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APPARATUS

Start the motor so the rotating disk with holes chops the laser beam.  The photodetector output is amplified and sent to a speaker.  Rotate the stool to bring different rows of holes into the path of the beam to produce notes of different frequencies.

The siren disk also demonstrates how pitch and intervals are related to vibration frequency.  One can show that changing the motor speed changes the pitch (frequency) but not the intervals (frequency ratios).

NOTES

Place the motor with siren disk on the rotating stool, with photodetector on a ringstand in front of the disk and the laser behind.  Set the beam at the same height as the axis of the disk so that when the stool is rotated, different rows of holes pass through the beam.  This way Bob can play Finlandia by swinging the turntable.

 

There are two siren disks with chucks.  The smaller one has concentric rings of 5, 10, 15, ... 45 holes.  The larger one has rings of 24, 27, 30, 32, 36, 40, 45, 48 holes and produces a major scale.

Set the motor rotation rate to 11 Hz.

The less musical version of this involves sweeping a piece of screen through the beam, or using a modulated laser connected to a function generator.  Set the function generator to produce a 1/2 V p-p, 1 kHz sine wave.