CA Convolution Reverb — Cellular Automaton IR Generator

Convolution reverb whose impulse response is grown from an Elementary Cellular Automaton (Wolfram rule 0-255). Live cells become signed impulses shaped by exponential decay; the rule chooses the room: chaotic rules give noise-like reverberation, structured rules give self-similar metallic resonators.

Author: Shai Cohen Affiliation: Department of Music, Bar-Ilan University, Israel Version: 0.3 (2026) License: MIT License Repo: https://github.com/ShaiCohen-ops/Praat-plugin_AudioTools

Contents:

What this does Quick start 5 Presets CA Theory Controls Applications

What this does

This script implements a convolution reverb whose impulse response is grown from an Elementary Cellular Automaton (Wolfram rule 0-255). Live cells become signed impulses shaped by exponential decay; the rule chooses the room: chaotic rules give noise-like (Schroeder-idealised) reverberation, structured rules give self-similar metallic resonators that no physical room could produce. The result is a new Sound object — the dry sound convolved with the CA-grown IR.

What is a Cellular Automaton? A cellular automaton is a grid of cells that evolve over discrete time steps according to a local rule. Each cell's next state depends on itself and its neighbours. Wolfram's elementary CA uses a 1D grid with a 3-cell neighbourhood (left, self, right) and 256 possible rules (0–255). Chaotic rules (e.g., 30, 45) produce unpredictable, noise-like patterns; structured rules (e.g., 90, 110) produce self-similar, fractal patterns. This script maps the CA's live cells to impulses in an impulse response, then convolves with your sound.

Key Features:

5 Presets — Chaotic Room (rule 30), Sierpinski Plate (rule 90), Complex Bloom (rule 110), Granular Traffic (rule 184), Cathedral Chaos (rule 45), plus Custom
Elementary CA (rule 0-255) — choose any Wolfram rule for the impulse structure
Frame decorrelation — seeded per-generation rotation kills the samplerate/width comb (room-like reverb)
Exponential decay — shapes the IR's amplitude envelope
Wet/dry mix — blend original and reverberated signals
Stereo support — dry sounds are convolved natively (mono IR broadcast across channels)
Visualisation — CA matrix image, IR waveform + spectrum, wet waveform, summary strip

v0.3 fix — no more muffled sound: Dead cells now map to silence (not -1), so the CA background is no longer a full-amplitude DC field. Live cells map to ±1 through a fixed seeded per-column sign vector — runs of live cells whiten (no LF buildup), and the background stops thumping. Row-to-row correlation survives, so the decorr-OFF comb feature is untouched.

Quick start

In Praat, select exactly one Sound object.
Run script… → CA_Reverb_IR.praat.
Choose a preset from the dropdown (Chaotic Room, Sierpinski Plate, Complex Bloom, Granular Traffic, Cathedral Chaos, or Custom).
If Custom, set Rule (0–255), CA_width, Seed_mode, Ir_duration, Decay_time, Frame_decorrelation.
Set Wet_dry_percent (0–100%).
Optionally enable Keep_IR_and_CA to inspect the IR and CA matrix.
Click OK — script grows the CA, builds the IR, convolves with your sound.
Result originalname_CAreverb_presetname appears in the Objects window.

Quick tip: Start with Chaotic Room (rule 30) for a natural-sounding, Schroeder-like reverb. Sierpinski Plate (rule 90) produces a metallic, comb-filtered resonance — turn Frame_decorrelation OFF to hear the samplerate/width comb. Cathedral Chaos (rule 45, width=1024, IR=4s) creates a vast, chaotic hall. Enable Keep_IR_and_CA to see the CA matrix that generated your reverb's impulse response.

Important: This script is pure Praat — no Python dependencies. The CA evolution uses Matrix operations and is fast (width 256, 1000 generations takes seconds). The convolution uses Praat's native Convolve command. Frame_decorrelation ON kills the samplerate/width comb (room-like); OFF preserves it (metallic resonator). The IR is shaped with exponential decay and a final fade-out to prevent clicks.

5 Presets

Preset	Rule	Width	Seed	IR (s)	Decay (s)	Decorr	Character
Chaotic Room	30	256	random	1.2	0.35	ON	Natural, noise-like, Schroeder-idealised reverb
Sierpinski Plate	90	512	centre	2.0	0.6	OFF	Metallic comb, self-similar fractal resonator
Complex Bloom	110	256	centre	2.5	0.9	ON	Swelling, complex, organic reverb
Granular Traffic	184	128	random	0.8	0.2	ON	Grainy, short, rhythmic texture
Cathedral Chaos	45	1024	random	4.0	1.4	ON	Vast, chaotic, cavernous hall

CA Theory — From Rule to Impulse Response

Elementary Cellular Automaton

A 1D binary grid (width W) evolves over discrete generations (t = 1..N).

Each cell's next state depends on its left neighbour, itself, and its right neighbour (3-bit neighbourhood).

Rule 0–255 maps each of the 8 possible neighbourhood patterns to a new state (0 or 1).

Example neighbourhood patterns and their rule bit positions:

111 → bit 7
110 → bit 6
101 → bit 5
100 → bit 4
011 → bit 3
010 → bit 2
001 → bit 1
000 → bit 0

CA → IR mapping (v0.3):

Dead cells (0) → silence (no impulse)
Live cells (1) → ±1 (sign from fixed seeded per-column vector)
Decorrelation (ON): each generation's row is circularly rotated by a seeded random offset — kills the samplerate/width comb.
Decorrelation (OFF): rows are read in natural order — preserves the comb at sampleRate/width Hz.
Exponential decay: amplitude × exp(-t / decay_time)
End fade: cosine fade-out over 30 ms or 10% of IR duration

Why v0.3 sounds different: In v0.2, dead cells mapped to -1, creating a full-amplitude DC background — the IR was dominated by low-frequency rumble. v0.3 maps dead cells to silence and live cells to ±1 through a fixed sign vector. Runs of live cells whiten (no LF buildup), and the row-to-row correlation that gives the comb feature survives. The result is a much cleaner, more musical impulse response.

Controls

CA & IR parameters (Custom mode)

Parameter	Range	Description
Rule	0–255	Wolfram elementary CA rule. Chaotic (30, 45) = noise-like; structured (90, 110) = self-similar.
CA_width	16–2048	Number of cells per row. Wider = longer IR (width × generations). Also sets the comb frequency when decorrelation OFF.
Seed_mode	single centre / random row	Centre = deterministic (single live cell at the centre). Random = density 0.5, uses Random_seed.
Random_seed	integer	Seed for random row initialisation and decorrelation rotations.
Ir_duration	0.25–10 s	Total impulse response length (clipped to CA width × generations).
Decay_time	0.1–5.0 s	Exponential decay time constant. Shorter = faster decay.
Frame_decorrelation	ON / OFF	ON = seeded circular rotation per generation (room-like). OFF = preserve samplerate/width comb (metallic).

Frame_decorrelation explained: With decorrelation OFF, the IR has a strong comb filter at sampleRate / width Hz. For width=512 at 44.1 kHz, the comb is at 86 Hz — a metallic, ringing character. With decorrelation ON, each generation is randomly shifted, breaking the comb and producing a smoother, room-like reverb. Both modes are musically useful — OFF for Sierpinski-like metallic textures, ON for natural reverb.

Applications

Natural-sounding reverb (Chaotic Room)

Use case: Add a realistic-sounding reverb to any sound.

Settings: Chaotic Room preset (rule 30, decorrelation ON). The chaotic CA produces a dense, noise-like impulse response that mimics Schroeder's idealised reverb.

Metallic / resonator effects (Sierpinski Plate)

Use case: Create metallic, ringing, self-similar resonances.

Settings: Sierpinski Plate preset (rule 90, decorrelation OFF). The self-similar CA pattern produces a comb-filtered impulse response with a metallic character.

Experimental / generative reverb

Use case: Explore the full range of CA rules — each rule produces a unique impulse response.

Settings: Custom mode, experiment with different rules (0–255). Rule 110 (Complex Bloom) produces swelling, organic textures; rule 184 (Granular Traffic) produces short, grainy responses.

Cathedral / vast spaces (Cathedral Chaos)

Use case: Create a huge, chaotic hall reverb.

Settings: Cathedral Chaos preset (rule 45, width=1024, IR=4s). The large width and long IR produce a vast, immersive space.

Workflow: Voice → Chaotic Room reverb

Source: Spoken word or vocal recording.
Settings: Chaotic Room preset, Wet_dry_percent = 40%.
Result: A natural, room-like reverb that doesn't colour the voice — the chaotic CA provides a dense, Schroeder-style impulse response.

Workflow: Piano → Sierpinski Plate resonator

Source: Piano recording.
Settings: Sierpinski Plate preset (rule 90, decorrelation OFF), Wet_dry_percent = 60%.
Result: The piano takes on a metallic, ringing character — like playing inside a fractal metal plate. The comb at 86 Hz adds a resonant low-mid emphasis.

Workflow: Drum loop → Granular Traffic

Source: Drum loop.
Settings: Granular Traffic preset (rule 184, width=128, IR=0.8s), Wet_dry_percent = 50%.
Result: A short, grainy reverb that adds texture without washing out the transients — the rule 184 CA produces a rhythmic, traffic-like pattern.

Troubleshooting:
• IR is silent or very quiet: Check that the CA has live cells. Centre seed + rule 0 (all cells die) produces silence. Use rule 30 or 90 for live patterns.
• Comb filter too strong: Turn Frame_decorrelation ON to break the samplerate/width comb. Or choose a different width to shift the comb frequency.
• Muffled / thumping sound: You may be using an older version. v0.3 fixes this: dead cells map to silence, not -1. Update the script.
• IR has click at end: The script applies a 30 ms fade-out. If clicks persist, increase the fade duration by editing the script.
• CA evolution is slow: Reduce CA_width or Ir_duration (fewer generations). The Matrix Formula operation is fast for widths ≤ 1024.

Visualisation

When Draw_visualization is enabled, the script generates:

Title strip — preset, rule, width, generations, decay time, decorrelation status
CA matrix image — 2D visualisation of the cellular automaton (generation × cell)
IR waveform — the impulse response (blue curve)
IR spectrum — frequency content (0–10 kHz), showing the comb or noise-like structure
Wet output waveform — the reverberated sound
Summary strip — rule, width, generations, seed, decorrelation, wet/dry, output details

The CA matrix image is the key visual: you can see the rule's pattern — chaotic (rule 30) vs. self-similar (rule 90) — and understand how the impulse response is shaped.