Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by nonlinearities in a system. The intermodulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies (integer multiples) of either, like harmonic distortion, but also at the sum and difference frequencies of the original frequencies and at multiples of those sum and difference frequencies.
Intermodulation is caused by non-linear behaviour of the signal processing (physical equipment or even algorithms) being used. The theoretical outcome of these non-linearities can be calculated by generating a Volterra series of the characteristic, while the usual approximation of those non-linearities is obtained by generating a Taylor series.
Practically all audio equipment has some non-linearity, so it will exhibit some amount of IMD, which however may be low enough to be imperceptible by humans. Due to the characteristics of the human auditory system, the same percentage of IMD is perceived as more bothersome when compared to the same amount of harmonic distortion.
Intermodulation is a 1966 jazz album by pianist Bill Evans and jazz guitarist Jim Hall. It is a follow-up to their 1963 collaboration Undercurrent.
Writing for Allmusic, music critic Michael G. Nastos wrote of the album: "A duet recording between pianist Bill Evans and guitarist Jim Hall is one that should retain high expectations to match melodic and harmonic intimacies with brilliant spontaneous musicianship. Where this recording delivers that supposition is in the details and intricacy with which Evans and Hall work, guided by simple framings of standard songs made into personal statements that include no small amounts of innovation... At only 32 and a half minutes, it's disappointing there are no bonus tracks and/or additional material for a CD-length reissue, but Intermodulation still remains a precious set of music from these two great modern jazz musicians."