The siren gets its unique tone, or timbre, from the from the shape of the sound wave it produces. The chopping action of the rotor produces a triangular waveform that is rich in both odd and even harmonics. The first few harmonics are especially strong. The waveform below is from a single-tone 580 Hz fire siren.

 

Below is the spectrogram of this single-tone siren sounding, from beginning to end. The bottom-most green line is the fundamental tone, which peaks at 580 Hz in the middle of the image. Each green line above this is a harmonic multiple of 580 Hz-- 1160 Hz, 1740 Hz, etc... As the frequency goes up the power in each harmonic is less, shown by the diminishing brightness of the lines.

 

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