Handbook for Sound Engineers

Microphones 499

microphone will seriously degrade the effective

high-frequency discrimination. An example of such

degradation can be seen in Fig. 16-18 where a head

object was placed 2 in (5 cm) in front of the micro-

phone. (The two curves have not been normalized.) This

sort of performance results from the head as a reflector

and is a common cause of feedback as one approaches a

microphone. This should not be considered as a short-

coming of the microphone, but rather as an unavoidable

result of the sound field in which it is being used. At

180°, for example, the microphone sees, in addition to

the source it is trying to reject, a reflection of that

source some 2 in (5 cm) in front of its diaphragm. This

phenomenon is greatly reduced at low frequencies

because the head is no longer an appreciable obstacle to

the sound field. It is thus clear that the effective discrim-

ination of any unidirectional microphone is greatly

influenced by the sound field in which it is used.

16.2.3.1.2 Types of Cardioid Microphones

Cardioid microphones are named by the way sound

enters the rear cavity. The sound normally enters the

rear of the microphone’s cavity through single or

multiple holes in the microphone housing, as shown in

Fig. 16-19.

16.2.3.1.3 Single-Entry Cardioid Microphones

All single-entrant cardioid microphones have the rear

entrance port located at one distance from the rear of the

diaphragm. The port location is usually within 1½ in

(3.8 cm) of the diaphragm and can cause a large prox-

imity effect. The Electro-Voice DS35 is an example of a

single-entrant cardioid microphone, Fig. 16-20.

The low-frequency response of the DS35 varies as

the distance from the sound source to the microphone

decreases, Fig. 16-21. Maximum bass response is

produced in close-up use with the microphone 1½ in

(3.8 cm) from the sound source. Minimum bass

response is experienced at distances greater than 2 ft

(0.6 m). Useful effects can be created by imaginative

application of the variable low-frequency response.

Another single-entrant microphone is the Shure

SM-81.^3 The acoustical system of the microphone oper-

ates as a first-order gradient microphone with two sound

openings, as shown in Fig. 16-22. Fig. 16-23 shows a

simplified cross-sectional view of the transducer, and

Fig. 16-23 indicates the corresponding electrical analog

circuit of the transducer and preamplifier.

Figure 16-17. An example of rapid variations in

high-frequency polar response for single-frequency

excitation. Courtesy Shure Incorporated.

Figure 16-18. An example of a head obstacle on a polar
response. Courtesy Shure Incorporated.

0 40 80 120 160 200 240 280 320 360

0

10

20

30

Angular position–degrees

Relative response–dB

3130 Hz

3150 Hz

0 40 80 120 160 200 240 280 320 360

0

10

20

30

Angular Position–Degrees

Relative response–dB

Figure 16-19. Three types of cardioid microphones.

Figure 16-20. Electro-Voice DS35 single-entrant micro-

phone. Courtesy Electro-Voice, Inc.

Source

Front Rear

C. Multiple-entry microphone.

A. Single-entry microphone.

B. Three-entry microphone.