Handbook for Sound Engineers

506 Chapter 16

does illustrate that even with a “perfect,” massless,
moving-coil microphone, “perfect” electrical wave-
forms will not be produced.

When sound waves vibrate the diaphragm, the voice
coil has a voltage induced in it proportional to the
magnitude and at the frequency of the vibrations. The
voice coil and diaphragm have some finite mass and
any mass has inertia that causes it to want to stay in the
condition it is in—namely, in motion or at rest. If the
stationary part of the diaphragm-magnet structure is
moved in space, the inertia of the diaphragm and coil
causes them to try to remain fixed in space. Therefore,
there will be relative motion between the two parts with
a resultant electrical output. An electrical output can be

obtained in two ways, by motion of the diaphragm from

airborne acoustical energy or by motion of the magnet

circuit by structure-borne vibration. The diaphragm

motion is the desired output, while the structure-borne

vibration is undesired.

Several things may be tried to eliminate the unde-

sired output. The mass of the diaphragm and voice coil

may be reduced, but there are practical limits, or the

frequency response may be limited mechanically with

stiffer diaphragms or electronically with filter circuits.

However limited response makes the microphone

unsuitable for broad-range applications.

16.3.3.1 Unidirectional Microphones

To reject unwanted acoustical noise such as signals

emanating from the sides or rear of the microphone,

unidirectional microphones are used, Fig. 16-7. Unidi-

rectional microphones are much more sensitive to vibra-

tion relative to their acoustic sensitivity than

omnidirectional types. Fig. 16-37 shows a plot of vibra-

tion sensitivity versus frequency for a typical omnidi-

rectional and unidirectional microphone with the levels

normalized with respect to acoustical sensitivity.

The vibration sensitivity of the unidirectional micro-

phone is about 15 dB higher than the omnidirectional

and has a peak at about 150 Hz. The peak gives a clue to

help explain the difference.

Figure 16-35. Effect of a varying sound pressure on a
moving-coil microphone.

a-b

c-d-e

f-g

a

b c

d

e f

g

a

b

c

d

e

a

b

c

d

e

A. Acoustical sine

waveform.

B. Electrical sine

waveform.

C. Acoustical square

waveform.

D. Electrical square

waveform.

Figure 16-36. Effect of a transient condition on a

moving-coil microphone.

a

a'

b c e d e a

a'

b d

A. Acoustical

Waveform.

c

B. Electrical

waveform.