Handbook for Sound Engineers

Microphones 497

diaphragm it is some angle between 0° and 180°, as

shown in Fig. 16-11B. If the rear pressure was at

0°, the output would be 0. It would be ideal if the

rear pressure were at 180° so that it could add to

the input, doubling the output.

The phase inversion is caused by the extra distance
the wave has to travel to reach the back of the
diaphragm. When the wave is coming from the rear of
the microphone, it hits the front and back of the
diaphragm at the same time and with the same polarity,
therefore canceling the output.

The frequency response of cardioid microphones is
usually rougher than an omnidirectional microphone
due to the acoustical impedance path and its effects on
the front wave response. The front and rear responses of
a cardioid microphone are not the same. Although the
front pattern may be essentially flat over the audio spec-
trum, the back response usually increases at low and
high frequencies, as shown in Fig. 16-12.

Discrimination between the front and back response
is between 15 and 30 dB in the mid frequencies and
could be as little as 5–10 dB at the extreme ends, as
shown in Fig. 16-12.

16.2.3.1 Proximity Effects

As the source is moved closer to the diaphragm, the

low-frequency response increases due to the proximity

effect, Fig. 16-13. The proximity effect^1 is created

because at close source-to-microphone distance, the

magnitude of the sound pressure on the front is appre-

ciably greater than the sound pressure on the rear. In the

vector diagram shown in Fig. 16-14A, the sound source

was a distance greater than 2 ft from the microphone.

The angle 2KD is found from D, which is the acoustic

distance from the front to the rear of the diaphragm and

K=2S/O. Fig. 16-14B shows the vector diagram when

used less than 4 inches to the sound source.

Figure 16-11. Cardioid microphone employing acoustical

delay.

Output

Front

A. Ideal

Front Output

Rear

B. Normal

Rear

Acoustical delay Figure 16-12. Frequency response of a typical cardioid

microphone.

Figure 16-13. Proximity effect variations in response with

distance between source and microphone for cardioid

microphones. Courtesy Telex Electro-Voice.

Figure 16-14. Vector diagram of a unidirectional micro-

phone. Courtesy Telex Electro-Voice.

+10

0



10



20



30

(^4050100500) 1K
Front
Rear
5K10K20K
dB
Frequency–Hz
Relative response–dB
+5
0

5
+5
0

5
20 50 100 1K
Microphone A
Microphone B
Frequency–Hz
Fg
2 KD
F 0
(^0) F 1
A. With the sound source at a distance
from the microphone.
Fg
F 2
F 0
0
F 0
F 1
B. With the sound source close to the microphone.