Siren disk

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A siren disk is used in pneumatic sirens and has holes which are variously spaced apart. When the disk is spun in front of a jet of air, the holes modulate the air-jet which produces a sound. The pitch of a siren is produced by "the frequency of the impulses of compressed air passing through the openings in a rotating disk."^[1] The pitch is therefore determined by the speed at which the disk rotates, the number of holes which air passes through, the size of the holes and their spacing apart.