Central Amplifier System (One-Way)

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

rattling.

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,

CENTRAL AMPLIFIER SYSTEM

However, for specification purposes and for checking

(ONE-WAY)

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,

efficiency.

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

tions.

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

ohms.

6MC using the AN/SIA-114B.

7-9