Compressed air jet blows through spinning disk with holes.

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(B)Waves - B+45: Sound Spectrum/Sources

Air through holes on rotating disk generates sound. This apparatus can be mounted horizontally, as shown, or vertically,- with the disk facing the class. Compressed air is blown through the holes in a ring as it rotates, making a sound. The frequency of the sound is higher toward the outer ring, and depends on the speed of rotation.
UCB Index: 
B+45+30
PIRA Index: 
3C20.30
Demo Diagram: 

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PIRA Taxonomy:

Video: 

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Comments

James Little

Fri, 06/01/2018 - 09:12

To demonstrate how we perceive pitch (our brains do an autocorrelation analysis): take two nozzles (close together) and aim them at the same ring of holes of the spinning disk. Their separation should be close to 1/2 the spacing of the holes. The frequency you perceive is of course twice the frequency of holes passing by because you hear each individual hole passing one nozzle and then the other. Now slowly aim the two nozzles closer together (they're still both aimed at the same circle of holes). The frequency you hear doesn't change. As you bring them closer together still, there comes a point where the sound you hear drops an octave, and that happens long before the two nozzles are aimed at the same hole. The transitions from hearing first the higher octave and then double pulses an octave lower to finally single pulses at the lower octave is a fascinating demonstration of the brain at work.