Handbook for Sound Engineers

532 Chapter 16

Fig. 16-89. This microphone is used for close micing
where high SPLs are commonly encountered. The
response below 30 Hz is rolled off to prevent
low-frequency blocking and can be switched to 200 Hz
to allow compensation for the bass rise common to all
directional microphones at close range.
The figure-eight characteristic is produced by means
of two closely spaced or assembled cardioid character-
istic capsules, whose principal axes are pointed in oppo-
site directions and are electrically connected in
antiphase.
These microphones are usually made with backplates
equipped with holes, slots, and chambers forming delay
elements whose perforations act as part friction resis-
tances and part energy storage (acoustic inductances
and capacitances), giving the backplate the character of
an acoustic low-pass network. In the cutoff range of this
low-pass network, above the transition frequency ft, the
membrane is impinged upon only from the front, and
the microphone capsule changes to a pressure or inter-
ference transducer.
The output voltage e(t) of a condenser microphone
using dc polarization is proportional to the applied dc
voltage Eo and, for small displacement amplitudes of
the diaphragm, to the relative variation in capacity
c(t)/Co caused by the sound pressure

(16-25)

where,
Eo is the applied dc voltage,
c(t) is the variable component of capsule capacity,
Co is the capsule capacity in the absence of sound pres-
sure,
t is the time.

The dependence of output voltage e(t) on Eo is

utilized in some types of microphones to control the

directional characteristic. Two capsules with cardioid

characteristics as shown in Fig. 16-90 are placed back to

back. They can also be assembled as a unit with a

common backplate. The audio (ac) signals provided by

the two diaphragms are connected in parallel through a

capacitor C. The intensity and phase relationship of the

outputs from the two capsule halves can be affected by

varying the dc voltage applied to one of them (the left

cartridge in Fig. 16-90). This can be accomplished

through a switch, or a potentiometer. The directional

characteristic of the microphone may be changed by

remote control via long extension cables.

If the switch is in its center position C, then the left

capsule-half does not contribute any voltage, and the

microphone has the cardioid characteristic of the right

capsule-half. In switch position A, the two ac voltages

are in parallel, resulting in an omnidirectional pattern.

In position E the two halves are connected in antiphase,

and the result is a figure-8 directional response pattern.

The letters A to E given for the switch positions in

Fig. 16-90 produce the patterns given the same letters in

Fig. 16-91.

16.6.9 Interference Tube Microphone

The interference tube microphone^8 as described by Olson

in 1938 is often called a shotgun microphone because of

its physical shape and directional characteristics.

Important characteristics of any microphone are its

sensitivity and directional qualities. Assuming a

constant sound pressure source, increasing the micro-

e(t) Eoct^

Co

= ---------

Figure 16-87. Cut-away view of a Sennheiser MD 431 handheld entertainment microphone. Courtesy Sennheiser Electronic
Corporation.

Three-layer

stainless-steel gauze

Dynamic transducer

capsule

Rigid outer

stainless-steel

Hum compensation coil

Shock absorber

Metal springs

Suspension system

Outer switch housing

Handle

Magnet

Switch button Switch locking device

Filter components Magnetic shield

Connector pins

Rim for quick-release

stand clip

Rear sound inlet