An Audible Signal is a representation of sound, typically as an electrical voltage. Audio signals have frequencies in the audio frequency range of roughly 20 to 20,000 Hz (the limits of human hearing).  Horns and sirens, also known as "sounders", are tone generators that output an audible signal when a voltage is applied to their input power terminals. Sounders are self-generating - meaning that the tones are pre-programmed into their circuitry and the sound is simply emitted when power is supplied.

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