Figure 7-2.--Dynamic microphone.

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.

amplifier.

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

diaphragm.

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,

diaphragm.

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.

7-4