Resonance — a spatial audio simulator in Three.js

Move a loudspeaker through a 3D room and hear exactly what distance, direction, obstacles and the room itself do to the sound. Your camera is the microphone: orbiting the view swings the sound around your head, walking away makes the room take over, and stepping behind a pillar muffles the source.

Every number on screen is the actual value driving the audio graph that frame.

npm install
npm run dev          # http://localhost:5173

Headphones strongly recommended — the binaural simulation collapses on speakers.

Bring your own audio. Drop a file onto the 3D view, or use Load a track from your computer in the Source panel — MP3, WAV, FLAC, OGG, M4A, whatever your browser decodes. It becomes the loudspeaker: set Motion to Orbit and your song circles your head, Dopplering as it goes, muffling when it passes behind the pillar. Files are summed to mono, because a loudspeaker standing at one point in a room radiates one signal; a stereo mix would leave half the image glued to your ears no matter where the source went. Nothing is uploaded — decoding happens locally in the page.

What it models

Effect	How
Distance attenuation	Inverse / exponential / linear laws. At rolloff 1.0 the inverse law is exact physics: −6 dB per doubling.
Directivity	Web Audio's inner/outer cone model. The lobe in the scene dims with the gain you are actually receiving.
Propagation delay	A delay line holding distance / speedOfSound. Sound genuinely arrives late.
Doppler	Emergent. Nothing computes a Doppler ratio to feed the audio — the delay line chasing a moving source resamples the signal, exactly as air does. The reported ratio is 1 − dD/dt, which is what a delay line actually produces.
Room reverberation	A synthetic impulse response built from the room's real geometry via Sabine's equation, T60 = 0.161 V / Sα. Early reflections land at the actual wall arrival times.
Direct-to-reverberant ratio	The reverb send is tapped before distance and directivity, because a room's reverberant field is roughly uniform. This is the effect that makes distance audible.
Occlusion	A bundle of rays across the head gives fractional occlusion, which sweeps a lowpass on the direct path only — reflected sound still gets through.
Air absorption	Distance-dependent treble loss. The default is realistic, which means subtle; the slider goes to 8× if you want to hear it.

The critical distance

The one idea the app is built around. The direct sound falls as 1/r; the reverberant field does not fall at all. They cross at the critical distance, rc = √(Q·R / 16π), drawn as a ring on the floor:

• Inside the ring the source dominates. You can point at it with your eyes shut.
• Outside it the room dominates. It does not get quieter as you back away — and that constancy is a large part of how your ears judge distance.

Q is the source's directivity factor — how much more intensity it puts on its axis than an omnidirectional source of the same total power. It belongs in that formula because the reverberant field is fed by radiated power, while the ring is measured against the on-axis direct sound. A cone that throws its energy forward excites the room far less than its on-axis level suggests: the default 70°/160° cone has Q = 5.07, a directivity index of 7.1 dB, which pushes the ring from 6.1 m out to 13.7 m. Drop Q and reflections stay conspicuously loud when you walk behind the speaker — the room is being driven as if the speaker radiated backwards as hard as it radiates forwards.

Press 3 for a guided walk through it.

Controls

WASD	walk (you are the mic)	Q E	down / up
↑↓←→	move the source	PgUp PgDn	source height
drag	orbit	Shift	move 3× faster
G / R	gizmo: move / turn	double-click	place the source
K	play / pause	H	hide panels
123	guided demos	?	all shortcuts

Camera panning is deliberately disabled: it would move the orbit target without moving the camera, quietly breaking the invariant that the camera is the ear.

The signal path

The bottom-left panel is the whole point. It lists every stage between the source and your ear with its loss in dB on one shared −60…0 dB scale, so the bars are directly comparable and the stage losses sum exactly to the total:

Source        Sawtooth       −3.1 dB   ██████████
Distance         12.24 m    −21.8 dB   ███████
Directivity   0.0° off-axis   0.0 dB   ██████████
Occlusion            clear    0.0 dB   ██████████
Air        tone only · LP 20.5 kHz  —
──────────────────────────────────────────────
At your ear   35.7 ms flight −21.8 dB  ████
Room (wet)          t60 1.63 s −9.1 dB ████████
Direct / room   room-dominant −12.7 dB

Air absorption has no broadband dB — it only shapes tone — so it deliberately reports no number rather than an invented one.

Architecture

src/
  physics/     Pure functions, no Web Audio, no Three.js. Fully unit-tested.
    vector · attenuation · cone · doppler · occlusion
  audio/
    AudioEngine   Node graph + per-frame spatial update + telemetry
    presets       Band-limited, seam-free looping source material
    reverb        Sabine acoustics and impulse-response synthesis
    userAudio     Decoding and mono-summing files from disk
  scene/
    Stage         Renderer, camera-as-listener, orbit + gizmo, occlusion rays
    Room · SoundSource · Annotations · motion
  ui/
    panel · signalPath · readouts · meters · onboarding · controls · format
  main.ts      One rAF loop: stage → engine → readouts

The audio graph:

source ▶ sourceGain ▶ delay ─┬─▶ air ▶ occlusion ▶ direct ▶ panner(HRTF) ─┐
                             │                                            ├▶ master ▶ limiter ▶ analyser ▶ out
                             └─▶ reverbSend ▶ convolver ──────────────────┘

AudioEngine.updateSpatial() computes the full acoustic state whether or not an AudioContext exists, so every readout is live and correct before the user has unlocked audio.

Source presets are built additively from a bounded harmonic series rather than sampled from ideal waveforms. That leaves an octave of clean headroom, so Doppler can pitch them up without folding harmonics back down as aliasing.

Tests

npm test          # 116 unit tests: physics, room acoustics, audio graph, presets, file loading
npm run test:e2e  # 47 checks driving real Chrome (starts its own dev server)
npm run build     # tsc + production bundle

The unit suite runs in Node against a mock AudioContext that records the graph topology, so it can assert things like "the reverb send is tapped before the distance gain" and "the listener's orientation is published, not just its position" — both of which were silently wrong before.

The browser suite covers what a mock cannot: that the view is actually sized, that orbiting swings the sound between the ears, that walking away attenuates the direct path while the reverberant field holds steady, that each demo does what it claims, and that the panel controls follow the demos rather than silently reverting them.

Known limitations

• Reverb is a single static impulse response per room. It does not update with listener position, so you cannot hear yourself walk into a corner.
• No precedence effect. Beyond the critical distance real ears still localise well because the first wavefront wins; here localisation degrades with the direct-to-reverberant ratio, so a very live room smears the image more than it would in life.
• Occlusion tests one obstacle, and models diffraction as a fixed broadband loss rather than as a frequency-dependent function of the obstacle's size.
• HRTF quality is whatever the browser's PannerNode provides.
• Dark theme only: the viewport is a lit 3D room, and a light chrome around it would need a second set of scene materials to not look like a picture frame.