Two open-ended hollow tubes can be made to vibrate at their fundamental frequencies with a small electric heater near their base, much like the Bunsen burner and the cardboard tube I showed last class. If the tube lengths are identical, they produce the same sound. But if I make one a little longer or shorter than the other, this changes the fundamental frequency. (A longer tube would have a longer fundamental wavelength and lower frequency, and vice versa.) The frequency difference shows up as distinct "beats" in the overall sound. This is how musical instruments are tuned. The player changes the length of his trumpet or tightens the string on the cello to make the beat frequency he hears so small that it effectively disappears. But remember: You can tune a piano, but you can't tuna fish. (editors note: ugh!)

See also: Beats - Visualization and Doppler Effect