Handbook for Sound Engineers

494 Chapter 16

16.2.1 Omnidirectional Microphones

The omnidirectional, or spherical, polar response of the
pressure microphones, Fig. 16-3 is created because the
diaphragm is only exposed to the acoustic wave on the
front side. Therefore, no cancellations are produced by
having sound waves hitting both the front and rear of
the diaphragm at the same time.

Omnidirectional microphones become increasingly
directional as the diameter of the microphone reaches

the wavelength of the frequency in question, as shown

in Fig. 16-4; therefore, the microphone should have the

smallest diameter possible if omnidirectional character-

istics are required at high frequencies. The characteristic

that allows waves to bend around objects is known as

diffraction and happens when the wavelength is long

compared to the size of the object. As the wavelength

approaches the size of the object, the wave cannot bend

sharply enough and, therefore, passes by the object. The

various responses start to diverge at the frequency at

which the diameter of the diaphragm of the microphone,

D, is approximately one-tenth the wavelength, O, of the

sound arriving as equation

(16-1)

The frequency, f, at which the variation begins is

(16-2)

where,

v is the velocity of sound in feet per second, or meters

per second,

D is the diameter of the diaphragm in feet or meters.

For example, a ½ inch (1.27 cm) microphone will

begin to vary from omnidirectional, though only

slightly, at

and will be down approximately 3 dB at 10,000 Hz.

Omnidirectional microphones are capable of having

a very flat, smooth frequency response over the entire

audio spectrum because only the front of the diaphragm

is exposed to the source, eliminating phase cancellations

found in unidirectional microphones.

For smoothness of response the smaller they are, the

better. The problem usually revolves around the

Figure 16-2. Air particle motion in a sound field, showing
relationship to velocity and acceleration.

Figure 16-3. Omnidirectional pickup pattern. Courtesy
Shure Incorporated.

a

b

c

d

Particle

displacement

Particle

velocity

Particle

acceleration

o

a = displacement 0

b = displacement max

c = displacement 0

d = displacement max

e = displacement 0

a = velocity max

b = velocity 0

c = velocity max

d = velocity 0

e = velocity max

a = acceleration 0

b = acceleration max

c = acceleration 0

d = acceleration max

e = acceleration 0

e

o

270 o

o

Figure 16-4. High-frequency directivity of an omnidirec-

tional microphone.

D O

10

----- -=

f v

10 D

=----------

Sealed microphone

10 kHz

1 kHz

1 kHz

10 kHz

f^1130

10 0.5

12

u------ -

©¹

§·

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

=2712 Hz