BEATS
The interference of two sound waves is also apparent when the frequency of one wave is near to that of the other. The interaction of the waves causes a periodic change in intensity (loudness) of the sound. This can be explained by adding graphically the amplitudes of the two waves at every instant of time, (Fig. 1.22). The resultant amplitude is maximum when the waves are in step and a minimum when they are out of step. If these frequencies differ by (say) five vibrations per second then the waves get in and out of step five times per second and the number of pulses or beats heard per second is five. In general
Up to a beat frequency ...
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