Chord Roughness (Chord IMD)

Why chords get mushy. Distorting one note adds harmonics — octaves and fifths above it, still musical. Distorting two notes does something uglier: the nonlinearity multiplies them together, inventing tones at sums and differences of their frequencies. Those intermodulation (IM) products mostly land outside the key, and they — not the harmonics — are the mush, growl, and fizz of a distorted chord. No single-note measurement can show this; this one plays intervals on purpose.

What it shows

Distorting one note adds harmonics — octaves and fifths, still musical. Distorting two notes also multiplies them together, creating sum and difference tones (f2−f1, 2f1−f2, …) that mostly land outside the key. The blue stems are the notes you played; everything else is invented. Even-order products (orange) sit far from the notes and read as growl or thickness; odd-order products (aqua) land close to the notes and read as sour, clashing grit. The louder and less pitch-aligned a stem, the uglier the interval sounds — this is why high-gain players move to power chords.

The spectrum sits on a musical pitch axis: every invented tone is placed at the note it lands on, labeled with its deviation in cents from the nearest real pitch. The axis window and tick unit follow the app-wide Range and Unit selectors — see Harmonic Distortion.

Chord Roughness view: two tall blue stems for the played notes A2 and E3, surrounded by orange even-order and aqua odd-order product stems on a pitch axis, with growl and sourness summary tiles — The bundled Fuzz-ish simulated rig fed a power chord (A2+E3): strong even-order products (orange) land on or near consonant pitches — 2f1 on A3, f1+f2 near C♯4, f2−f1 on A1 — thickness rather than dissonance, which is why fifths survive high gain.from sim-imd-fifth.json

Reading the product list

Each row is one invented tone: its recipe (2f1−f2 means twice the low note minus the high note), the pitch it lands on, and its level below the notes you played (dBc — 0 dBc would be as loud as the louder note). Rows within a few cents of a real pitch are the lucky ones; the rest fall between the frets.

Modes

Musical interval (the differentiator): pick two notes — a power chord’s fifth, a major third — and see exactly what your pedal makes of them. Measure the same pedal on E2+B2 and then E3+G♯3 and the “thirds sound bad through dirt” folklore turns into visible, labeled spikes.
SMPTE-style (110 Hz + 1.3 kHz, 4:1): the standard lab protocol, adapted to guitar-relevant frequencies — good for comparing against published measurements.
CCIF twin tone: two close, equal tones; the difference products fall low and isolated, making subtle IMD easy to spot.

What’s musically meaningful

Fifths are forgiving because their IM products land on or near the harmonic series the two notes already share — the project’s reference Rat, fed a power chord, put its strongest product (f1+f2) almost exactly on the just major third above the root: extra thickness, not dissonance. Thirds and sevenths scatter products between the frets. That’s the physics behind arrangement wisdom every guitarist absorbs: power chords for the gain channel, triads for the clean one. How much a pedal punishes intervals — its IMD level at matched drive — is a real, comparable character trait: “chewy vs. fizzy” made measurable.

Hear it

The Hear-it panel is the whole argument in four buttons, loudness-matched: the clean interval, the distorted interval, the products only — every invented tone isolated from the notes and their harmonics — and distorted minus products, the same distortion with the mush surgically removed. Play a major third and toggle: the products-only lane is the clatter you could never quite point at; the minus-products lane is how polite the same pedal would be if physics allowed it. The panel states its interval (“A2 + E3 — from this record”) and a picker swaps it for any §2.3 musical interval on the same root — thirds mush, fifths mostly don’t, and now you can hear exactly that through the same measured model (the audio re-renders on change). Headphones or full-range monitors recommended — see Listening conditions.

A major third (E3+G♯3) through the bundled simulated rig's measured model.

Listen for: Under the two notes there's a rough, beating undergrowth that neither note explains — that's the intermodulation, about to be isolated below.

The same render with everything except the invented IM products masked away.

Listen for: Only the undergrowth remains: a dissonant, clattering chord of tones the pedal invented. Through a clean bypass this lane is near-silence — the products are made by the distortion, not the notes.

▶ Hear this in PedalScope

Common misreadings

IMD is not THD. Two pedals with identical single-note harmonic recipes can treat chords differently. Don’t extrapolate chord behavior from the Harmonic Distortion; this measurement exists because you can’t.
dBc is relative. Products are quoted below the played notes’ level; a −40 dBc product at bedroom volume and at stage volume are the same measurement. What changes product levels is drive, so compare at matched drive levels.
Near-pitch products aren’t harmless — a product 8 cents off the major third still beats against fretted notes. “Lands on a pitch” is better than “lands between frets”, not a free pass.

Try it on a record: open a Chord IMD measurement.