Electric Sirens
A simple method that has been embraced by many police departments is an electric siren. This siren is merely a speaker that is connected to a power source. This way, the sound of the siren is as easy to turn on as the flashing lights, needing only a flick of a switch. These are programmed to make the sound that was originally made with mechanical sirens.
The Mechanical Siren
The traditional mechanical siren is made up of a few parts. At the bottom, there is an electric motor that is connected to a fan. Outside of this fan is metal drum with holes cut out of it on the sides in two distinct alternating rows. At the top of the siren is a large cone, there to take in any air that's nearby.
How Does It Work?
When the mechanical police siren is turned on, the motor starts. The motor is connected to a fan or rotor, which starts to move. The movement of the fan starts sucking air in through the air intake cone. When the air enters the drum with fan, it is then expelled out the sides through the small holes in the drum. Because the fan is moving at high speeds (some in excess of 7000 rpm), the air is pushed out through the hole quickly, and this causes the metal to vibrate quickly and strongly. This vibration creates the familiar loud siren sound. The alternating tones come from the alternating rows of holes on the drum.
Tags: alternating rows, drum with, holes drum, that connected
