A siren is a loud noisemaker designed to alert people to a variety of different situations. They include tornado sirens, fire sirens, ambulance sirens and police sirens. Emergency sirens such as the tornado sirens can alert an entire community of impending danger whereas police sirens are most often used to instruct individuals to stop or slow down in traffic situations.
The word siren first originated in Greek mythology, was used to refer to mermaids and was often associated with dangerous temptations. Today the word siren describes a warning signal that is used to instruct a person to stop. According to Greek mythology, listening to a siren could have disastrous consequences, but these days, ignoring a police siren can have the dangerous consequences.
In the early 19th century the very first sirens were made of pneumatic tubes and two disks. One disk was perforated and one disk rotated. A sound was made when air flowed from the tube to the fixed disk and caused the other disk to rotate. Later sirens were made using slotted cylinders, or electroplating hangers. All types of modern police sirens are simple modifications of the first basic pattern.
The two basic types of sirens are the pneumatic siren, similar to the early models, and electronic sirens. Pneumatic sirens and their energy requirements are much higher compared to electronic sirens. Pneumatic sirens are also known as mechanical sirens. Electronic sirens use different types of sounds which are synthesised with the combined action of sound modulators, oscillators and amplifiers. Most police sirens today are electronic sirens, although some police cars may be fitted with both types of sirens.
Some countries in Europe have developed an in-between type of police siren known as the electro-mechanic siren. It uses two air horn sets with both low and high pitches. The air compressor switches between both sets of horns so the sound alters between a high and low pitch. This type of police siren was developed to produce a specific sound patten, and these types of sirens also don't use as much power as a simple mechanical siren.