Question: What is Intermodulation Distortion?
Answer:
Intermodulation Distortion, or IMD is an amplifier or pre-amplifier specification that measures non-harmonic frequencies added to the signal. IM distortion is the result of two or more signals mixing together that are not harmonic frequencies. These signals mix to create additional non-harmonic frequencies that are undesirable. Using the same middle C note example from above, if an amplifier creates a non-harmonic frequency of 300Hz along with the fundamental frequency of 440Hz, a third frequency of 740Hz will be reproduced (440 + 300), and 740Hz is not a harmonic of 440Hz. Thus, it is termed intermodulation distortion because it is between harmonic frequencies.
How is it Expressed?
IMD is measured the same way as THD, as a percentage of the total output signal. Like THD, lower numbers are better.
Why is it Important
Like THD, IMD is usually so low that it is imperceptible. However, since IMD is not harmonic (it is discordant) it is a more meaningful measurement, but in general, IMD is low enough and most amplifiers are designed well enough to make IMD insignificant. Once again, your ears are the better judge of sound quality, not the printed IMD specification.
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