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A siren disk is a disk with holes variously spaced, such that when spun in front of an air jet, the holes modulate the jet in a primarily binary (on/off) fashion, to directly produce sound.

Contents

Typical usages of siren disks

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

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The harmonic portion of the siren disk

Most siren disks include several 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 hole-circles include uniformly spaced holes, in which the number of holes increases along a harmonic series, so that eight notes of a musical scale can be played by directing an air jet sequentially at each circle.

The nonharmonic portion of the siren disk

Typically there are further holes near the central part of the disk, with various non-uniform patterns, such as random spacing, and the like.

Typical numbers of holes in siren disks

The most common hole configurations are 6, 7, 8, 9, 10, 12, 15, and 16, a space, 24, 27, 30, 32, 36, 40, 45, 48, which give the following musical notes:

The 12 notes of the most common kind of siren disk.

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

The corresponding letters of the major scale given by the last 8 holes of the siren disk are:
C, D, E, F, G, A, B, C

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

There are commas between these numbers so you can "paste" them into Octave, e.g. to get the number of holes to make a minor-scale siren disk:
[1, 9/8, 6/5, 4/3, 3/2, 8/5, 9/5, 2]*8*5*3

120 135 144 160 180 192 216 240

Examples of hole configurations include the Federal Signal Model 2(6 hole), STL-10(7 hole), American Signal Tempest T-128 (8 hole), American Signal Screamer (9 hole), Federal Signal T-22 (Low tone=10 hole), Federal Signal Thunderbeam (12 hole), Sentry Siren 40V2T (High note=15 hole), and Sentry-M (16 hole).

Typical siren disk setup

The typical siren disk setup consists of a motor, pulleys, belt, and air jet.

The setup at the Ontario Science Centre, for example, includes organ-style keys so that children can play music on the siren disk by pressing the keys to open and close a series of eight valves that direct the air selectively at the eight harmonic circles of uniformly spaced holes of the siren disk.

See also

Siren (noisemaker)


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