Rotating hole patterns convert air puffs into precisely determined complex tones

Evidence

Each puff is a monopole source and leads to a pressure wave traveling away from the siren. Knowing the revolutions per second and the number of holes, the frequency of the sound can be determined precisely—a boon in the days before electronics. A rotating disk with 20 equally spaced holes at 10 times per second will produce equally spaced puffs separated by 1/200th of a second—that is, a 200 Hz complex tone with many partials—since the discrete puffs are certainly not sinusoidal. One can supply air to two sets of holes on the same disk, one with perhaps 10 evenly spaced holes at one radius, and another with 15 evenly spaced holes at a larger radius. If the disk is rotating 10 times per second, 100 Hz and 150 Hz tones result, an absolutely perfect fifth apart at a ratio of 3:2. There is visual proof on the face of the siren of the ratio of the two frequencies being heard. — Eric J. Heller, Why You Hear What You Hear- An…, 582

Categorization

• XREF: Connects to the pufferfish-of-frequency idea from the siren discussion and to how mechanical rotation maps ratio pitch intervals (3:2 perfect fifth) directly to geometry.
• CANDY: A disk with 10 holes at one radius and 15 at another spinning 10x/sec produces a literal perfect fifth — visual geometry becomes musical ratio.