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the set of simple harmonic waves into which a sound wave can be resolved. The spectrum expresses the frequency composition of the sound and is obtained by analyzing the sound. A sound spectrum is usually represented in a coordinate plane where the frequency f is plotted along the axis of abscissas and the amplitude A, or intensity, of a harmonic component with a given frequency is plotted along the axis of ordinates.
Pure tones, sounds with a periodic wave form, and sounds resulting from the superposition of several periodic waves have line spectra (Figure 1). Musical sounds, for example, have such spectra; a musical sound’s timbre depends on the sound’s spectrum.
Acoustic noise, single pulses, and damped sounds have continuous spectra (Figure 2).
Composite spectra are characteristic of the noise produced by some mechanisms. For example, the rotation of a motor may yield separate frequency components that are superposed on a continuous spectrum. Such spectra are also characteristic of keyboard musical instruments (Figure 3) that have, especially in the upper register, a noise coloration due to the impact of the hammers.