Handbook for Sound Engineers

500 Chapter 16

One sound opening, which is exposed to the sound
pressure p 1 , is represented by the front surface of the
diaphragm. The other sound opening, or rear entry,
consists of a number of windows in the side of the trans-
ducer housing where the sound pressure p 2 prevails. The
diaphragm has an acoustical impedance Z 0 , which also
includes the impedance of the thin air film between the
diaphragm and backplate. The sound pressure p 2 exerts

its influence on the rear surface of the diaphragm via a

screen mounted in the side windows of the transducer

housing, having a resistance R 1 and inertance L 1 ,

through the cavity V 1 with compliance C 1. A second

screen has a resistance R 2 and inertance L 2 , through a

second cavity V 2 with compliance C 2 , and finally

through the perforations in the backplate.

The combination of circuit elements L 1 , R 1 , C 1 , L 2 ,

R 2 , C 2 forms a ladder network with lossy inertances, and

is called a lossy ladder network. The transfer character-

istic of this network enforces a time delay on the pres-

sure p 2 imparting directional (cardioid) characteristics

for low and medium frequencies. At high frequencies

the attenuation caused by the network is large, and the

resulting pressure arriving at the back of the diaphragm

due to p 2 is small. The microphone then operates much

like an omnidirectional system under the predominant

influence of p 1. At these frequencies directional charac-

teristics are attained by diffraction of the sound around a

suitably shaped transducer housing.

A rotary low-frequency response shaping switch

allows the user to select between a flat and a

6 dB/octave roll-off at 100 Hz or an 18 dB/octave cutoff

at 80 Hz. The 100 Hz roll-off compensates for the prox-

imity effect associated with a 6 in (15 cm) source to

microphone distance, while the 80 Hz cutoff signifi-

cantly reduces most low-frequency disturbances with

minimal effect on voice material. In the flat position the

microphone has a 6 dB/octave electronic infrasonic

roll-off, with 3 dB at 10 Hz to reduce the effects of

inaudible low-frequency disturbances on microphone

preamplifier inputs. Attenuation is provided for opera-

tion at high sound pressure levels (to 145 dB SPL) by

means of a rotary capacitive switch (see Section

16.3.4.1).

A final example of a single entry cardioid micro-

phone is the Shure Beta 57 supercardioid dynamic

microphone, Fig.16-24. Both the Shure Beta 57 and the

Beta 58 use neodymium magnets for hotter output and

incorporate an improved shock mount.

16.2.3.1.4 Three-Entry Cardioid Microphones

The Sennheiser MD441 is an example of a three-entry

cardioid microphone, Fig. 16-25. The low-frequency

rear entry has a d (distance from center of diaphragm to

the entry port) of about 2.8 inches (7 cm), the

mid-frequency entry d is about 2.2 inches (5.6 cm) and

the high-frequency entry d is about 1.5 inches (3.8 cm),

with the transition in frequency occurring between

800 Hz and 1 kHz. Each entry consists of several holes

around the microphone case rather than a single hole.

Figure 16-21. Frequency response versus distance for an
Electro-Voice DS35 single-entrant cardioid microphone.
Courtesy Electro-Voice, Inc.

Figure 16-22. Simplified cross-sectional view of the Shure
SM81 condenser transducer. Courtesy Shure Incorporated.

Figure 16-23. Electrical equivalent circuit of the Shure
SM81 condenser transducer and preamplifier. Courtesy
Shure Incorporated.

Frequency–Hz

Response–dB

20 100 200 500 1k 2k 5k 10k 20k

Blackplate V 1

Air film

R 2 L 2

R 1 L 1

V 2 P 2

P 1

Diagram

Spacer Transducer

housing

ET

Transducer Connector Preamplifier

CP C 1 EP

CA

CP RP