<P> 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). </P> <P> 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 . </P> <P> Beginning in 1952, the Bell and Lights Air Raid Warning System, developed by AT&T, was made available to provide automated transmission of an expanded set of alert signals: </P> <Ul> <Li> Red Alert (attack imminent) </Li> <Li> Yellow Alert (attack likely) </Li> <Li> White Alert (all - clear) </Li> <Li> Blue Alert (undefined) </Li> </Ul>

What does a air raid siren sound like