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In shipboard announcing and public-address
CAPACITY.-- The load-carrying capacity
systems, a matching transformer is built into each
of a loudspeaker is usually expressed in terms
loudspeaker to transform the low voice-coil impedance
of the maximum electrical power that should be
to a higher value suitable for connection to loudspeaker
applied to it. This power is limited by heating,
distribution lines. Because loudspeakers in a system are
mechanical strength, and the production of nonlinear
connected and operated in parallel, the combined
distortion that is caused by excessive diaphragm
impedance of a large number of low-impedance voice
amplitudes or excessive acoustical pressures in
coils without matching transformers would be so low
the sound passages. Excessive power causes the
compared with the resistance of the connecting cables
diaphragm to strike portions of the magnet or
that an appreciable portion of the amplifier output power
supporting frame and may produce buzzing or
would be dissipated in the cables. Thus, matching
transformers are provided to reduce this loss. These
transformers have several taps to vary the loudspeaker
EFFICIENCY.-- The loudness of the sound
impedance. Changing the loudspeaker impedance
obtainable from a loudspeaker at any particular
changes the power absorbed by the loudspeaker from
listening point is not a factor of load-carrying
the lines, and thus provides a means of varying the
capacity alone. Other important factors are the
loudness of the loudspeaker.
efficiency and the amount that the sound is spread out.
The definition of absolute efficiency of a loudspeaker
is not subject to simple practical interpretation,
However, for specification purposes and for checking
the performance of naval loudspeakers, a specified
The one-way central amplifier amounting system
voltage is applied to the input terminals and the
provides a means of transmitting general orders,
output sound pressure is measured at a given distance
information and, in some cases, alarm signals to various
from the loudspeaker on the loudspeaker axis using
locations simultaneously by microphones and
various test frequency signals. These measurements
loudspeakers connected through a central amplifier.
a r e combined with off-axis sound pressure
Two examples of the one-way central amplifier
measurements to evaluate the relative loudspeaker
announcing system are the general announcing system,
circuit 1MC, and the intership announcing system,
circuit 6MC.
When satisfactory frequency in a loudspeaker
One of the more common types of central amplifiers
is limited to a small angle about the axis, the
installed on board ship today for use with circuits 1MC
absolute efficiency at high frequencies is con-
and 6MC is the AN/SIA-114B amplifier-oscillator
siderably lower than at low frequencies. The use
group. The AN/SIA-114B, with other equipment,
o f diffusing arrangements with these loud-
comprises a shipboard announcing system capable of
speakers to spread out the high frequencies
both circuit 1MC and circuit 6MC operation. The major
usually results in spreading out the small amounts
components of the system are the control rack and the
of available high-frequency energy to such an ex-
power rack. The AN/SIA-114B contains two distinct
tent that the response is unsatisfactory at all loca-
amplifier channels, A and B. Each channel is made up
of a preamplifier located in the control rack and a
500-watt power amplifier located in the power rack.
IMPEDANCE.-- The impedance of a loud-
There are two identical oscillator groups, 1 and 2,
speaker is usually measured between the voice coil
located in the control rack for generating alarm signals.
terminals at some average frequency, such as 1000 Hz,
There is also additional sound equipment used with the
in the usable range. This impedance varies with the
system and facilities available for muting and
frequency, rising with increasing frequency. The usual
attenuating the ship's entertainment system. Figure 7-8
value of voice coil impedance varies from 3 to 15
is a simplified block diagram of the circuits 1MC and
6MC using the AN/SIA-114B.

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