Lesson 4/Learning Event 3
In dual horns, one horn with a low pitch is blended with another horn with a high pitch. These
horns, although operated electrically, produce a sound closely resembling that of an air horn. The
sound frequency of the low-pitch horn is controlled by a long air column and the high-pitch horn
sound frequency is controlled by a short air column. The air column is formed by the projector
and a spiral passage cast into the base of the horn.
The current draw of a horn is very high; therefore, it is usually operated by a relay. The control
switch usually is mounted on the steering wheel and may be controlled by a button or a horn ring.
The horn receives electric current through the ignition switch and the relay contact points. The
relay contact points normally are open, keeping the horn from operating.
The relay contact points are closed whenever the magnetic coil is energized. The magnetic coil
receives positive battery current from the ignition switch. The horn switch completes the circuit to
The air horn is a trumpet-like device that operates from compressed air. Air horns usually are used
on vehicles that are equipped with air brakes because they can operate from the vehicle's
compressed air system. Air horns also may have their own compressed air system. The air horn is
controlled by an air valve.
Large vehicles sometimes use an air or electric horn at the rear that is actuated when the
transmission is shifted to reverse.
The purpose of this is to warn pedestrians when the vehicle is backing up.