Civil defense siren

A civil defense siren (also known as an air-raid siren or tornado siren) is a siren used to provide emergency population warning of approaching danger and sometimes to indicate when the danger has passed. Some (that are mostly located in small towns) are also used to call the volunteer fire department to go fight a fire. Initially designed to warn of air raids in World War II, they were adapted to warn of nuclear attack and of natural destructive weather patterns such as tornadoes. The generalized nature of the siren led to many of them being replaced with more specialized warnings, such as the Emergency Alert System.

A mechanical siren generates sound by spinning a slotted chopper wheel to interrupt a stream of air at a regular rate. Modern sirens can develop a sound level of up to 135 decibels at 100 feet (30 m). The Chrysler air raid siren, driven by a 331-cubic-inch Chrysler Hemi gasoline engine, generates 138 dB at 100 feet away.

By use of varying tones or on/off patterns of sound, different alert conditions can be signaled. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.

Many warning sirens have a sound that is made distinguishable from that used by emergency vehicles by use of two simultaneous tones, with pitches usually in a 5:6 frequency ratio (an untempered minor third).

In the United States, several sets of warning tones have been used which varied over time, by government structure, and by manufacturer. The initial alerts used during World War II were the Alert Signal (a 3–5 minute steady continuous siren tone), and the Attack Signal (a 3–5 minute wail siren tone, or series of short tone bursts on devices incapable of wavering, such as whistles). The Victory Siren manual stated that when manual generation of the warbling tone was required, it could be achieved by holding the Signal switch on for 8 seconds and off for 4 seconds. In 1950, the Federal Civil Defense Administration revised the signals, naming the alert signal "red alert" and adding an "all-clear" signal, defined as three one-minute steady blasts, with two minutes of silence between blasts.

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