Compression
Touch response. This measurement steps a single note from whisper to roar and records what comes back at each step. The result is the curve your picking hand already knows: where playing harder stops getting louder.
What it shows
The dashed line is a pedal that doesn’t compress: 1 dB in, 1 dB out. Where the measured curve flattens away from it, playing harder stops getting louder — that’s the squash you feel under your fingers. The knee marks where that starts; the cleanup point is the input level below which distortion falls under 1% (roll your guitar volume down to get there). The shaded band is roughly where guitar pick attacks land at sane interface gain — it shifts with your gain knob.
Levels here are time-averaged (RMS, in dB) — not the instantaneous ± voltages of the Transfer Curve.
How to read it
The knee is where the squash begins. A knee below the guitar band means the pedal is always compressing your playing; a knee above it means you only touch the ceiling on hard attacks. The value is fitted — two line segments through the whole measured curve, with the breakpoint free to land between probe levels — and its displayed precision follows what the fit actually resolves (tenths of a dB for a crisp knee, whole dB for a smeared one).
“Knee ≤ …” is a bound, not a reading. Some pedals — high-gain fuzzes especially — are already compressing at the quietest level the measurement probes. Their true knee lies below the measured range, and no number in range can locate it, so the app reports the honest upper bound (“knee ≤ −50 dBFS, below the measured range”) at low confidence instead of dressing the lowest probe level up as a point estimate. Summaries phrase these knees “at or below”.
The slope after the knee is the character of the limit. A slope near 1 is clean bleed-through — pick attack survives (screamer-style). A slope near 0 is a true limiter: everything past the knee comes out the same. The project’s reference Rat measured a slope of 0.00 over its top 15 dB — a brick wall, which is exactly the “endless sustain, flattened dynamics” Rat feel.
The cleanup point is the roll-your-volume-down number: below it, THD drops under 1 % and the pedal is effectively clean. A fuzz that “never cleans up” simply has no crossing in range.
The companion THD curve
Total harmonic distortion at the probe note as drive increases. A pedal that “cleans up” drops below 1% at low levels; a fuzz that is never clean stays high across the whole range.
Read the two together: the level curve says what your volume does, the THD curve says what your tone does at the same moment. Touch-sensitive pedals show a wide region where both are in motion.
What’s musically meaningful
Compression here isn’t a compressor pedal’s pumping — it’s the feel dimension of drive. Where the knee sits relative to your pick attack decides whether dynamics live or die; the slope decides whether digging in adds edge or just adds sameness; the cleanup point decides whether the guitar’s volume knob is a second channel or just a mute.
Hear it
Knee walk plays a run of plucks stepping in level uniformly across the whole measured range, through the pedal’s model and through a clean bypass. Below the knee the two lanes track each other; past it the pedal’s plucks stop getting louder and start getting thicker — limiting, as a feeling instead of a slope readout. While it plays, a pulsing marker sits on the touch-response curve at each pluck’s attack level — the level the curve is measured from — so you can watch the plucks step across the knee as you hear them. Each marker holds its position and fades out at the note’s own decay rate; the next pluck moves it to its new spot on the curve. The comparison is loudness-matched as a whole, so the flattening you hear is the curve, not a volume trick. Headphones or full-range monitors recommended — see Listening conditions.
▶ Move a knee yourself: swap Si for LED diodes in Simulation mode
Common misreadings
The guitar band is approximate. It marks where pick attacks land at sane interface gain staging — your interface gain knob moves your playing within (or out of) it. Comparisons between pedals at the same rig are fair; absolute claims need the level calibration taken with salt.
The probe is one note (default 220 Hz). Circuits with strong tone filters compress differently across the neck; run the probe at another pitch before generalizing.
Distortion rising as you get quieter would look like an error on this chart but is real behavior for some circuits (crossover distortion in certain amps). For pedals, suspect gating or a dying battery instead.
Try it on a record: open a Compression measurement. To judge two devices’ touch response against each other, compare two Compression records on shared axes — solo pages autoscale per record, which flatters flat curves.