The periodic rise and fall of the loudness of the resultant sound wave produced by the superposition of two progressive sound waves of equal amplitude but of slightly different frequencies are called beats.
Characteristics of Beats:
1. Two progressive waves with equal or nearly equal amplitudes, but differing slightly in frequencies are allowed to superpose. The resultant amplitude rises and falls periodically and beats are formed.
2. Rise and fall of resultant amplitude result in rise and fall of intensity. This corresponds to periodic increments and decrements of the loudness of sound waves and the brightness of light waves.
3. If the resultant intensity goes to a maximum of n times per second, then n is called the number of beats per second or beat frequency.
4. The persistence of hearing of the human ear is 1/10 s. This means that if more than one sound of the same type comes to our ear within 1/10 s, we can’t distinguish them. So, if the beat frequency is more than 10 Hz, we can’t feel the effect of the rise and fall of the intensity. We may conclude that to feel the effect of beats, the two superposing waves should have a frequency difference of less than 10 Hz.
5. For sound waves, two sources having a frequency difference of less than 10 Hz are often realized in practice. But for light waves, individual frequencies are of the order of 1015 Hz in the visible range. So, a frequency difference of less than 10 Hz is practically impossible to observe. For this reason, the formation of beats is a phenomenon useful for sound waves but has no physical significance for light waves.