Beats
If two sounds, vibrations, or electrical signals have nearly the same frequency and they are linearly added together, their combined amplitude will fluctuate up and down at a rate equal to the difference frequency between them. This phenomenon is called “beating”, and is very commonly seen in practice. For instance, a musician tunes his instrument by listening for beats between two tones that are nearly the same pitch.
A beating waveform looks very much like amplitude modulation, but it is actually completely different. A spectrum analysis of beats produces only the two frequency components that are combined -- there are no new frequencies such as sidebands present. It is easy to confuse beats with amplitude modulation, but a spectrum analysis will show the difference. In general, beats are benign, and do not imply faults in machines. For example, the sound of two similar machines running side by side at slightly different speeds will often produce audible beats. This is simply the sounds made by the machines combining in air to produce the amplitude fluctuations.
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