Emergency Vehicle Siren is a loud noise making device. Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks. Sirens are used on emergency service vehicles such as ambulances, police cars and fire trucks. There are three general types: Acoustic siren、pneumatic siren and electronic siren.

Acoustic sirens
By converting the audio signal to sound energy by amplifying the low frequency power. That is, electrons can be converted to sound.
The acoustic siren was one of the fundamental tools of 19th century science. Invented by the French scientist Cagniard de la Tour, in 1819, the siren used a bellows apparatus to force air through two brass disks. As the air passed through holes in the discs, one of which was rotating, it produced a series of regular pressure waves that we perceive as a musical tone. The construction of the siren allowed that tone to be raised or lowered by simply increasing or decreasing the speed of the rotating disc.

Pneumatic Sirens
By converting electrical energy into mechanical energy, mechanical energy can be converted into gas energy. The gas can be converted into sound.
Pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a chopper, siren disk or rotor), such that the material between the holes interrupts a flow of air from fixed holes on the outside of the unit (called a stator). As the holes in the rotating disk alternately prevent and allow air to flow it results in alternating compressed and rarefied air pressure, i.e. sound. Such sirens can consume large amounts of energy. To reduce the energy consumption without losing sound volume, some designs of pneumatic sirens are boosted by forcing compressed air from a tank that can be refilled by a low powered compressor through the siren disk.

Electronic Sirens
By converting electrical energy into mechanical energy, and mechanical energy can be converted into sound.
Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, yelp, pierce/priority/phaser, hi-lo, scan, airhorn, manual, and a few more) which is played through external speakers. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens. Often, police sirens also use the interval of a tritone to help draw attention. The first electronic siren that mimicked the sound of a mechanical siren was invented in 1965 by Motorola employees Ronald H. Chapman and Charles W. Stephens