Siren Design for Producing Controlled Wave Form with Amplitude Modulation

A siren has been developed which permits the generation of a sound pressure wave with harmonics more than 15 db below the fundamental over a broad frequency range and over a range of power output exceeding 40 db. The large dynamic range was obtained by the use of a shutter which changes the maximum open area of the siren ports and simultaneously alters the time variation of area so that the output signal remains substantially sinusoidal at any shutter position. The shutter can be operated rapidly to provide a means for programed amplitude modulation of the output signal with a minimum of harmonic distortion.