Intermodulation distortion (IMD) refers to the unwanted extra frequencies that are created when two or more audio signals interact in a non-linear device like an amplifier.
For example, if you send a 100Hz and 6kHz sine wave into an amp, intermodulation may generate frequency components at 5.9kHz (6kHz – 100Hz) and 6.1kHz (6kHz + 100Hz). These new signals are byproducts of the amplifier’s slight nonlinearities.
In a live sound system, IMD is typically caused by overdriving amplifiers, mixing signals at excessive levels, or using low-quality gear. It leads to muddiness and a lack of clarity in the audio.
IMD is often described in terms of the order of distortion. Second-order IMD refers to the two frequencies directly above and below the interacting frequencies (like 5.9kHz and 6.1kHz). Third-order IMD components are at 2F1 ± F2 (so 200Hz and 5.8kHz in this example). Higher orders of IMD are usually less audible.
To minimise IMD, sound engineers should:
- Avoid clipping amps by keeping levels below maximum
- Use higher-quality amplifiers and pro audio gear
- Allow enough amplifier headroom for transient peaks
- High-pass filter sub-bass to reduce low-frequency overload
- Strategically limit/compress certain signals like the kick drum
Controlling levels and gain staging attentively helps reduce intermodulation distortion in a live sound mix.