What Is Intermodulation Distortion (IMD)?

When it comes to maintaining audio purity, there are a number of different elements to watch out for and take into consideration. Although lesser-known to many, Intermodulation Distortion (abbreviated as IMD) can be quite the rogue when it rears its ugly, cacophonous head. Unlike some other types of music-related distortion, Intermodulation Distortion is very obvious to the ear and can be one of the most difficult to mitigate in audio systems.

Intermodulation Distortion is often found as an amplifier or pre-amplifier specification (but can exist for other audio components like speakers, CD/DVD/media players, etc.) that quantifies the non-harmonic frequencies added to an input signal. Similar to Total Harmonic Distortion, Intermodulation Distortion is measured and represented as a percentage of the total output signal. And just as with Total Harmonic Distortion, lower numbers are better for improved performance.

Intermodulation Distortion can occur when two or more signals are mixed through a non-linear amplifier device. Each of the tones interact with each other, producing altered (or modulated) amplitudes. This results in the formation of frequencies – often referred to as "sidebands" – not present in the original signal. Since these sideband frequencies pop up at the sum and difference of the original tones, they are considered non-harmonic and highly undesirable due to the unmusical nature.

To illustrate, say that instrument one plays a note and produces a fundamental frequency of 440 Hz. Harmonic frequencies (integer multiples of the fundamental) for instrument one occur at 880 Hz, 1220 Hz, 1760 Hz, and so on. If an amplifier creates a non-harmonic frequency of 300 Hz along with the fundamental frequency of 440 Hz, a third frequency of 740 Hz will be reproduced (440 Hz + 300 Hz), and 740 Hz is not a harmonic of 440 Hz. Thus, it is termed Intermodulation Distortion because it is between harmonic frequencies.

Since Intermodulation Distortion is discordant (not harmonic), it's a more meaningful measurement. And when present, it's far easier to pick up on by ear than harmonic distortion, since harmonics are generally present in audio signals anyway. But at lower volume levels and/or with more simple music, Intermodulation Distortion may not be so noticeable. Separate tones can still be clearly heard. But once volume increases to a point where non-linearity happens within the amplifier, the alteration and unwanted generation of frequencies muddies or blurs the original signal.

This effect is also compounded with more complex music genres (e.g. orchestra) where there is greater interaction between all the frequencies. And the result can be the creation of a noise floor that effectively erodes sonic detail and precision. At best, Intermodulation Distortion leads to dull-, veiled-, or lifeless-sounding music. At worst, everything sounds harsh and/or grossly distorted.

However, as with Total Harmonic Distortion, Intermodulation Distortion is usually so low that it is imperceptible. Most modern amplifiers are designed well enough to make Intermodulation Distortion quite insignificant. Just remember that your ears are the better judge of sound quality, so don't judge components solely by the specification for Intermodulation Distortion.