Wvwwvvvwwvvvj Wvwmaaaaaaw

wave 2: /2 = 55 Hz mean freq: /1 + /2 = 57.5 Hz silence loudest sound silence beat frequency = 60 Hz - 55 Hz = 5 Hz, the difference between the two frequencies

Figure 7.8 Beats — an example of amplitude modulation.

silence loudest sound silence beat frequency = 60 Hz - 55 Hz = 5 Hz, the difference between the two frequencies

Figure 7.8 Beats — an example of amplitude modulation.

7.3.2 Beats

When two notes of almost the same frequency are played simultaneously, the result is a throbbing note with periodic intervals of sound and silence called beats. The effect is best demonstrated with an example, such as in Figure 7.8.

Suppose that waves of 60 Hz and 55 Hz are superimposed. The 60 Hz wave produces 6 vibrations in 0.1 seconds but the 55 Hz wave produces only 5.5 vibrations in the same time interval. If the two waves are in phase at some particular time they will be completely out of phase 0.1 seconds later. After a further 0.1 seconds they are in phase again, and so on, giving intermittent periods of sound and silence.

If several notes with almost the same frequency are played simultaneously, the bursts of loud sound are shorter than with just two notes. In between, there are fainter sounds when only some of the waves are in phase. When the number of waves is extremely large, there is only one noticeable burst of sound at a time when all the waves are in phase. The number of waves in phase at any other time becomes very small.

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