How is a wave beat characterized?

Answer 1

If two waves are simply added together, the result is the same two waves. If two waves are combined together in a non-linear fashion, then they are said to beat together. If the waves have frequencies f1 and f2, the result of a non-linear combination is f1, f2, f1+f2, f1-f2. If the combining process is to multiply one wave by the other, the above is the only result. However, if the combining is done in any other nonlinear way, for instance by passing the waves through anything which will distort them, then the result will include the above, but also multiples of f1, multiples of f2, and the sums and differences of all the multiples.