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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
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
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.
In this type of loudspeaker (fig. 7-5), the diaphragm acts
directly on the medium, which is air. Both sides of the
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.

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