Quantcast Figure 7-2.--Dynamic microphone.

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coal, known as the piezoelectric effect. The bending of
When a rarefaction wave strikes the diaphragm, the
armature is deflected to the left. The flux path now is
the crystal resulting from the pressure of the sound wave
directed from the north pole of the magnet, up through
produces an electromotive force (emf) across the faces
the armature through the reduced gap at the upper left,
of the crystal. This emf is applied to the input of an
and back to the south pole.
Thus, the vibrations of the diaphragm cause an
The crystal microphone (fig. 7-3) consists of a
alternating flux in the armature. The alternating flux cuts
diaphragm that is cemented to one surface of the crystal.
the stationary coil wound around the armature and
Metal plates, or electrodes, are attached to the other
induces an alternating voltage (approximately 10
surface of the crystal. When sound waves strike the
millivolts at a 150-ohm load) in it. This voltage has the
same waveform as the sound waves striking the
diaphragm, the vibration of the diaphragm produces a
varying pressure on the surface of the crystal and
induces an emf across the electrodes. The emf has the
The magnetic microphone is the type most widely
same waveform as the sound waves striking the
used in shipboard amounting and intercommunicating
systems because it is more resistant to vibration, shock,
and rough handling.
Rochelle salt is most commonly used in crystal
Dynamic Microphone
microphones because of its relatively high-voltage
output. The crystal microphone can produce an output
The dynamic, or moving-coil, microphone (fig. 7-2)
voltage of from 0.01 to 0.03 volt into a load of 1 megohm
consists of a coil of wire attached to a diaphragm, and a
or more, when subjected to a sound pressure of a normal
radial magnetic field in which the coil is free to vibrate.
tone within a few inches of the crystal. However, this
Sound waves impinging on the diaphragm cause the
crystal microphone is seldom used in naval announcing
diaphragm to vibrate. This vibration moves the voice
and intercommunicating systems because of the
coil through the magnetic field so that the turns cut the
lines of force in the field. This action generates a voltage
sensitivity of the crystal element to high temperature,
in the coil that has the same waveform as the sound
humidity, and rough handling.
waves striking the diaphragm.
The dynamic microphone requires no external
voltage source. It has good fidelity and produces an
output volt age of about 0.05 volt when spoken into in a
normal tone within a few inches of the diaphragm.
Crystal Microphone
The crystal microphone utilizes a property of
certain crystals, such as quartz, Rochelle salt, sugar, or
Figure 7-3.--Crystal microphone.
Figure 7-2.--Dynamic microphone.

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