The Hearing Machine
An acid box, a rack of open-source Eurorack modules, a step sequencer, a mixing console, a looper and an arranger, all in one machine. Click any output jack, then any input jack or knob, to run a cable. Everything you play goes through a model cochlea, so you can watch your inner ear respond.
The ear71 places along the basilar membrane · what the whole machine sounds like to your inner ear
Play a track in another Chrome tab (a jungle tune, say), press this, pick that tab and tick “Share tab audio”: the ear shows what it looks like. Works when this page is opened on its own (localhost).
Mix metersLoudness the way streaming services measure it, peaks, stereo and the spectrum of exactly what you hear
LUFS is how loud a track feels, the number Spotify and Apple Music use: they turn every track to about −14 LUFS, so louder doesn't win there; club masters of melodic house sit around −8 to −6 LUFS short-term in the drop. Keep peaks under −1 dB. Integrated is the whole song since Reset (press Reset, then play the song through). Stereo near +1 is close to mono, around 0 is very wide; below 0 parts cancel out on a club's mono speakers. The spectrum shows the mix right now (bright line) and its average over the last few seconds (filled), and the bands below compare each range with the mix's average, per octave: a balanced mix reads near 0 dB in all five; a big plus in Sub or Bass means the low end is heavy, a big minus in Air means it's dull. The picture is tilted 4.5 dB per octave, the usual slope on mixing analysers, so a balanced mix looks roughly level; most club tracks still read a few dB up in Sub and Bass.
MasteringThe last stage: glue, drive, width and the limiter, and how loud the finished track should be
Master is the listening volume. In order, after the Console: the master EQ (on the Console), Glue (a gentle compressor that pulls the mix together), Drive (warm saturation), Width (the stereo sides: 100% is as mixed, more is wider; the middle, where kick and bass sit, never changes), then the limiter, which never lets a peak above the Ceiling. Pick a starting point, play the loudest part of the song and press Match it: it sets this patch's level so the track lands on the target, measured on the short-term meter above. The settings and level are saved with the patch and the song.
ArrangeScenes and song · turn loops and patches into a whole track
Store saves everything the rack is doing (every knob, cable and pattern, plus which looper tracks are playing) into a scene; Go switches to it on the next bar. In the song, each section plays a scene for a number of bars, then moves on exactly on the bar line: intro, build, drop, breakdown. Scenes and the song are kept in this browser.
LooperEight tracks · each records from the next bar line, then loops in time · build a whole track layer by layer
Load a patch, press ● Rec on a track and it records from the next bar for the length you chose, then plays it back on a loop. Load another patch (or change this one) and record the next track while the first plays: drums, then bass, then chords. + Dub adds one more pass on top of a track. While any track holds a loop, loading a patch keeps the tempo so everything stays in time. Loops live in this page only: they are gone when you reload.
KeysPlay notes into Keys with the mouse, your computer keyboard (A to K) or a MIDI keyboard
Sound devices and modulesDrums and synths first, then the effects, then the modules that move things
How patching worksThree kinds of cable · anything into anything · every route tested
Any sound output into an audio input (or the master). Inputs add up, so two cables into one input mix. Amount sets the level. Feedback loops are allowed: each hop runs 2.7 ms late, the way a real patch cable would if sound were slow.
Any output into a knob's small jack. The knob stays where you set it and the cable swings it: amount 100% moves it across its whole range. Pitch knobs follow 1 volt per octave, so Chance or Keys at 100% play exact notes of their scale. Selects and switches have no jack; set those by hand.
Any output can fire a trigger, like on a real Eurorack: it fires when the signal rises above 0.3 and re-arms once it drops below 0.1. The system clocks give 16ths, beats, offbeats, the backbeat (2 and 4) and the bar; Chance's t1/t2 and Keys' gates are the other tidy ones, but an LFO or a drum works too.
Some knobs only matter in a mode, just as on the modules: Voicebox Decay and Colour need a trigger and an engine without its own envelope (not Vowels and speech); Segments D, S and R need the AD or ADSR preset; Modal's bow and blow tone need Bow or Blow turned up; Cross-mod's Note needs its internal carrier; Keys' octave range needs the arpeggiator on; the Acid bass's Slide needs a gate on its slide input.
Plug a USB keyboard in before opening the page, press Connect a MIDI keyboard and allow MIDI. Works in Chrome, Edge and Firefox on a computer; Safari and iPhone/iPad browsers do not support Web MIDI, so use the on-screen keys or your computer keyboard (A to K) there.
Every possible single cable was patched on the real engine and measured: 192,134 routes across all 50 modules (last checked 19 October 2026), none broken, none running away.