How and Why Microphones Work

Microphones appear in an almost endless variety of shapes, sizes,
and design types, but no matter what their physical attributes,
their purpose is the same—to convert acoustic vibrations (in the
form of air pressure) to electrical energy so it can be amplified
or recorded. Most achieve this by the action of the air vibrating a
diaphragm connected to something that either creates or allows a
small electron flow.

There are three basic mechanical techniques that are used in
building microphones for professional audio purposes, but all
three types have the same three major parts:

A Diaphragm—The sound waves strike the diaphragm, causing it
to vibrate in sympathy with the sound wave. In order to accurately
reproduce high frequency sounds, it must be as light as possible.

A Transducer—The mechanical vibrations of the diaphragm are
converted into an electronic signal by the transducer.

A Casing—As well as providing mechanical support and protection
for the diaphragm and transducer, the casing can also be
made to help control the directional response of the microphone.

Let’s take a look at the three types of microphones.

The Dynamic Microphone

The dynamic microphone is the workhorse of the microphone
breed. Ranging from really inexpensive to moderately expensive,
there’s a dynamic model to fit just about any application.

In a moving coil (or more commonly called “dynamic”) microphone,
sound waves cause movement of a thin metallic diaphragm
and an attached coil of wire that is located inside a permanent
magnet. When sound waves make the diaphragm vibrate, the
connected coils also vibrate in the magnetic fi eld, causing current
to fl ow. Since the current is produced by the motion of the diaphragm
and the amount of current is determined by the speed of
that motion, this kind of microphone is known as velocity sensitive
(see Figure 1).

figure 1

The ability of the microphone to respond to transients and
higher frequency signals is dependant upon how heavy the moving
parts are. In this type of microphone, both the diaphragm and
the coil move, so that means it’s relatively heavy. As a result, the
frequency response falls off above about 10kHz.

The microphone also has a resonant frequency (a frequency or
group of frequencies that is emphasized) that is typically somewhere
from about 1 to 4kHz. This resonant response is sometimes
called the presence peak, since it occurs in the frequency region
that directly affects voice intelligibility. Because of this natural
effect, dynamic microphones are often preferred by vocalists,
especially in sound reinforcement.

These microphones tend to be expensive because they’re somewhat
complex to manufacture, but they’re generally very robust
(you can actually hammer nails with some of them—and they’ll
still work!) and insensitive to changes in humidity.
Robust and durable, can be relatively inexpensive, insensitive to
changes in humidity, need no external or internal power to operate,
can be made fairly small.
Resonant peak in the frequency response, typically weak highfrequency
response beyond 10kHz.

The Ribbon Microphone
How this microphone works
The ribbon microphone operates almost the same as the moving
coil microphone. The major difference is that the transducer is
a strip of extremely thin aluminum foil wide enough and light
enough to be vibrated directly by the moving molecules of air of
the sound wave, so no separate diaphragm is necessary. However,
the electrical signal generated is very small compared to a moving
coil microphone, so an output transformer is needed to boost the
signal to a usable level. (See Figures 2 and 2A)

figure 2

figure 2A

Like the dynamic microphone, the high frequency response
is governed by the mass of the moving parts. But because the
diaphragm is also the transducer, the mass is usually a lot less than
a dynamic type. As a result, the upper frequency response tends
to reach slightly higher, to around 14kHz. The frequency response
is also generally fl atter than for a moving coil microphone.
All good studio ribbon mics provide more opportunity to
EQ to taste since they “take” EQ well. Ribbon mics have their
resonance peak at the bottom of their frequency range, which
means that a ribbon just doesn’t add any extra high frequency
hype like condenser mics do.

Relatively flat frequency response, extended high frequency response
as compared to dynamics, needs no external or internal
power to operate.

Fragile—requires care during operation and handling, moderately

The Condenser Microphone
How this microphone works
The condenser microphone has two electrically charged plates:
one that can move, which acts as a diaphragm, and one that is
fixed, called a backplate. This is, in effect, a capacitor (or “condenser”)
with a positively and negatively charged electrode and
an air space in between. Sound depresses the diaphragm, causing
a change in the spacing between it and the backplate. This change
in capacitance and distance between it and the back plate cause a
change in voltage potential that can be amplifi ed to a usable level.
To boost this small voltage, a vacuum tube or FET transistors are
used as an amplifi er. This is why a battery or phantom power is
needed to charge the plates and also to run the preamp. Because
the voltage requirements to power a vacuum tube are so high and
therefore require some large and heavy components, some microphones
have the power supply in a separate outboard box. (See
Figure 3)

A condenser has an omnidirectional pickup pattern in its native
state. In order to make it directional, little holes are punched in the
backplate. The object of the holes is to delay the arrival of sound at
the rear of the diaphragm to coincide with the same sound at the
front, which then cancels the sound out. The size and position of
the holes determine the frequencies that will be cancelled.
Most large diaphragm condensers are multi-pattern microphones.
This design is comprised of a single backplate placed
between two diaphragms. By varying how much signal from each
diaphragm is fed to the preamp, the microphone can have selectable
patterns ranging from a tight cardioid to a fi gure-8 to full
Condenser mics, however, always ring (resonate) a bit, typically
in the 8 to 12kHz range. A condenser mic’s pattern of resonances
is a major part of its character. Their built-in top end response
bump limits the EQ you might want to add, since a little bit of high
frequency boost can start to sound a bit “edgy” rather quickly.

Excellent high frequency and upper harmonic response, can have
excellent low frequency response.
Moderate to very expensive, requires external powering, can be
relatively bulky; low cost (and some expensive) models can suffer
from poor or inconsistent frequency response, two mics of the
same model may sound quite different, humidity and temperature
affect performance.
How a microphone works has been brought to you by computer music man

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