Intermodulation — why chords mush
Single notes through heavy distortion sound glorious. Full chords through the same pedal can collapse into a fighting, beating mess — “mush”. The mechanism has a name, intermodulation, and once you see it you can predict which intervals survive a given amount of dirt. The Chord Roughness chart measures it; this page explains it.
One note versus two
Distortion is a nonlinearity: output is a bent function of input. Feed a bent function one note and you get that note’s harmonics — musically related overtones, all multiples of the same frequency. That’s the pleasant kind of new content.
Feed it two notes at once and something extra happens: the bending multiplies the notes against each other, creating combination tones at the sums and differences of their frequencies (and of their harmonics’ frequencies). Play E3 (165 Hz) and G♯3 (208 Hz) — a major third — and the pedal invents tones at 43 Hz (the difference), 373 Hz (the sum), 122 Hz, 251 Hz, and on up the ladder. Almost none of those are notes in any key you’re playing. That inharmonic undergrowth is the mush.
Why fifths survive and thirds don’t
The intermodulation products of an interval land in tune only when the two notes’ frequencies form a simple ratio:
A perfect fifth is 3:2. Its products (differences, sums, and their harmonics) land on multiples of a common fundamental one octave below the low note — musically, the products reinforce the chord. This is why a power chord (root + fifth) through a wall of gain sounds huge and coherent instead of dirty.
A major third (5:4) generates products a musical third-party everywhere — near notes, but not on them, with slow beating between close neighbors. The ear reads that beating texture as roughness.
Anything more complex (minor seconds, extended voicings) fares worse in proportion.
The practical rule every metal player discovers empirically: the dirtier the tone, the simpler your intervals want to be. Now it’s a measured rule — the app’s two-tone measurement drives your pedal with an interval and reports exactly how much energy lands outside it.
What to do with it
Voice for the gain. At high drive, prefer roots, fifths, octaves; save thirds for lower-gain passages or higher registers (products fall further from the notes when the notes are further apart in Hz).
Compare pedals on chords, not just leads. Two pedals with similar single-note harmonic recipes can differ sharply in two-tone behavior — that difference is the one your rhythm sound lives with.
Hear it isolated. The Chord Roughness page’s Hear-it panel can play the products alone — the sound of everything the pedal invented — and the chord with the products removed, which is roughly what a perfectly polite pedal would do.
▶ Feed the virtual Rat a third vs. a fifth in Simulation mode
For the curious: where the products come from
Expand the nonlinearity as a polynomial y = a₁x + a₂x² + a₃x³ + …, and feed it x = sin(2πf₁t) + sin(2πf₂t). The x² term produces components at f₂−f₁, 2f₁, 2f₂, f₁+f₂; the x³ term adds 2f₁−f₂, 2f₂−f₁, 2f₁+f₂, 2f₂+f₁, and so on — order k mixes every combination |mf₁ ± nf₂| with m+n ≤ k. For f₂/f₁ = 3/2, every such combination is a multiple of f₁/2: the products stay on a harmonic grid. For 5/4 the grid is f₁/4 — four times denser, mostly non-chord tones. The two-tone measurement reports the measured power in these product families versus the notes themselves.