12. Audio Engineering & Practice · Concept 12 of 12
Ugly new tones that appear when two different sounds clash inside overloaded gear and breed unrelated notes.
Two clean tones hit a clipping stage and breed unrelated sum and difference tones that clash with the music.
What it is
Ugly, unrelated new tones created when two or more frequencies clash inside overloaded gear.
Key facts
- IMD = sum and difference tones; two inputs f1 and f2 breed f1-f2, f1+f2, 2f1-f2, 2f2-f1, and more
- Third-order products 2f1-f2 and 2f2-f1 are the nasty ones: they land close to the originals and are hard to filter out
- Example: 1,000 Hz + 1,100 Hz overdriven makes a 100 Hz difference tone plus junk at 900 Hz and 1,200 Hz
- Difference frequency formula: f_diff = f2 - f1 (higher tone minus lower tone), in hertz
- Unlike harmonic distortion (THD), IMD tones are NOT musically related to the notes, so they clash and sound harsh fast
- Specs are quoted as IMD or THD+N in percent (%); hi-fi power amps aim under 0.1%, often under 0.01%
- Standard test SMPTE IMD uses 60 Hz + 7,000 Hz mixed 4:1; CCIF twin-tone uses 19 kHz + 20 kHz
- Clipping is the trigger: drive a stage past 0 dBFS digital or its rail voltage and IMD explodes
- +6 dB = double voltage; +3 dB = double power; -3 dB = half power (the corner point); +10 dB sounds about twice as loud
- Audible band 20 Hz to 20,000 Hz; difference tones often dump energy into the low-mids where they muddy a mix
How it works
- Two different frequencies (f1, f2) enter the same gain stage at once.
- The stage is overdriven past clipping so it stops responding in a straight line.
- That non-linearity multiplies the two tones together, not just adds them.
- Out pop brand-new sum (f1+f2) and difference (f2-f1) tones.
- Higher-order products (2f1-f2, 2f2-f1) crowd in near the originals.
- These unrelated tones clash with the music: harsh, gritty, muddy.
Real examples
- Full festival mix slammed into a clipping amp: kick plus vocal breed low-mid mud no EQ can dig out.
- Two synth notes 1,000 Hz and 1,100 Hz overdriven create an audible 100 Hz difference rumble.
- Cheap powered speaker run at the limiter wall: cymbals plus bass turn fizzy and harsh.
- A DI box clipping its input makes a clean acoustic guitar sound gritty and unmusical.
How it helps in live sound
- Keep input gain so peaks sit around -12 to -6 dBFS; never let the red clip LED light.
- Mind gain staging at EVERY stage: mic pre, channel, bus, amp; one clipped stage breeds IMD for the whole mix.
- Watch amp and powered-speaker limiter LEDs; if they pin, turn down, do not push through.
- Dense full mixes reveal IMD worst, so set headroom on the loudest chorus, not the quiet intro.
- If a loud PA sounds harsh or muddy but EQ won't fix it, suspect clipping and pull levels before reaching for EQ.
- Use a -10 or -20 dB pad on hot sources (kick mic, DI) to stop the front end clipping.
Everyday analogy
Like two people shouting over each other so badly you start hearing a garbled third voice that belongs to neither of them.
Watch out
Myth: distortion just adds nice warm harmonics. Truth: IMD adds UNRELATED clashing tones (sum/difference), so it sounds nasty and muddy, not warm.
Fun fact
Your own ears generate intermod difference tones too: play two loud high notes and you can literally hear a phantom low note no instrument is playing.
Key takeaways
- IMD only appears when gear is overdriven past linear; keep levels sane and it vanishes.
- It creates sum and difference tones unrelated to the music, unlike harmonic distortion.
- Third-order products (2f1-f2, 2f2-f1) sit near the originals and are the hardest to fix.
- Difference tones often dump energy into the low-mids, the source of 'muddy loud PA'.
- Fix is gain staging and headroom, not EQ; aim peaks near -12 to -6 dBFS.