1-37
Microphones
If an rf carrier is to convey intelligence, some feature of the carrier must be varied in accordance
with the information to be transmitted. In the case of speech intelligence, sound waves must be converted
to electrical energy.
A MICROPHONE is an energy converter that changes sound energy into electrical energy. A
diaphragm in the microphone moves in and out in accordance with the compression and rarefaction of the
atmosphere caused by sound waves. The diaphragm is connected to a device that causes current flow in
proportion to the instantaneous pressure delivered to it. Many devices can perform this function. The
particular device used in a given application depends on the characteristics desired, such as sensitivity,
frequency response, impedance matching, power requirements, and ruggedness.
The SENSITIVITY or EFFICIENCY of a microphone is usually expressed in terms of the electrical
power level which the microphone delivers to a matched-impedance load compared to the sound level
being converted. The sensitivity is rated in dB and must be as high as possible. A high microphone output
requires less gain in the amplifiers used with the microphone. This keeps the effects of thermal noise,
amplifier hum, and noise pickup at a minimum.
For good quality sound reproduction, the electrical signal from the microphone must correspond in
frequency content to the original sound waves. The microphone response should be uniform, or flat,
within its frequency range and free from the electrical or mechanical generation of new frequencies.
The impedance of a microphone is important in that it must be matched to the microphone cable and
to the amplifier input as well as to the amplifier input load. Exact impedance matching is not always
possible, especially in the case where the impedance of the microphone increases with an increase in
frequency. A long microphone cable tends to seriously attenuate the high frequencies if the microphone
impedance is high. This attenuation is caused by the increased capacitive action of the line at higher
frequencies. If the microphone has a low impedance, a lower voltage is developed in the microphone, and
more voltage is available at the load. Because many microphone lines used aboard ship are long, low-
impedance microphones must be used to preserve a sufficiently high voltage level- over the required
frequency range.
The symbol used to represent a microphone in a schematic diagram is shown in figure 1-32. The
schematic symbol identifies neither the type of microphone used nor its characteristics.
Figure 1-32.—Microphone schematic symbol.
CARBON MICROPHONE.—Operation of the SINGLE-BUTTON CARBON MICROPHONE
figure 1-33, view (A) is based on varying the resistance of a pile of carbon granules located within the
microphone. An insulated cup, referred to as the button, holds the loosely piled granules. It is so mounted
that it is in constant contact with the thin metal diaphragm. Sound waves striking the diaphragm vary the
pressure on the button which varies the pressure on the pile of carbon granules. The dc resistance of the
carbon granule pile is varied by this pressure. This varying resistance is in series with a battery and the
