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TM 9-8000
Section II. HORNS
an air horn. The sound frequency of the low-pitch horn is
17-9. Principles of Operation (Fig. 17-23). The most
controlled by a long air column and the high-pitch horn
common type of horn is the vibrator type, in which the
sound frequency is controlled by a short air column. The
general principle of operation is the same as that of a
air column is formed by the projector and a spiral
vibrating coil. A vibrating diaphragm is operated by the
passage cast into the base of the horn.
coil that also operates the contacts that break the circuit.
17-11. Controls (Fig. 17-24). The current draw of a
Magnetism from the coil pulls the diaphragm toward it
when the contacts are closed. The contacts then are
horn is very high, therefore, usually it is operated by a
pulled open by the coil, reducing the magnetism and
relay. The control switch usually is mounted on the
allowing the diaphragm to return to its normal position.
steering wheel and may be controlled by a button or a
When the contacts are closed again, a new surge of
horn ring.
current induces magnetism in the coil and starts a
a. The horn receives electric current through the
second movement of the diaphragm. This cycle is
repeat- ed rapidly. The vibrations of the diaphragm within
ignition switch and the relay contact points. The relay
an air column produce the note of the horn. Tone and
contact points normally are open, keeping the horn from
volume adjustments are made by loosening the adjusting
locknut and turning the adjusting nut. This very sensitive
b. The relay contact points are closed when- ever the
adjustment controls the current consumed by the horn.
Increasing the current increases the volume. However,
magnetic coil is energized. The magnetic coil receives
too much current will make the horn sputter and may
positive battery current from the ignition switch. The horn
lock the diaphragm.
switch completes the circuit to ground.
17-10. Dual Horns. In dual horns, one horn with a low
17-12. Air Horns (Fig. 17-25). The air horn is a trumpet
pitch is blended with another horn with a high pitch.
like device that operates from compressed air. Air horns
These horns, although operated electrically, produce a
usually are used on vehicles that are equipped with
sound closely resembling that of
airbrakes because they can operate from the vehicle's
Figure 17-23. Electric Horn, Vibrator Type


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