Handbook for Sound Engineers

Microphones 507

Unidirectional microphones are usually differential
microphones; that is, the diaphragm responds to a pres-
sure differential between its front and back surfaces.
The oncoming sound wave is not only allowed to reach
the front of a diaphragm but, through one or more open-
ings and appropriate acoustical phase-shift networks,
reaches the rear of the diaphragm. At low frequencies,
the net instantaneous pressure differential causing the
diaphragm to move is small compared to the absolute
sound pressure, Fig. 16-38. Curve A is the pressure
wave that arrives at the front of the diaphragm. Curve B
is the pressure wave that reaches the rear of the
diaphragm after a slight delay due to the greater
distance the sound had to travel to reach the rear entry
and some additional phase shift it encounters after
entering. The net pressure actuating the diaphragm is
curve C, which is the instantaneous difference between
the two upper curves. In a typical unidirectional micro-
phone, the differential pressure at 100 Hz will be about
one-tenth of the absolute pressure or 20 dB down from
the pressure an omnidirectional microphone would
experience.

To obtain good low-frequency response, a reason-
able low-frequency electrical output is required from a

unidirectional microphone. To accomplish this, the

diaphragm must move more easily for a given sound

pressure. Some of this is accomplished by reducing the

damping resistance to less than one-tenth used in an

omnidirectional microphone. This reduction in damping

increases the motion of the mechanical resonant

frequency of the diaphragm and voice coil, around

150 Hz in Fig. 16-37, making the microphone much

more acceptable to structure-borne vibrations. Since the

diaphragm of an omnidirectional microphone is much

more heavily damped, it will respond less to inertial or

mechanical vibration forces.

To eliminate unwanted external low-frequency noise

from effecting a unidirectional microphone, some kind

of isolation such as a microphone shock mount is

required to prevent the microphone cartridge from expe-

riencing mechanical shock and vibration.

16.3.4 Capacitor Microphones

In a capacitor or condenser microphone the sound pres-

sure level varies the head capacitance of the microphone

by deflecting one or two plates of the capacitor, causing

an electrical signal that varies with the acoustical signal.

The varying capacitance can be used to modulate an RF

signal that is later demodulated or can be used as one

leg of a voltage divider, Fig. 16-39, where R and C form

the voltage divider of the power supply ++ to .

The head of most capacitor microphones consists of

a small two-plate 40–50 pF capacitor. One of the two

plates is a stretched diaphragm; the other is a heavy

back plate or center terminal, Fig. 16-39. The back plate

is insulated from the diaphragm and spaced approxi-

mately 1 mil (0.001 in) from, and parallel to, the rear

surface of the diaphragm. Mathematically the output

from the head may be calculated as

(16-3)

where,

Figure 16-37. Vibration sensitivity of microphone cartridge.

Figure 16-38. Differential pressure at low frequencies on
unidirectional microphones.

0 dB = 0.001 V for l g.

30

20

10

0

10

Relative response–dB

Frequency–Hz k 20k

Unidirectional

Omnidirectional

Relative pressure amplitude

10" A

B

C

Figure 16-39. Voltage divider type of capacitor

microphone.

C

R

+

++

EO

Epa^2 P

8 dt

= ---------------