Air horns, so named because they rely on air moving through a collection of trumpet-like horns, are true to their name in that they rely on air forced through the horns to produce sound. Because there is considerable resistance in an air horn, powerful electric air compressors generate tremendous amounts of air at a very high pounds-per-square-inch (PSI) pressure. This air is routed through tubes to the horns, where it will ultimately become a source of sound.
Compressors Are Activated by Switches
The compressors that power air horns are not, of course, always on; if they were, the horns would always be producing sound. Instead, the compressors are wired to a power source with a simple on/off toggle switch, much like the one used to turn on a lamp or roll up a power window, interrupting the flow of electricity. When the switch is moved into its "On" position, it completes an electrical circuit between the air compressor and its power source, springing the compressor into action and generating the air necessary to produce sound.
Air Flows Through Horns
When the compressor is activated and large amounts of air flow to the horns at very high pressure, the horns allow the air to quickly pass through and escape. A metal (or, in typical automotive use, plastic) diaphragm inside the horn oscillates between its open and closed position as the air passes through. This vibration is carried to and magnified by the bell of the horn, which then vibrates at the same frequency. The vibrations are carried through the air and, ultimately, received by the human ear as sound. Because air horns typically use a collection of horns powered by one or more compressors, and because the entire horns vibrate to transfer powerful vibrations to the air, the sound produced by air horns is heard as significantly louder than typical automotive horns.