Which unit measures the frequency of sound waves?

The hertz, one cycle per second, named for Heinrich Hertz - who proved electromagnetic waves exist by generating and detecting them in his laboratory in the 1880s, confirming Maxwell's theory. Asked what use it might have, he reportedly said none whatsoever, and that it was simply an experiment proving Maxwell right.

Radio was built on it within a decade of his death at 36. The unit replaced cycles per second officially in 1960, which is why older equipment is marked in kc and Mc rather than kHz and MHz.

Human hearing runs from about 20 hertz to 20,000, and the top end falls with age - most adults lose the highest frequencies steadily, which is the basis of the mosquito alarm sold to deter teenagers from loitering because people over about 25 cannot hear it. The same idea was inverted into a ringtone so students could receive messages in class inaudibly to teachers, which is the more satisfying use of the physics.