Since sound is a wave motion, it can be focussed by reflection (and less easily by refraction), and interference can
FIG. 6.1.1 Sound sources. A point source at S produces a calculable intensity at a. The sound waves can set an elastic membrane or partition (like a large window) at W into vibration. This large source can produce roughly planar sound waves, which are radiated outward with little change in form but are distorted and dispersed as they pass the solid barrier B.
occur, as can standing wave patterns. These effects are important in noise control and in auditorium acoustics. Another wave-motion phenomenon, the coincidence effect, affects partition behavior.
When two wave forms of the same frequency are superimposed, if they are inphase, they add and reinforce each other; while if they are of opposing phase, the resultant signal is their difference. Thus, sound from a single source combined with its reflection from a plane surface can produce widely varying sound levels through such interference. If reflective surfaces are concave, they can focus the sound waves and produce high sound levels at certain points. Dispersion often partially obscures these patterns.
Sound from a single source can be reinforced by reflection between two walls if their separation is a multiple of the wavelength; this standing-wave pattern is described by Figure 6.1.2.
These phenomena are important in auditorium design, but they cannot be ignored in noise control work. Reinforcement by the addition of signals can produce localized high sound levels which can be annoying in themselves and are also likely to produce mechanical vibra-tions—and thus new, secondary noise sources.
Random noise between parallel walls is reinforced at a series of frequencies by the formation of standing waves; this reinforcement partially accounts for the high noise level in city streets.
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