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If you have ever tuned a guitar by ear, you must have used beats to know when a string is tuned to the previous one. When we hear two sounds that are very close in frequency and similar in amplitude, the result is that we hear just one sound the periodically changes its volume, hence the impression of 'beats'. The frequency of these beats is equal to the difference in frequency of the two sounds. So, if both a 440Hz and a 450Hz are playing, we will hear only one sound whose beats will take place 10 times per second (10Hz).

Let's try:

In the picture below, we can see the sum of the two waves: