What is intermodulation distortion (IMD) and how does it affect sound?

Intermodulation distortion (IMD) happens when two or more signals of different frequencies pass through a nonlinear system and interact to create new frequencies that weren’t in the original signal.

Here’s the mechanism.

Doing the math

A perfectly linear system just scales its input:

output = A × input

But real components (amplifiers, mixers, speakers, even air at high SPL) have a transfer function with higher-order terms:

output = A·x + B·x² + C·x³ + …

This is where things start getting spicy.

When you feed in two tones, say f1 and f2, those nonlinear terms do something nasty:

• The squared term (x²) creates sum and difference frequencies: f1 + f2 and f2 - f1.
• The cubed term (x³) creates the real troublemakers: 2f1 - f2 and 2f2 - f1.

These third-order products are the villains. They sit close to the original frequencies, which means you can’t filter them out without killing your actual signal.

Why it sounds dirty

Feed 1000 Hz and 1100 Hz into a slightly nonlinear amplifier and you’ll get extra junk like:

• 900 Hz (2·1000 - 1100)
• 1200 Hz (2·1100 - 1000)

Plus:

• 100 Hz (difference)
• 2100 Hz (sum)

Now imagine this with music or voice. Thousands of frequencies interacting, every pair generating its own IMD products. The result is a harsh, muddy, “dirty” sound.

And here’s the key insight:

• Harmonic distortion is musically related, so it sometimes sounds “pleasant”.
• IMD is not musically related, so it sounds like grit, chaos, mess.

That’s why IMD is way more offensive to the ear.

Where it shows up in your workflow

Pure digital math is perfectly linear. So in your DAW itself, no IMD is happening.

But the moment you load anything that emulates analog:

• Tape emulation
• Tube preamp
• Saturator
• Compressor with “character”

you are introducing nonlinearity on purpose. Which means you are adding IMD on purpose.

And that’s the whole game. That “analog flavor” you love, that thickness, that vibe, comes with a bit of controlled chaos.