Intermodulation

An interaction between a pair of mixed signals within a circuit, such that the output contains additional frequencies, in some amount, that are the sum and difference of the frequencies of the signals in the input. Intermodulation results from non-linearities in the circuit and is usually undesirable, although some devices (notably distortion circuits) produce intermodulation as an intended result. (The “power chord”, consisting of the root and perfect fifth, when played into a distortion box intermodulates in such a way so as to add a minor third to the output; this is a fundamental characteristic of heavy metal and other forms of rock music that rely on heavily distorted guitar.) See also ring modulation.