Handbook for Sound Engineers

Microphones 495

smallest diaphragm possible versus the lowest
signal-to-noise ratio, SNR, or put another way, the
smaller the diaphragm, the lower the microphone sensi-
tivity, therefore, the poorer the SNR.
Omnidirectional microphones have very little prox-
imity effect. See Section 16.2.3.1 for a discussion on
proximity effect.
Because the pickup pattern is spherical, the random
energy efficiency is 100%, and the ratio of front
response to back or side is 1:1, therefore signals from
the sides or rear will have the same pickup sensitivity as
from the front, giving a directivity index of 0 dB. This
can be helpful in picking up wanted room characteris-
tics or conversations around a table as when recording a
symphony. However, it can be detrimental when in a
noisy environment.
Omnidirectional microphones are relatively free
from mechanical shock because the output at all
frequencies is high; therefore, the diaphragm can be
stiff. This allows the diaphragm to follow the magnet or
stationary system it operates against when subjected to
mechanical motion (see Section 16.3.3).

16.2.2 Bidirectional Microphones

A bidirectional microphone is one that picks up from
the front and back equally well with little or no pickup
from the sides. The field pattern, Fig. 16-5, is called a
figure eight.
Because the microphone discriminates between the
front, back, and sides, random energy efficiency is 33%.
In other words, background noise, if it is in a rever-
berant field, will be 67% lower than with an omnidirec-
tional microphone. The front-to-back response will still
remain one; however, the front-to-side response will
approach infinity, producing a directivity index of 4.8.
This can be extremely useful when picking up two
conversations on opposite sides of a table. Because of
the increased directional capabilities of the micro-
phone, pickup distance is 1.7 times greater before feed-
back in the direct field than for an omnidirectional
microphone. The included pickup cone angle shown in
Fig. 16-6 for 6 dB attenuation on a perfect bidirectional
microphone is 120° off the front of the microphone and
120° off the rear of the microphone. Because of diffrac-
tion, this angle varies with frequency, becoming
narrower as the frequency increases.

16.2.3 Unidirectional Microphones

Unidirectional microphones have a greater sensitivity to
sound pickup from the front than any other direction.

The average unidirectional microphone has a

front-to-back ratio of 20–30 dB; that is, it has 20–30 dB

greater sensitivity to sound waves approaching from the

front than from the rear.

Unidirectional microphones are usually listed as

cardioid or directional, Fig. 16-7, supercardioid, Fig.

16-8, or hypercardioid, Fig. 16-9. The pickup pattern is

called cardioid because it is heart shaped. Unidirec-

tional microphones are the most commonly used micro-

Figure 16-5. Bidirectional pickup pattern. Courtesy

Sennheiser Electronic Corporation.

Figure 16-6. Polar pattern of a typical bidirectional ribbon

velocity microphone showing the narrowing pattern at high

frequencies.

Directional

characteristic

of a typical velocity

microphone

Mic

Decibels