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ing partition even though its amplitude is only a few micro inches.

REFLECTION, DISPERSION, ABSORPTION, AND REFRACTION

Sound traversing one medium is reflected when it strikes an interface with another medium in which its velocity is different; the greater the difference in sound velocity, the more efficient the reflection. The reflection of sound usually involves dispersion or scattering.

Sound is dispersed or scattered when it is reflected from a surface, when it passes through several media, and as it passes by and around obstacles. Thus, sound striking a building as plane waves usually is reflected with some dispersion, and plane waves passing an obstacle are usually somewhat distorted. This effect is suggested by Figure 6.1.1. The amount of dispersion by reflection depends on the relationship between the wavelength of the sound and the contour of the reflecting surface.

The absorption of sound involves the dissipation of its mechanical energy. Materials designed specifically for that purpose are porous so that as the sound waves penetrate, the area of frictional contact is large and the conversion of molecular motion to heat is facilitated.

Sound waves can be refracted at an interface between media having different characteristics; the phenomenon can be described by Snell's law as with light. Except for events taking place on a large scale, refraction is usually distorted by dispersion effects. In the tracking of seismic waves and undersea sound waves, refraction effects are important.

In engineering noise analysis and control, reflection, refraction, and dispersion have pronounced effects on directivity patterns.

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