The siren produces sounds of
different frequencies. A rotating disk with holes
interrupts an
air jet, producing
a sound whose frequency depends on the rotation rate of the
disk.
A
counter keeps track of the number of rotations. The
frequency of
the
sound can be determined from the number of holes x the number
of
rotations/duration
of the sound.
REFERENCE
Tom
Greenslade's website
on early physics teaching apparatus.
