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Section 15.4 Quality of Sound

Any vibrating object will be a source of sound. If an object vibrates at a single frequency, it will produce a pure tone of that frequency. For instance, a U-shaped tuning fork can be constructed that produces a single-frequency sound when struck. Smaller tuning forks produce higher frequencies than the larger ones.

Musical instruments produce discrete set of frequencies, the fundamental and overtones, whose frequencies are integral multiples of the fundamental frequency. When a particular key is played on an instrument, it generally produces both the fundamental frequency and the overtones. You may have noticed that the same key, for instance the middle C of piano, played on different instruments, sound different to our ears. The physical reason for the difference is the different mixture of overtones and their relative amplitudes produced in each. Different mixtures of overtones give the sound its quality. In case of musical instruments this referreed to as tone color or timber.

Not every sound produced are like those of musical instruments, such as when you drop a stone or a rubber ball on a concrete floor. Clearly you can tell whether a stone or a rubber ball hit the concrete floor due to the difference in quality of the two sound, but you cannot discern a pitch in either. Such sounds are called noise.

Figure 15.4.1. Power spectra of (a) white noise and (b) a musical instrument. Musical instruments produce fundamental and overtones while noise contains almost continuum of frequencies. The shapes of power spectra of different musical instruments are different.

The reason you are not able to find a pitch in a noise is that it usually consists of a continuous sound spectrum unlike sound of a musical note which consists of a particular frequency and its overtones. If you plot the intensity versus frequency in a noise, you will get a continuous graph, rather than spikes at the fundamental and overtones in the case of musical notes (see Figure 15.4.1). The plot of the intensity versus frequency is also called the power spectrum.