturers have several model numbers for what is essentially the same
device. They build a batch, and then test the mics and charge a premium price
for the good ones. The really big names throw away mic capsules that don't meet
their standards. (If you buy one Neumann mic, you are paying for five!)
No mic is perfectly linear; the best you can do is find one with distortion that
complements the sound you are trying to record. This is one of the factors of
the microphone mystique discussed later.
A flat frequency response has been the main goal of microphone companies
for the last three or four decades. In the fifties, mics were so bad that
console manufacturers began adding equalizers to each input to compensate. This
effort has now paid off to the point were most professional microphones are
respectably flat, at least for sounds originating in front. The major exceptions
are mics with deliberate emphasis at certain frequencies that are useful for
some applications. This is another part of the microphone mystique. Problems in
frequency response are mostly encountered with sounds originating behind the
mic, as discussed in the next section.
Microphones produce a very small amount of current, which makes sense when
you consider just how light the moving parts must be to accurately follow sound
waves. To be useful for recording or other electronic processes, the signal must
be amplified by a factor of over a thousand. Any electrical noise produced by
the microphone will also be amplified, so even slight amounts are intolerable.
Dynamic microphones are essentially noise free, but the electronic circuit built
into condensor types is a potential source of trouble, and must be carefully
designed and constructed of premium parts.
Noise also includes unwanted pickup of mechanical vibration through the body of
the microphone. Very sensitive designs require elastic shock mountings, and mics
intended to be held in the hand need to have such mountings built inside the
The most common source of noise associated with microphones is the wire
connecting the mic to the console or tape deck. A mic preamp is very similar to
a radio reciever, so the cable must be prevented from becoming an antenna. The
basic technique is to surround the wires that carry the current to and from the
mic with a flexible metallic shield, which deflects most radio energy. A second
technique, which is more effective for the low frequency hum induced by the
power company into our environment, is to balance the line:
Current produced by the microphone will flow down one wire of the twisted pair,
and back along the other one. Any current induced in the cable from an outside
source would tend to flow the same way in both wires, and such currents cancel
each other in the transformers. This system is expensive.
As I said, microphone outputs are of necessity very weak signals, generally
around -60dBm. (The specification is the power produced by a sound pressure of
10 uBar) The output impedance will depend on whether the mic has a transformer
balanced output . If it does not, the microphone will be labeled "high
impedance" or "hi Z" and must be connected to an appropriate
input. The cable used must be kept short, less than 10 feet or so, to avoid
If a microphone has a transformer, it will be labeled low impedance, and will
work best with a balanced input mic preamp. The cable can be several hundred
feet long with no problem. Balanced output, low impedance microphones are
expensive, and generally found in professonal applications. Balanced outputs
must have three pin connectors ("Cannon plugs"), but not all mics with
those plugs are really balanced. Microphones with standard or miniature phone
plugs are high impedance. A balanced mic can be used with a high impedance input
with a suitable adapter.
There is a transformer at
the input of the console preamp. (Or, in lieu of a transformer, a complex
circuit to do the same thing.) This is the most significant difference between
professional preamplifiers and the type usually found on home tape decks. You
can buy transformers that are designed to add this feature to a consumer deck
for about $20 each. (Make sure you are getting a transformer and not just an
adapter for the connectors.) With these accessories you can use professional
quality microphones, run cables over a hundred feet with no hum, and because the
transformers boost the signal somewhat, make recordings with less noise. This
will not work with a few inexpensive cassette recorders, because the strong
signal causes distortion. Such a deck will have other problems, so there is
little point trying to make a high fidelity recording with it anyway.
Many people have the misconception that microphones only pick up sound from
sources they are pointed at, much as a camera only photographs what is in front
of the lens. This would be a nice feature if we could get it, but the truth is
we can only approximate that action, and at the expense of other desirable
These are polar graphs of the output produced vs. the angle of the sound
source. The output is represented by the radius of the curve at the incident
The simplest mic design will pick up all sound, regardless of its point of
origin, and is thus known as an omnidirectional microphone. They are very easy
to use and generally have good to outstanding frequency response.
It is not very difficult to produce a pickup pattern that accepts sound
striking the front or rear of the diaphragm, but does not respond to sound from
the sides. This is the way any diaphragm will behave if sound can strike the
front and back equally. The rejection of undesired sound is the best achievable
with any design, but the fact that the mic accepts sound from both ends makes it
difficult to use in many situations. Most often it is placed above an
instrument. Frequency response is just as good as an omni, at least for sounds
that are not too close to the microphone.
This pattern is popular for sound reinforcement or recording concerts where
audience noise is a possible problem. The concept is great, a mic that picks up
sounds it is pointed at. The reality is different. The first problem is that
sounds from the back are not completely rejected, but merely reduced about 10-30
dB. This can surprise careless users. The second problem, and a severe one, is
that the actual shape of the pickup pattern varies with frequency. For low
frequencies, this is an omnidirectional microphone. A mic that is directional in
the range of bass instruments will be fairly large and expensive. Furthermore,
the frequency response for signals arriving from the back and sides will be
uneven; this adds an undesired coloration to instruments at the edge of a large
ensemble, or to the reverberation of the concert hall.
A third effect, which may be a problem or may be a desired feature, is that the
microphone will emphasize the low frequency components of any source that is
very close to the diaphragm. This is known as the "proximity effect",
and many singers and radio announcers rely on it to add "chest" to a
basically light voice. Close, in this context, is related to the wavelength of
the sound and the size of the microphone diaphragm, so the nice large mics with
even back and side frequency response exhibit the strongest presence effect.
Most cardioid mics have a built in lowcut filter switch to compensate for
proximity. Missetting that switch can cause hilarious results. Bidirectional
mics also exhibit this phenomenon.
It is posible to exaggerate the directionality of cardioid type microphones, if
you don't mind exaggerating some of the problems. The Hypercardioid pattern is
very popular, as it gives a better overall rejection and flatter frequency
response at the cost of a small back pickup lobe. This is often seen as a good
compromise between the cardioid and bidirectional patterns. A "shotgun"
mic carries these techniques to extremes by mounting the diaphragm in the middle
of a pipe. The shotgun is extremely sensitive along the main axis, but posseses
pronounced extra lobes which vary drastically with frequency. In fact, the
frequency response of this mic is so bad it is usually electronically restricted
to the voice range, where it is used to record dialogue for film and video.
You don't need a special microphone to record in stereo, you just need two (see
below). A so called stereo microphone is really two microphones in the same
case. There are two kinds: extremely expensive professional models with
precision matched capsules, adjustable capsule angles, and remote switching of
pickup patterns; and very cheap units (often with the capsules oriented at 180
deg.) that can be sold for high prices because they have the word stereo written
Use of a single microphone is pretty straightforward. Having chosen one with
appropriate sensitivity and pattern, (and the best distortion, frequency
response, and noise characteristics you can afford), you simply mount it where
the sounds are. The practical range of distance between the instrument and the
microphone is determined by the point where the sound overloads the microphone
or console at the near end, and the point where ambient noise becomes
objectionable at the far end. Between those extremes it is largely a matter of
taste and experimentation.
If you place the microphone close to the instrument, and listen to the results,
you will find the location of the mic affects the way the instrument sounds on
the recording. The timbre may be odd, or some notes may be louder than others.
That is because the various components of an instrument's sound often come from
different parts of the instrument body (the highest note of a piano is nearly
five feet from the lowest), and we are used to hearing an evenly blended tone. A
close in microphone will respond to some locations on the instrument more than
others because the difference in distance from each to the mic is proportionally
large. A good rule of thumb is that the blend zone starts at a distance of about
twice the length of the instrument. If you are recording several instruments,
the distance between the players must be treated the same way.
If you place the microphone far away from the instrument, it will sound as if it
is far away from the instrument. We judge sonic distance by the ratio of the
strength of the direct sound from the instrument (which is always heard first)
to the strength of the reverberation from the walls of the room. When we are
physically present at a concert, we use many cues beside the sounds to keep our
attention focused on the performance, and we are able to ignore any distractions
there may be. When we listen to a recording, we don't have those visual clues to
what is happening, and find anything extraneous that is very audible annoying.
For this reason, the best seat in the house is not a good place to record a
concert. On the other hand, we do need some reverberation to appreciate certain
features of the music. (That is why some types of music sound best in a stone
church) Close microphone placement prevents this. Some engineers prefer to use
close miking techniques to keep noise down and add artificial reverberation to
the recording, others solve the problem by mounting the mic very high, away from
audience noise but where adequate reverberation can be found.
Stereo sound is an illusion of spaciousness produced by playing a recording back
through two speakers. The success of this illusion is referred to as the image.
A good image is one in which each instrument is a natural size, has a distinct
location within the sound space, and does not move around. The main factors that
establish the image are the relative strength of an instrument's sound in each
speaker, and the timing of arrival of the sounds at the listener's ear. In a
studio recording, the stereo image is produced artificially. Each instrument has
its own microphone, and the various signals are balanced in the console as the
producer desires. In a concert recording, where the point is to document
reality, and where individual microphones would be awkward at best, it is most
common to use two mics, one for each speaker.
The simplest approach is to assume that the speakers will be eight to ten feet
apart, and place two microphones eight to ten feet apart to match. Either omnis
or cardioids will work. When played back, the results will be satisfactory with
most speaker arrangements. (I often laugh when I attend concerts and watch
people using this setup fuss endlessly with the precise placement of the mics.
This technique is so forgiving that none of their efforts will make any
The big disavantage of this technique is that the mics must be rather far
back from the ensemble- at least as far as the distance from the leftmost
performer to the rightmost. Otherwise, those instruments closest to the
microphones will be too prominent. There is usually not enough room between
stage and audience to achieve this with a large ensemble, unless you can suspend
the mics or have two very tall stands.
There is another disadvantage to the spaced technique that appears if
the two channels are ever mixed together into a monophonic signal. (Or broadcast
over the radio, for similar reasons.) Because there is a large distance between
the mics, it is quite possible that sound from a particular instrument would
reach each mic at slightly different times. (Sound takes 1 millisecond to travel
a foot.) This effect creates phase differences between the two channels, which
results in severe frequency response problems when the signals are combined. You
seldom actually lose notes from this interference, but the result is an uneven,
almost shimmery sound. The various coincident techniques avoid this problem by
mounting both mics in almost the same spot.
This is most often done with two cardioid microphones, one pointing slightly
left, one slightly right. The microphones are often pointing toward each other,
as this places the diaphragms within a couple of inches of each other, totally
eliminating phase problems. No matter how they are mounted, the microphone that
points to the left provides the left channel. The stereo effect comes from the
fact that the instruments on the right side are on-axis for the right channel
microphone and somewhat off-axis (and therefore reduced in level) for the other
one. The angle between the microphones is critical, depending on the actual
pickup pattern of the microphone. If the mics are too parallel, there will be
little stereo effect. If the angle is too wide, instruments in the middle of the
stage will sound weak, producing a hole in the middle of the image.
[Incidentally, to use this technique, you must know which way the capsule
actually points. There are some very fine German cardioid microphones in which
the diaphragm is mounted so that the pickup is from the side, even though the
case is shaped just like many popular end addressed models. (The front of the
mic in question is marked by the trademark medallion.) I have heard the results
where an engineer mounted a pair of these as if the axis were at the end. You
could hear one cello player and the tympani, but not much else.]
You may place the microphones fairly close to the instruments when you use this
technique. The problem of balance between near and far instruments is solved by
aiming the mics toward the back row of the ensemble; the front instruments are
therefore off axis and record at a lower level. You will notice that the height
of the microphones becomes a critical adjustment.
The most elegant approach to coincident miking is the M.S. or middle-side
technique. This is usually done with a stereo microphone in which one element is
omnidirectional, and the other bidirectional. The bidirectional element is
oriented with the axis running parallel to the stage, rejecting sound from the
center. The omni element, of course, picks up everything. To understand the next
part, consider what happens as instrument is moved on the stage. If the
instrument is on the left half of the stage, a sound would first move the
diaphragm of the bidirectional mic to the right, causing a positive voltage at
the output. If the instrument is moved to center stage, the microphone will not
produce any signal at all. If the instrument is moved to the right side, the
sound would first move the diaphragm to the left, producing a negative volage.
You can then say that instruments on one side of the stage are 180 degrees out
of phase with those on the other side, and the closer they are to the center,
the weaker the signal produced.
Now the signals from the two microphones are not merely kept in two channels and
played back over individual speakers. The signals are combined in a circuit that
has two outputs; for the left channel output, the bidirectional output is added
to the omni signal. For the right channel output, the bidirectional output is
subtracted from the omni signal. This gives stereo, because an instrument on the
right produces a negative signal in the bidirectional mic, which when added to
the omni signal, tends to remove that instrument, but when subtracted, increases
the strength of the instrument. An instrument on the left suffers the opposite
fate, but instruments in the center are not affected, because their sound does
not turn up in the bidirectional signal at all.
M.S. produces a very smooth and accurate image, and is entirely mono
compatabile. The only reason it is not used more extensively is the cost of the
special microphone and decoding circuit, well over $1,000.
The above techniques work well for concert recordings in good halls with
small ensembles. When recording large groups in difficult places, you will often
see a combination of spaced and coincident pairs. This does produce a kind of
chorusing when the signals are mixed, but it is an attractive effect and not
very different from the sound of string or choral ensembles any way. When
balance between large sections and soloists cannot be acheived with the basic
setup, extra microphones are added to highlight the weaker instruments. A very
common problem with large halls is that the reverberation from the back seems
late when compared to the direct sound taken at the edge of the stage. This can
be helped by placing a mic at the rear of the audience area to get the ambient
sound into the recording sooner.
A complete description of all of the procedures and tricks encountered in the
recording studio would fill several books. These are just a few things you might
see if you dropped in on the middle of a session.
This provides the engineer with the ability to adjust the balance of the
instruments at the console, or, with a multitrack recorder, after the musicians
have gone home. There may be eight or nine mics on the drum set alone.
The microphones will usually be placed rather close to the instruments.
This is partially to avoid problems that occur when an instrument is picked up
in two non-coincident mics, and partially to modify the sound of the instruments
(to get a "honky-tonk" effect from a grand piano, for instance).
The interference that occurs when when an instrument is picked up by two
mics that are mixed is a very serious problem. You will often see extreme
measures, such as a bass drum stuffed with blankets to muffle the sound, and
then electronically processed to make it sound like a drum again.
Studio musicians often play to "click tracks", which are not
recorded metronomes, but someone tapping the beat with sticks and occasionally
counting through tempo changes. This is done when the music must be synchronized
to a film or video, but is often required when the performer cannot hear the
other musicians because of the isolation measures described above.
Recordings require a level of perfection in intonation and rhythm that is
much higher than that acceptable in concert. The finished product is usually a
composite of several takes.
Some microphones are very sensitive to minor gusts of wind--so sensitive in
fact that they will produce a loud pop if you breath on them. To protect these
mics (some of which can actually be damaged by blowing in them) engineers will
often mount a nylon screen between the mic and the artist. This is not the most
common reason for using pop filters though:
Vocalists like to move around when they sing; in particular, they will lean
into microphones. If the singer is very close to the mic, any motion will
produce drastic changes in level and sound quality. (You have seen this with
inexpert entertainers using hand held mics.) Many engineers use pop filters to
keep the artist at the proper distance. The performer may move slightly in
relation to the screen, but that is a small proportion of the distance to the
There is an aura of mystery about microphones. To the general public, a
recording engineer is something of a magician, privy to a secret arcana, and
capable of supernatural feats. A few modern day engineers encourage this
attitude, but it is mostly a holdover from the days when studio microphones were
expensive and fragile, and most people never dealt with any electronics more
complex than a table radio. There are no secrets to recording; the art is mostly
a commonsense application of the principles already discussed in this paper. If
there is an arcana, it is an accumulation of trivia achieved through experience
with the following problems:
There is no wrong microphone for any instrument. Every engineer has
preferences, usually based on mics with which he is familiar. Each mic has a
unique sound, but the differences between good examples of any one type are
pretty minor. The artist has a conception of the sound of his instrument,
(which may not be accurate) and wants to hear that sound through the speakers.
Frequency response and placement of the microphone will affect that sound;
sometimes you need to exaggerate the features of the sound the client is looking
It is easy to forget that the recording engineer is an illusionist- the
result will never be confused with reality by the listener. Listeners are in
fact very forgiving about some things. It is important that the engineer be able
to focus his attention on the main issues and not waste time with interesting
but minor technicalities. It is important that the engineer know what the main
issues are. An example is the noise/distortion tradeoff. Most listeners are
willing to ignore a small amount of distortion on loud passages (in fact, they
expect it), but would be annoyed by the extra noise that would result if the
engineer turned the recording level down to avoid it. One technique for
encouraging this attention is to listen to recordings over a varitey of sound
systems, good and bad.
Many students come to me asking for a book or a course of study that will
easily make them a member of this elite company. There are books, and some
schools have courses in recording, but they do not supply the essential quality
the professional recording engineer needs, which is experience.
A good engineer will have made hundreds of recordings using dozens of different
microphones. Each session is an opportunity to make a new discovery. The
engineer will make careful notes of the setup, and will listen to the results
many times to build an association between the technique used and the sound
achieved. Most of us do not have access to lots of professional microphones, but
we could probably afford a pair of general purpose cardioids. With about $400
worth of mics and a reliable tape deck, it is possible to learn to make
excellent recordings. The trick is to record everything that will sit still and
make noise, and study the results: learn to hear when the mic is placed badly
and what to do about it. When you know all you can about your mics, buy a
different pair and learn those. Occasionally, you will get the opportunity to
borrow mics. If possible, set them up right alongside yours and make two
recordings at once. It will not be long before you will know how to make
consistently excellent recordings under most conditions.
Peter Elsea 1996