sound.synth

The synthesiser Hollowbell's sound is made of. Pure arithmetic over a mono f32 canvas: no source files, no service, no network, so it runs identically in a browser, headless, and offline.

local Synth = require("sound.synth")

local c = Synth.canvas(0.4, 32000)          -- 0.4 seconds at 32 kHz
Synth.tone(c, { freq = 220, amp = 0.5, decay = 0.15 })
Synth.noise(c, { amp = 0.3, lp = 4000, lpEnd = 600, decay = 0.04 })
Synth.reverb(c, { mix = 0.25 })
Synth.normalize(c, 0.88)

Every call adds to the canvas, so a sound is written as a stack of layers and the order does not matter.

What it makes

• tone — a sinusoid with an attack, an exponential decay, and optional pitch glide, vibrato and FM. The FM is what turns a sine into something metallic or growling.
• bell — bronze. Eleven inharmonic partials at the ratios a cast bell actually rings at, each with its own decay so the high ones die first, plus the clapper's knock. crack adds a detuned twin on the mid partials whose beating is the buzz you hear in a cracked bell — which is the whole premise of this game, so it is a first-class argument rather than an effect.
• noise — filtered noise through a one-pole pair that can sweep. This is every impact, whoosh, footstep and breath.
• reverb — four combs and two allpasses. A crypt is small, hard and wet, so the combs are short and the tail is long.

What makes a loop loop

Audio.looping wraps the whole clip, so the join between the last sample and the first is heard once a lap. Two calls are built to survive it:

• drone snaps its frequency to a whole number of cycles across the buffer, so a sustained partial arrives back exactly where it started.
• loopNoise generates two laps of noise and cross-fades the second over the first, so the wrap is continuous rather than a step.

Interior loop points are not the answer here: a .soundClip stores loopStart / loopEnd and no playback surface reads them, so a bed has to be seamless end to end. audio.loopSeam(zaud) reads whether it is.

Baking without dropping a frame

A bed is hundreds of thousands of samples. Set Synth.cooperative = true inside a task and each layer hands the frame back once it has spent its sample budget, so a long bake costs frames rather than one stall. Set it false to bake in one pass when the caller would rather take the hitch.

Finishing

normalize scales to a target peak, saturate is a gentle soft clip so a stacked layer never arrives as a hard one, trimEdges takes the first and last few milliseconds to zero (a one-shot that starts on a non-zero sample ticks — but never call it on a loop, where an edge fade IS the dip), and measure reports peak, RMS and how many samples came out non-finite, which the codec refuses.