Ring Modulation:

When two frequencies (say two sine tones) are fed into each input of a ring modulator what we hear is not the frequencies as the primary tones but the sum of the tones and the difference tone.

Example: 300Hz and 100hz sine tones are fed in, what we hear is primarily a 400Hz and 200 Hz tone. However there are other tones of the 'second order' which colour the sound and are much fainter: e.g. 300Hz+ 400Hz= 700Hz (secondary summation tone) 300Hz- 200Hz= 100HZ (secondary difference tone) From two sine tones, eight tones of varying intensity are produced. This produces a rich effect, which was of interest to early electronic composers as a way of 'enlivening' oscillator produced sound. It continues to be used by contemporary composers. More complex signals such as square waves can be fed into the ring modulator, but the danger is that distortion may result! A piano makes an interesting inputted 'carrier' signal which is modulated by the other (internal modulation) signal.