As we see from Equation (6.8), the frequencies of all the harmonics of a taut
string are exact multiples of the fundamental frequency and form a harmonic series.
For most vibrating systems this is not the case. These will also vibrate at a series of
higher frequencies in addition to the fundamental frequency. These higher frequencies
are called overtones. However, in general, the frequencies of these overtones
will not be an exact multiple of the fundamental: they are not harmonic. A bell,
for example, will have overtones whose frequencies are not exact multiples of
the fundamental. When the bell is struck, the overtone frequencies will be heard
in addition to the fundamental. The skill of the bell maker is to ensure that the
combination of the fundamental and the overtones produces a sound that is not
discordant to the ear. (Of course, the term overtone can also be applied to a taut
string but in this case the overtones are harmonic.)
We have used the example of a taut string to explore the physical characteristics
of standing waves. However, standing waves occur in many different physical
situations and the ideas we have been discussing are important to a wide range of
physical phenomena. In a microwave oven, electromagnetic waves reflect from the
walls of the oven to form standing wave patterns in the oven compartment. This
means that there will inevitably be places in the compartment where the intensity
of the microwave radiation is reduced and the food will not be properly cooked. To
reduce the effects of these ‘cold spots’ the food is placed on a rotating turntable. In
a laser, the light forms a standing wave between the two mirrors placed at the ends
of the laser tube. In this way the wavelength of the laser light is well defined, i.e.
monochromatic. In a very different example, in the realm of quantum mechanics,
the discrete energy levels of atoms can be thought of as the standing-wave solutions
of the Schrodinger equation.