Driving Mechanism

impedance rises with increasing frequency. The actual

use in the Navy are (1) the direct radiator type, which

impedance of the microphone in shipboard applications

radiates sound directly from a vibrating member into the

is of importance only as it relates to the input load

air, and (2) the horn type, which consists of a driving

impedance into which the microphone is designed to

unit combined with a horn to couple the unit to the air.

All loudspeakers consist essentially of a driving

operate. If the microphone is mismatched with the input

impedance, the microphone input is reduced and

mechanism.

distortion occurs. All specifications and acceptability

tests for naval microphones are based on the designed

Driving Mechanism

input load impedance.

The driving mechanism changes electrical

SENSITIVITY.-- The sensitivity or eficiency of

vibrations into mechanical vibrations that are

a microphone is usually expressed in terms of the

transmitted to a diaphragm or other vibrating source.

electrical power level that the microphone delivers to a

This vibrating source is coupled to the air, either directly

terminating load, the impedance of which is equal to the

or by a horn, and causes sound to be radiated. The

rated impedance of the microphone, compared to the

dynamic, or moving-coil, driving mechanism is the

acoustical intensity level or pressure of the sound field

basic type used in Navy loudspeakers. The design of this

that is being picked up.

unit is similar to that of the dynamic microphone, but

Most systems rate the microphone in the electrical

the principle of operation is the reverse of that of the

power level (in decibels below 1 milliwatt (mW)

dynamic microphone.

produced by an acoustical pressure of 1 dyne per square

A coil of wire is attached to a diaphragm and rests

centimeter. For example, a crystal microphone rated at

in a magnetic field. When a varying electric current

80 dB means that for an input acoustical pressure of 1

flows through the coil, a force is exerted on the coil,

dyne per square centimeter, the electrical output is 80

causing it to move back and forth in the magnetic field.

dB below 1 mW, or 10 mW. Other systems rate the

The consequent motion of the diaphragm causes the

microphone in terms of the voltage delivered to a

radiation of sound waves that correspond to the

specified terminating load impedance for an acoustical

variations in the electric current. The electrodynamics

pressure input of 1 dyne per square centimeter.

and the permanent-magnet types are the two variations

It is important to have the sensitivity of the

in the dynamic loudspeaker. These types differ only in

microphone as high as possible. High sensitivity means

the method employed for obtaining the magnetic field.

a high electrical power output level for a given input

In the electrodynamics loudspeaker, the magnetic

sound level. High microphone output levels require less

field is established by passing a direct current through a

gains in the amplifiers used with them and thus provide

field coil that is wound on an iron core. This type

a greater margin over thermal noise, amplifier hum, and

requires a source of filtered direct voltage and two

noise pickup in the line between the microphone and the

additional conductors to carry the field current to the

amplifier.

loudspeaker.

When a microphone must be used in a noisy

In the permanent-magnet dynamic loudspeaker, a

location, it should favor sounds coming from a nearby

permanent magnet establishes the magnetic field. All

source over random sounds coming from a relatively

loudspeakers used by the Navy are of the permanent-

greater distance. Microphones of this type cancel

magnet dynamic type.

random sounds and pick up only those sounds

originating a short distance away. When talking into this

type of microphone, hold your lips as close as possible

Direct-Radiator Loudspeaker

to the diaphragm. Directional characteristics that favor

sound coming from one direction only also aid a

The direct-radiator loudspeaker, sometimes called

microphone in discriminating against background

a cone loudspeaker, is the simplest form of loudspeaker.

noise.

In this type of loudspeaker (fig. 7-5), the diaphragm acts

directly on the medium, which is air. Both sides of the

LOUDSPEAKERS

diaphragm are open to the air so sound radiates behind

as well as in front of the loudspeaker. At the instant the

A loudspeaker is a device that converts electrical

diaphragm is moving in an outward direction, the front

energy into sound energy and radiates this energy into

surface of the diaphragm produces a compression wave

the air in the form of waves. The loudspeakers in general

and the back surface produces a rarefaction wave.

7-6