Figure 7-4.--Schematic diagram of a carbon microphone.

secondary turns and also upon the change in primary

Carbon Microphone

current. Normal output voltage of a typical circuit is

from 3 to 10 volts peak at the secondary terminals.

The carbon microphone (fig. 7-4) operates on the

principle that a changing pressure of a diaphragm

The carbon microphone is not used in shipboard

applied to a small volume of carbon granules changes

announcing equipment because it requires a polarizing

its electrical resistance according to the vibrations of the

current and has a tendency to amplify certain

sound waves striking the diaphragm.

frequencies more than others.

The carbon microphone consists of a diaphragm

Characteristics of Microphones

mounted against a mass of carbon granules that are

contained in a small cup. To produce an output voltage,

Microphones are rated according to their (1)

this microphone is connected in a series circuit

frequency response, (2) impedance, and (3) sensitivity.

containing a battery and the primary of a transformer.

FREQUENCY RESPONSE.-- Shipboard an-

When a direct current follows through the carbon

nouncing and intercommunicating systems are designed

granules, the varying resistance changes the amplitude

to produce maximum speech intelligibility under

of the current and produces an alternating voltage in the

conditions of high background noise. To achieve this

secondary of the transformer. This voltage has the same

objective, the overall frequency response characteristic

waveform as the sound waves striking the diaphragm.

of the system is altered by cutting off the system

The current through this microphone may be as great as

response at some lower limit, such as 500 Hz, and by

0.1 ampere. The resistance may vary from about 50 to

employing an emphasized frequency response

90 ohms. The voltage developed across the secondary

characteristic that rises with increasing frequency at a

depends upon the ratio of the transformer primary and

rate of approximately 6 dB per octave. The output sound

pressure is doubled each time the frequency is doubled

for a constant level input to the system. The emphasized

speech tends to sound thin and sometimes harsh, but

when the masking due to background noise is almost as

high as the speech level, the speech appears to cut

through the noise.

For good quality, a microphone must convert sound

waves into electrical waves that have the same relative

magnitude and frequency without introducing any new

frequencies. The frequency range of the microphone

must be at least as wide as the desired overall response

limits of the system with which it is used.

Except in the case of the emphasized system in

which it may be desirable for the microphone to have a

rising frequency-response characteristic, the micro-

phone response should be uniform or flat, within its

frequency range, and free from sharp peaks or dips, such

as those caused by mechanical resonances.

IMPEDANCE.-- Crystal microphones have

impedances of several hundred thousand ohms, whereas

the magnetic and dynamic microphones have

impedances that range from 20 to 600 ohms. The

impedance of a microphone is usually measured

between its terminals at some arbitrary frequency within

the useful range, such as 1000 Hz.

The impedance of magnetic and dynamic

microphones varies with frequency in much the same

Figure 7-4.--Schematic diagram of a carbon microphone.

manner as that of any coil or inductance; that is, the

7-5