Pitch and Frequency
In the previous chapter it was mentioned that the pitch of a note, i.e., its position in the musical scale depends on the frequency of vibration of its source. This was demonstrated early in the nineteenth century by Felix Savart, who held a card against a rotating toothed wheel and showed that the pitch of the note emitted depends on the speed of rotation (Fig. 28.1). As the teeth strike the card it vibrates with a frequency equal to the number of teeth multiplied by the number of revolutions of the wheel per second.
If four wheels with teeth numbers in the ratio 4: 5: 6: 8 are run at constant speed on the same shaft the notes given out are doh, me, soh, doh’. This well-known sequence of notes can be recognized whatever the constant speed of the shaft, showing that the musical relation between notes depends on the ratio of their frequencies rather than their actual frequencies.
Another device, the disc siren, is a rotating metal plate with holes drilled in concentric rings. When a jet of air is directed against the plate, puffs of air escape through the holes and produce a note of frequency equal to the product of the number of holes in a ring and the number of revolutions per second.