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.
Siren disks were once used as the active element of sirens, but the majority of siren disks currently manufactured are used as instructional aids in teaching the mechanics of sound and music. For example, the Ontario Science Centre has a metal siren disk set up as an interactive teaching exhibit for children.
Siren disk hole spacing
The harmonic portion of the siren disk
Most siren disks include multiple concentric circles of spaced holes, so that an air-jet can be directed at different radii to get different effects. For example, the outermost eight circles of holes include uniformly spaced holes, in which the number of holes increases along a harmonic series, so that the eight notes of a musical scale can be played by directing an air jet sequentially at each circle.
The non-harmonic portion of the siren disk
Typically there are further holes nearer the disk's centre with various non-uniform patterns, such as random spacing or arpeggios, etc.[clarification needed]
