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A civil defense siren (also referred to as an air raid siren, tornado siren,, or outdoor warning siren) is an electrically-powered mechanical device for generating sound to provide warning of approaching danger and to indicate when the danger has passed. more...

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Initially designed to warn of air raids, they were adapted to warn of nuclear attack and of natural phenomena 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.

Sound is generated by a motor driving a shaft with fans on each end, one fan having a few more blades than the other. Around each fan is a housing with a number of cut slots to match the number of fan blades. The blades draw air in at the end and force it out through the slots in the housing in rapid pulses, which produces the sound. Modern sirens can reach up to, but not commonly, 135 decibels when measured 100 feet away from the siren.

Many warning sirens have a sound that is distinct from that used by emergency vehicles due to use of two simultaneous tones, usually in a 5:6 frequency ratio (a minor third).

Some newer sirens have the ability to broadcast voice messages over large areas, depending on winds and noise. These electronic sirens are similar to electromechanical sirens; however, they rely on a series of large loudspeakers to produce sound. However, there is some question about the ability of a system of electronic sirens to broadcast a voice message with sufficient intelligibility over long distances - not only does the sound echo off some surfaces, the sound could have multiple arrival times from widely-spaced siren sites.

However, the relative simplicity, efficiency, and easily-serviceable design of modern electronic warning systems, in addition to the ability to broadcast any type of warning signal desired, makes them an attractive alternative for many municipalities.

Historical warning tones

During World War II for a "Red Warning" of approaching danger, the siren produced a tone that rose and fell regularly between one high and one low tone. A "White Warning" (All Clear) was a single continuous tone. Another alternating tone signified a "take cover" warning for immediate danger. After World War II, two further warnings were introduced for nuclear attack - a "Grey Warning" indicated approaching nuclear fallout with a 2½ minute warning of long steady tones divided by equal periods of silence, the silence being created with a manual shutter or electric solenoid. A "Black Warning", also for manual sirens, was either a Morse code 'D' (–··) or three quick tones, indicating imminent danger of fallout. These were not used in the United States.

Current warning tones

Today, the most common tones sirens in the United States produce are "Alert," "Attack," and "Hi-Lo."

Read more at Wikipedia.org