8.12.18. STRUDEL-18 — SFX Lab: a procedural sound-effect workbench

The SFX Lab is an interactive workbench for authoring one-shot sound effects — kicks, snares, hats, zaps, impacts — entirely procedurally, with no recorded samples. A sound is a stack of layers, each an sfxr (subtractive, the classic 8-bit “jsfxr” voice) or modal (struck / resonant) generator with its own gain and onset delay. You build the stack from docked panels, audition each layer or the whole mix, watch the rendered waveform + spectrum update live, optionally load an external .wav to mimic, and — the payoff — export the finished sound as a drop-in daslang render function that needs no editor to run.

It is built on the daStrudel audio engine (the strudel_sfxr / strudel_modal / strudel_sfx modules under modules/dasAudio/) and a dasImgui / ImPlot GUI. The tool lives at examples/daStrudel/sfx_lab/.

8.12.18.1. Running it

The GUI depends on two daspkg packages — dasImgui and dasImguiImplot — declared in the tool’s .das_package. Install them once (this clones and builds the C++ shared modules into modules/), then run the tool with no -load_module flags:

cd examples/daStrudel/sfx_lab
daslang ../../utils/daspkg/main.das -- install
daslang -project_root . main.das

The same main.das runs windowed (above), headless (-- --headless --headless-frames N for CI / scripted renders), and live (daslang-live -project_root . main.das for imgui_snapshot / playwright inspection) — audio is wired in init() so every mode has sound.

8.12.18.2. The workbench

../../_images/workbench.png — The docked workbench: **Layers** (the stack), **Editor** (the selected layer’s voice parameters), **Preview** (the summed mix as waveform + spectrum), plus **Reference** and **Postprocess**. Panels re-dock freely; the layout persists in `imgui.ini`.

Five panels, all re-dockable:

Layers — the layer stack. Add an sfxr or modal layer, select one, solo / mute it, duplicate it (optionally dup-linked so edits to the source mirror into the copy), reorder, delete, and play an individual layer.
Editor — the selected layer’s voice parameters (every sfxr / modal field), plus its gain, onset delay, and link toggle. A free-form label sits at the top of the editor — handy when you run two copies of the tool side by side and need to tell them apart.
Preview — the summed mix. The Wave + Spectrum tab shows the rendered waveform and its magnitude spectrum (ImPlot); the Spectrogram tab shows the time–frequency view.
Reference — load an external .wav to mimic. Its waveform and spectrum overlay the mix so you can match a real sound by eye, and Play → match reference loudness levels your audition to the reference’s loudness so you compare timbre, not volume.
Postprocess — a ten-effect master chain (filter, band-pass, drive, bit-crush, compressor, chorus, phaser, tremolo, delay, reverb) applied to the mix.

8.12.18.3. The authoring loop

A typical session:

Build — add layers and dial in each voice. A kick is often one modal “body” layer plus a short sfxr “click”; a snare adds a noisy sfxr layer on top of a modal tone.
Audition — play a single layer, or Play → Play mix for the whole stack. Turn on autoplay to hear every edit as you make it.
Match — load a reference .wav and overlay it; toggle match reference loudness so the A/B is fair.
Ear-tune — nudge the parameters until the waveform, the spectrum, and your ears agree.
Export — save the .sfx, or export a drop-in .das render function (below).

8.12.18.4. The `.sfx` format

File → Save sound writes a .sfx file — JSON describing the layer stack, the per-layer voice parameters, the post chain, a master gain, an optional mimic_target reference path, and a free-form label. A game loads it at runtime with render_sound_file("foo.sfx"). The example kit drums live in examples/daStrudel/sfx_lab/sounds/*.sfx — open them to see how each was built.

8.12.18.5. Exporting a drop-in `.das` render function

File → Export .das render function turns the current sound into real, readable daslang — a self-contained function with the standard one-shot drum signature that you can paste straight into your own code (or into strudel_synth) and call with no editor and no .sfx file:

// Generated by the SFX Lab. Drop-in render function; requires:
//   require strudel/strudel_sfx   // re-exports strudel_modal + strudel_sfxr
def render_bd(duration, gain : float) : array<float> {
    var layers : array<Layer>
    var l0 = modal_layer("body", ModalVoice(pitch = 55.0, n_modes = 2, ring = 0.18))
    layers |> emplace(l0)
    var l1 = sfxr_layer("click", SfxrSound(env_decay = 0.06, base_freq = 0.5))
    l1.gain = 0.4
    layers |> emplace(l1)
    var out <- render_sound(layers)
    return <- sfx_conform(out, 44100, 48000, duration, gain * 1.0)
}

The filename you choose is the function name (export render_bd.das → def render_bd(...)). The emitter writes only the non-default fields, rebuilds the layers and post chain, calls the engine, and conforms the result to the host’s 48 kHz rate with a tiny fade so the one-shot never clicks on attack or release. Pass base_freq as a third argument (the parametric form) to pitch a modal sound at call time — that is exactly how the kit’s pitched tom is generated. The eleven strudel_synth drum renderers (render_bd / render_sd / render_hh / …) were re-authored in the SFX Lab and exported this way.

8.12.18.6. A/B comparison with `drum_compare`

examples/daStrudel/drum_compare/ lays out the eleven kit drums as pads and plays each one either from the SFX Lab .sfx kit or from the General-MIDI percussion of a SoundFont — press S to toggle the whole grid between the two banks, so you can A/B the procedural drum against its reference and tune by ear.

../../_images/drum_compare.png — `drum_compare` — click a pad (or press its number key) to audition that drum; the green chip shows the active bank, and S toggles between the SFX Lab kit (`SYNTH`) and the SoundFont reference (`SF2`).