Spectrum, Sound

Spectrum, Sound 

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.

Figure 1. Spectrum of a musical sound

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.

Figure 2. Spectrum of a damped vibration

Acoustic noise, single pulses, and damped sounds have continuous spectra (Figure 2).

Figure 3. Sound spectrum of a keyboard instrument

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.