It is also referred to as a civil defense siren, air raid siren, fire siren/whistle, tsunami siren, flood siren, weather siren, outdoor warning siren, or time/Curfew siren. It is electrical or mechanical device for generating or producing the sound to provide warning of danger time. Sometimes it is also used when the danger has passed. In some areas of USA, civil defense sirens may generate sound in the late morning and early afternoon on a daily basis. It was the first use in World War 2 from the warning of nuclear attack. It can also be used for many cases, such as in Emergency Alert System. In term of mechanical Siren, sound is generated by a motor with a special fan on both ends. It is also known as chopper or rotor. One fan contains few more blades than the other one. Around, each chopper is housing with a number of holes to match the no of rotor blades. This housing is also called or known as stator. These holes are in rectangular in shape. At the end, of this blade has plate and the shape of each plate are matched with the circular cure and rectangular holes of the sector.
Modern sirens can reach up to 30 meter away from the siren. The Chrysler Air Raid Siren is the loudest siren producing 138 decibel around 100 feet or 30 meter. Most of the sirens are mounted on poles. These poles are 30 to 50 feet off the ground. Most of the warning sirens are distinct from the other sirens. It can be used to deliver the broadcast voice messages over large areas. It depends on the noise and winds. These sirens are different from the mechanical sirens because it can produce a large amount of sound than the other one. This device is very useful in critical situations especially in emergency situations. It can also be used in automobiles like a fire truck, ambulances and police cars. It is a signal for people to out away from the way. It can also be used in natural disasters. From this, people are aware from the natural disasters. It is very effective alarm and signaling device because it produces more sound than the horns. The sound of siren depends on the amount of power on it. It is useful when the person is in the ambulance. It is also being useful for the emergency vehicles. It can also be used commercial as a car alarm.