Handbook for Sound Engineers

Microphones 539

16.6.12.1 Distance Factor

The fact that microphone directivity determines the

perceived distance can be explained from the viewpoint

of the distance factor. Fig. 16-103 shows the sound

pressure level at the position of an omnidirectional

microphone versus the distance between the micro-

phone and a sound source S, with ambient noise evenly

distributed. Suppose the distance is 23 ft (7 m) and the

ambient noise level is at 1. If the microphone is replaced

by one that has a narrow directivity with the same

on-axis sensitivity, less noise is picked up, so the

observed noise level is lowered to 2. For an omnidirec-

tional microphone, the same effect can be obtained at a

distance of 7.5 ft (2.3 m). From a different standpoint,

the same SNR as for an omnidirectional microphone at

20.6 ft (6.3 m) can be obtained at a distance of 65 ft

(20 m). The ratio of actual-to-observed distance is

called the distance factor.

16.6.12.2 Operation of Zoom Microphones

By changing the sensitivity and directivity of a micro-

phone simultaneously, an acoustical zoom effect is real-

ized, and more reality becomes possible in sound

recording. Fig. 16-104 is the basic block diagram of a

zoom microphone system. The system consists of three

unidirectional microphone capsules (1 through 3)

arranged on the same axis. The three capsules have the

same characteristics, and capsule 3 faces the opposite

direction. The directivity can be varied from omnidirec-

tional to second-order gradient unidirectional by

varying the mixing ratio of the output of each capsule

and changing the equalization characteristic accord-

ingly. An omnidirectional pattern is obtained by simply

combining the outputs of capsule 2 and 3. In the process

of directivity change from omnidirectional to unidirec-

tional, the output of capsule 3 is gradually faded out,

while the output of capsule 1 is kept off. Furthermore,

the equalization characteristic is kept flat, because the

on-axis frequency response does not change during this

process. In the process of changing from unidirectional

to second-order gradient unidirectional, the output of

capsule 3 is kept off. The second-order gradient unidi-

rectional pattern is obtained by subtracting the output of

capsule 1 from the output of capsule 2. To obtain the

second-order gradient unidirectional pattern with

minimum error, the output level of capsule 1 needs to be

trimmed. Since the on-axis response varies according to

the mixing ratio, the equalization characteristics also

have to be adjusted along with the level adjustment of

the output of capsule 1. The on-axis sensitivity increase

of second-order gradient setup over the unidirectional

setup allows the gain of the amplification to be

unchanged.

Figure 16-103. Relationship between sound pressure level
and distance in an evenly distributed noise environment.

Sound-pressure level—dB

10

0

10

20

30

1 2 3 64  2010 40

Distance—m

Direct sound

Noise

Noise

Figure 16-104. Configuration of the zoom microphone.

Sound

source

#1 mic

#2 mic

#3 mic

Phase compensator

VR 1

+

3

(A)

123

VR 2

EQ

(B)

(^1) (C)
(D)
Out
C. D.
(^21)
dB^3
+6
0
+6
dB
1 2 3
Level
+12
0
Level
f
Hz
f
Hz
f
Hz
Level B.
dB
0
1 2
3
f
Hz
Phase A.
0 o

180 o

90 o
fc
D
2