United States thumb|upright|left|Federal Signal Model SD-10 thumb|Federal Signal Model 500AT thumb|upright|Federal Signal Model 2001In the United States, several sets of warning tones have been used that have varied due to age, government structure, and manufacturer. The initial alerts used during World WarII were the alert signal (a 3–5-minute steady, continuous siren tone) and the attack signal (a 3–5-minute warbling tone, or series of short tone bursts on devices incapable of warbling, such as whistles). The Victory Siren manual stated that when a 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, characterized by three 1-minute steady blasts with 2 minutes of silence between the blasts.
United States thumb|upright|left|Federal Signal Model SD-10 thumb|Federal Signal Model 500AT thumb|upright|Federal Signal Model 2001In the United States, several sets of warning tones have been used that have varied due to age, government structure, and manufacturer. The initial alerts used during World WarII were the alert signal (a 3–5-minute steady, continuous siren tone) and the attack signal (a 3–5-minute warbling tone, or series of short tone bursts on devices incapable of warbling, such as whistles). The Victory Siren manual stated that when a 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, characterized by three 1-minute steady blasts with 2 minutes of silence between the blasts.