Spectral Decay Reverb — User Guide

Poisson-IR convolution reverb with exponential decay, linear chirp modulation and Hann-band spectral shaping.

Author: Shai Cohen Affiliation: Department of Music, Bar-Ilan University, Israel Version: 0.3.3 (2026) License: MIT License Repo: https://github.com/ShaiCohen-ops/Praat-plugin_AudioTools

Contents:

What this does Quick start What is convolution reverb? Presets Parameters IR design Channels, timing and normalization Visualization Output

What this does

Spectral Decay Reverb creates a synthetic impulse response from a Poisson point process, shapes that IR with exponential amplitude decay, a true linear-frequency chirp modulation and a Hann-smoothed bandpass, then convolves the source with the resulting IR.

The wet convolution is generated separately from a dry path of fixed requested duration, faded and peak-protected before the final wet/dry mix.

Quick start

Select one mono or stereo Sound.
Run Spectral_Decay.praat.
Choose a preset or Custom.
Adjust the IR duration/density and bandpass if needed.
Set Wet/Dry and run.

Output duration is always source duration + Tail_duration_s. The convolution itself may be longer, but wet audio after the requested output end is explicitly muted.

What is convolution reverb?

An impulse response (IR) describes how a system responds over time to a very short excitation. Convolution applies that response to every moment of the input signal, producing the corresponding echoes, coloration and decay.

Here the IR is not measured from a physical room. It is generated algorithmically: random Poisson-distributed impulses are amplitude-shaped, chirp-modulated and band-limited before convolution. The result is therefore a designed spectral reverb rather than a room measurement.

Presets

Preset	Tail	IR	Density	Decay base	Band	Smoothing	Wet
Subtle	1.5 s	2.0 s	1200/s	150	120–3500 Hz	80 Hz	35%
Medium	2.0 s	3.0 s	2000/s	110	100–4000 Hz	100 Hz	50%
Heavy	3.0 s	4.5 s	3000/s	80	80–4500 Hz	120 Hz	65%
Extreme	4.5 s	6.5 s	4500/s	50	60–5000 Hz	150 Hz	80%

Preset fade times are 0.8, 1.2, 1.8 and 2.5 seconds respectively. Presets also override Wet/Dry; visualization and playback remain user-controlled.

Parameters

Parameter	Default	Meaning
Tail_duration_s	2.0	Requested output extension beyond the source.
Impulse_duration_s	3.0	Length of the generated left/mono Poisson IR; stereo right IR uses 93% of this duration.
Poisson_density	2000	Average event rate in the IR. Stereo right uses 95% of the entered density.
Decay_base	110	Exponential envelope base; must be >1. The unmodulated envelope is `base^(-t/T)`, reaching 1/base at the end of the IR.
Low_cutoff_Hz / High_cutoff_Hz	100 / 4000	Hann-band pass limits. High cutoff is reduced automatically to 95% of Nyquist if necessary.
Smoothing_Hz	100	Transition width for the Hann bandpass.
Wet_dry_percent	50	Linear dry/wet mix, clamped to 0…100%.
Fadeout_duration_s	1.2	Wet-tail fade, capped to Tail_duration_s.

IR design

The left/mono IR uses fixed chirp settings:

instantaneous modulation frequency = 150 + 20t Hz modulation depth = 0.70 IR envelope = decay_base^(-t/T) × [1 + 0.70 sin(2π(150t + 0.5×20t²))]

Stereo right uses 140 Hz + 22 Hz/s, depth 0.65, a slightly shorter/sparser Poisson process, decay_base × 0.95, and a slightly shifted bandpass.

The Poisson generation is not seeded by the script, so a new IR is normally produced on each run.

Channels, timing and normalization

Supports mono or stereo only.
The source is copied to a zero-based working time domain before convolution; audio content and sample rate are preserved.
Channel count is preserved.
Wet convolution uses the source only, not the appended silent dry tail.
Wet peaks above 0.95 are scaled down before mixing; stereo uses one shared wet gain.
After wet/dry mixing, peaks above 0.98 are scaled down when wet > 0.
At 0% wet, a fast path returns the dry source plus the requested silent tail without convolution or peak normalization.

Visualization

The display shows the dry waveform, the complete result including the tail, the intended Hann-band filter shape, the analytical chirp-modulated decay envelope and the underlying exponential envelope, followed by a summary strip.

The IR-envelope panel is a model of the deterministic amplitude/chirp law. It does not plot the individual random Poisson impulses generated for that run.

Output

Object name: <source>_spectral_<Custom|Subtle|Medium|Heavy|Extreme>.