a. Crystal microphones can be divided into two types--the diaphragm type and the sound cell
type. The crystal element used in either type can be permanently damaged by high temperatures. This
limits the number of useful applications. However, the crystal microphone is still widely used as a high-
quality microphone for communications, both military and commercial. Figure 2-4A is a diagram
showing the bimorph crystal unit; sound waves striking the diaphragm cause the diaphragm to vibrate.
These vibrations are transferred to the surface of the crystal by means of the connecting pin. The
fidelity of this type instrument is approximately equal to that of most double-button carbon
microphones; however, the frequency response extends to a much higher range. In the crystal
microphone, there is no background hiss or noise generated in the microphone itself. However, noise
pickup on cables which are longer than 30 feet does limit the use of crystal microphones in television.
b. The sound cell is another type of crystal microphone, as shown in Figure 2-4B. The back-to-
back crystal elements are enclosed within a rectangular bakelite frame sealed by two flexible
membranes. No diaphragms are required in a sound cell microphone, because the membrane imparts the
sound pressure directly to the crystals, which produce the resultant AC voltage.
Figure 2-4. Crystal microphone (A) cross sectional view of diaphragm and crystal and (B) cross
sectional view of single sound cell unit
7.
Capacitor microphone. A capacitor microphone generally consists of two electrodes separated
by a very thin dielectric, usually air. One electrode is the diaphragm, while the other is a rigid plate
which has the same area as the diaphragm. The diaphragm motion changes the spacing between the two
electrodes, varying the capacitance.
a. If a DC voltage is applied across the combination, the changes in spacing produce changes in
the capacitor charge which can be obtained as an AC voltage.
b. This device has a very linear pressure response and a wide frequency response and is
relatively insensitive to mechanical noise because of its stiffness of construction.
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