An air raid siren is a electrically powered mechanical device for sound generation to provide warning of approaching danger and to indicate when the danger has passed. 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 them being largely replaced with more considered warnings, such as the U.S. Emergency Broadcast System.
Sound is generated by having a motor drive a shaft at either end of which are mounted fans, 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 are designed to draw air in at the end and force it out through the slots in the housing. Due to the design, the air output is cut on and off alternately thus producing the sound. Modern sirens can reach 140 dB at 30 metres.
A number of different sound forms could be created. During WW II for a Red Warning of approaching danger the siren would be run normally producing a tone that rose and fell regularly between one high and one low tone, corresponding to the number of blades on each fan and the speed at which they turned. A White Warning (All Clear) was a single continuous tone. Sometimes there was a Take Cover warning for immediate danger, the power to the motor was cut for a moment at intervals to change the tone produced. Post WW II two further warnings were introduced for nuclear attack - Grey Warning indicated approaching fall out with a 2.5 minute warning of long steady tones divided by equal periods of silence, the silence being created with a manual shutter. Black Warning, also for manual sirens, was either a morse code 'D' (-..) or three quick tones, indicating imminent danger of fall out.
