Siren disk

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." 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.