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Up to now we looked at traveling waves. These are waves that are moving in a specific direction; but sometimes waves are stuck and don't move. We call those waves standing waves . A good example is the wave you get on the string of a guitar (Figure 1).

** Note: You can tell which harmonic it is by counting the number of antinodes, which are the places where the amplitude is at its maximum.

Explanation Follow-up

The upshot and most important thing to remember is that only a specific set of wavelengths are allowed on a string (or in a tube open at both sides). For a string or tube of length L, the possible wavelengths are given by the formula

(\lambda_n = \frac{2L}{n})

for n = 1, 2, 3, ....

So, for example, the first harmonic, the fundamental, has a wavelength equal to (\lambda_1 = 2L.) This makes sense since as you can see from Figure 2, the first harmonic looks like half a wavelength on the whole string.