Siren is a device used to sound warning signals. Ambulances, fire engines, and police cars use sirens to alert traffic. Sirens called foghorns are used on lighthouses and ships to warn other ships away during foggy and other bad weather. Sirens are also used for air-raid warnings.
One type of siren has two cylinders, one inside the other. The cylinders are perforated (punched with holes). This type of siren makes its sound when the outside cylinder revolves around the inner cylinder, and air is forced through the holes. An electric motor turns the cylinder in sirens on police cars, fire engines, and other vehicles. When the holes in the cylinders are lined up, puffs of air escape and cause vibrations. As the cylinder is turned faster, the puffs become more frequent and produce a wailing sound. Some sirens have perforated disks instead of cylinders, and some of these can be turned by hand.
A newer type of siren produces its wailing sound electronically. It has no moving parts, and it does not wear out as quickly as older types of sirens.
