What Is Intermodulation Distortion (IMD)?

Intermodulation Distortion is an amplifier or pre-amplifier specification that quantifies the non-harmonic frequencies added to an input signal. It can also exist for other audio components like speakers and CD or Blu-ray/DVD players, and it can be one of the most difficult music-related distortions to mitigate in audio systems.

How Intermodulation Distortion Works

Similar to Total Harmonic Distortion, Intermodulation Distortion is measured and represented as a percentage of the total output signal. And 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 interacts with one another, 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 their 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.

Why Intermodulation Distortion Is Important

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

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

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