Diffraction (Fig. 53-3) is the physical process by which sound passes around obstructions and through very small openings. Any point on a sound wave can establish a new wave when it passes an obstacle. When most of the wave is blocked, the portion that gets through a small opening starts a new wave. A small hole, there-


Figure 53-3 Diffraction.

fore, can block long wavelengths (low frequencies) more than short ones (high frequencies).

Sound diffracts around or over a barrier. The best location for a barrier is either very close to the source or close to the receiver. The worst position is halfway between source and listener. A massively thick barrier is only slightly better than a moderately thick one, so there is a practical limit to thickness. Absorptive material on the source side of the barrier will reduce noise reflected back to the source, but will not help the receiver much.

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