Harmonic Tension Engine

Feb 25, 2026 · live

Harmonic Tension Engine is a minimalist browser instrument for building rich, beating sine textures out of embarrassingly simple ingredients.

It’s basically:
16 free‑running sine oscillators arranged as 8 detuned pairs, then given just enough structure (scale/key quantization, note distribution, and some modulation) to feel musical while still drifting like it doesn’t respect your plans.

It runs as a single HTML file with no backend. So you can host it anywhere static, or just open it locally and ruin your evening.

Try it

What it does

Each “voice” is a pair of sine oscillators tuned slightly apart around the same center note.
That tiny offset creates beating: the pulsing/interference effect you get when close frequencies rub against each other.

Stack 8 pairs across a scale and you get a controllable drone machine that can sound:

calm and tonal
alive and breathing
dissonant and shifting
weird in the good way

Key features (real ones, not marketing ones)

1) 8 sine pairs (16 oscillators)

8 independent voice pairs
16 total free‑running oscillators (organic phase interaction)
per‑pair amplitude slider (main mix control)

2) Global detune / beating control

A global detune control sets how far apart each pair’s two oscillators are (in cents):

subtle movement: ~1–5 cents
obvious beating: ~5–15 cents
chorus-ish weirdness: 15+ cents

3) Scale quantization + key

Each pair’s center note is quantized to a chosen scale + key:

major/minor modes, pentatonic, chromatic, etc.
keeps the drone organized while detune adds motion

4) Base note + spread (smart note distribution)

base note sets the leftmost pair
spread sets how far the rightmost pair extends
intermediate pairs are distributed across the selected scale

This makes it easy to go from tight clusters to wide harmonic spreads.

5) Per-pair note offset

A per‑pair semitone offset lets you break symmetry locally:

introduce tension
double notes
shift modal color
make it “wrong” on purpose

6) Per-pair amplitude modulation (sine LFO)

Each pair has its own AM section:

AM amount
AM rate (slow to very slow)

Important detail: AM updates are direct/unslewed.
Randomize/Zero slew does not affect AM behavior, so the LFO stays responsive instead of turning into mush.

7) Hall-ish reverb

Simple built‑in reverb on the master output:

dry/wet
size
decay

It adds space without turning this into a plugin chain.

8) Stereo width

Global width spreads the 8 pairs across the stereo field:

narrow for centered drones
wide for immersive textures

9) Headroom-friendly amplitude mapping

Sliders show 0–100%, but internal gain is capped (so stacking sines doesn’t instantly clip):

easier gain staging
fewer “why is everything exploding?” moments

10) Randomize / zero amplitudes + slew

Performance tools for the 8 amplitude sliders:

randomize
zero
slew (fade/glide time)

Great for evolving textures and scene changes without clicks.

11) Local presets (browser storage)

Preset management via localStorage:

save / load / delete
export/import JSON

So you can reproduce a patch later, instead of trusting your memory (which is a bad system).

12) MIDI input (receive only)

Optional Web MIDI support (browser-dependent):

Note On can control base note
CC controls the 8 amplitude sliders
default CC70–CC77 (configurable start CC)

Technical format

single HTML file
Web Audio API
optional Web MIDI
no backend
works on most modern browsers (iPad included, as long as storage isn’t blocked)

Use cases

ambient drone design
harmonic beds for composition
sound meditation / sleep textures
experimental sound art
learning detuning + beating + modulation

Notes / caveats

This is sine-only on purpose. If you want “interesting timbre,” use a synth. If you want pure interference, welcome.
Web MIDI support depends on browser permissions and support. The engine doesn’t care. Humans do.

Status

Live. Still evolving. Like the beating itself.