F Galton devised this whistle in order to investigate the upper limit of audible sound in humans. It is constructed from a bras tube which has an internal diameter of approximately 2 millimetres. The whistle was operated by pasing a jet of gas through an orifice into a resonating cavity. By moving a plunger, the size of this cavity could be changed, enabling the pitch of the sound made by the whistle to be altered from the audible up into the ultrasonic range. Galton was able to determine that the normal upper limit of human hearing was about 18kHz.
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