When a wave is established in a medium and is reflected off a surface, a standing wave can be produced. It is called a standing wave because it does not appear to move. If a tube is closed at one end, as in our activity, standing waves occur when where l is the wavelength of the sound, is the length of the pipe and d is the inside diameter of the pipe. The vibrational frequencies that produce standing waves give rise to the patterns pictured in Figure 8. Standing waves in open and closed tubes.
V represents the velocity of the wave and represents the resonant length.
We can apply what we have learned thus far about resonance by considering the perplexing results of earthquakes. A recent article in the journal American Scientist describes how an understanding of resonance can explain earthquake patterns in the geological formations known as sedimentary basins. Mexico City, which suffered a devastating earthquake in 1985, and San Francisco, which experienced one in 1989, are both located in such basins. An earthquake generates powerful waves at its epicenter that travel in all directions. When these waves reach a sedimentary basin, they may resonate with one of the natural frequencies of the basin.