If a string is vibrating so that it moves back and forth sixty times in one second, we say that it has a frequency of sixty cycles per second. The unit of measurement, cycles per second, is often called Hertz (abbreviated Hz) after Heinrich Hertz, the German theoretical physicist who was the first to transmit radio waves (a dyed-in-the-wool theoretician, when asked what practical use radio waves might have, he reportedly shrugged, “None”). If you were to try to mimic the sound of a fire engine siren, your voice would sweep through different pitches, or frequencies (as the tension in your vocal
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