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.

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.

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.

Typically there are further holes nearer the disk's centre with various non-uniform patterns, such as random spacing or arpeggios, etc.

Unlike the hypothetical disk discussed above having just 5 concentric rings of equally space holes, a more typical disk might have 16 concentric rings instead. For the example shown below then the hole configurations need to be 8, 10, 12 and 16 then a space followed by 24, 27, 30, 32, 36, 40, 45, 48, which will give the four-note F-major arpeggio followed by the higher C-major scale:

The last eight circles of holes give the following temperament of a major (Ionian mode) musical scale (divide each of the above number of holes by 24): 1, 9/8, 5/4, 4/3, 3/2, 5/3, 15/8, 2 (Ptolemy's intense diatonic scale).

A similarly generated minor scale can be made by making a siren disk with the following ratios of hole numbers: 1, 9/8, 6/5, 4/3, 3/2, 8/5, 9/5, 2. (just minor scale).

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Wikipedia