sound-producing diaphragms. Figure 9-10 is an
illustration of a motor-operated horn.
Sirens are used in very noisy spaces or to sound
urgent alarms. The sound is produced by an electric
motor driving a multiblade rotor past a series of ports
or holes in the housing. The air being forced through
the ports gives a siren sound, the frequency of which
depends upon the number of ports, the number of
rotor blades, and the motor speed. Figure 9-11 is an
illustration of a siren.
Electronic Signal Units
The IC/E3D2 electronic signal unit (fig. 9-12) is
designed for use with alarm switchboards. The unit
consists of a solid-state oscillator and an amplifier.
The unit is capable of generating three distinct tones:
a wailing or siren tone, a steady tone, and a pulsating
Visual sig nals ar e used with many alar m,
safety , and warning sy stems to prov ide an
additional means of identifying the alarm
being sounded. Audible and v isual sig nals are
often used tog ether . In noisy spac es, audible
signals are supplemented by visual signals;
Figure 9-12.--Type IC/E3D2 electronic signal
and in brightly lighted spaces, visual signals are
supplemented by audible signals. In many
instruments, the same audible device is used in
combination with several visual signals. The
principal type of visual signals are lamp
Standard watertight lamp indicators are
designed as single-dial, two-dial, four-dial, and
six-dial units. Two 115-volt lamps are connected
in parallel and mounted behind each dial. The
use of two lamps in parallel provides protection
against the loss of illumination in case one lamp
bums out. A colored-glass disk and sheet-brass
target engraved with the alarm identification
are illuminated from the rear by the two lamps.
The 115-volt lamps are in parallel with
Figure 9-10.--Motor-operated horn.