Fast Envelope Shaper — User Guide

Real-time amplitude modulation: uses efficient RMS envelope extraction to create dynamic effects like gates, expanders, and reverse-envelope swells without complex processing.

Author: Shai Cohen Affiliation: Department of Music, Bar-Ilan University, Israel Version: 2.0 (2025) License: MIT License Repo: https://github.com/ShaiCohen-ops/Praat-plugin_AudioTools

Contents:

What this does Quick start Envelope Shaping Theory Processing Pipeline Parameters Guide Applications

What this does

This script implements fast envelope shaping — a real-time amplitude modulation technique that extracts the RMS envelope of a signal and uses it to dynamically control the signal's own amplitude. The system provides two complementary effects: (1) Expander/Gate mode: Amplifies loud sections and attenuates quiet sections, emphasizing transients and creating punch. (2) Reverse envelope mode: Attenuates loud sections and amplifies quiet sections, creating swelling "ghost" effects and bringing out background content. The implementation uses an efficient "square-and-smooth" RMS method that avoids computationally expensive FFT processing while maintaining high quality.

Key Features:

Dual Processing Modes — Expander/gate and reverse-envelope effects
RMS Envelope Extraction — Accurate amplitude tracking without FFT
Performance Optimization — Optional downsampling for speed
Adjustable Smoothness — Control envelope response time
Real-time Capable — Efficient processing suitable for live use
Stereo Compatible — Automatic mono conversion with phase coherence

What is envelope shaping? Envelope shaping uses a signal's own amplitude envelope to dynamically control its gain. In expander mode, the envelope acts as a control signal: when amplitude is high, gain increases; when amplitude is low, gain decreases. This emphasizes transients and creates punch. In reverse mode, the relationship is inverted: high amplitude reduces gain, low amplitude increases gain. This creates swelling effects and reveals background content. The RMS (Root Mean Square) method provides an accurate representation of perceived loudness by measuring power rather than peak amplitude. Advantages: (1) Musical dynamics: Creates natural-sounding compression/expansion. (2) Content-aware: Automatically adapts to signal characteristics. (3) Efficient processing: No complex analysis required. (4) Versatile applications: Useful for music production, sound design, and audio restoration.

Technical Implementation: (1) Signal preparation: Convert stereo to mono, optionally downsample for speed. (2) Envelope extraction: Square samples (power calculation), apply lowpass filter (smoothing), take square root (RMS conversion). (3) Mode selection: Apply envelope directly or reverse it based on user choice. (4) Modulation: Multiply original signal by envelope control signal. (5) Output processing: Normalize peak level, cleanup intermediate objects. The system uses Praat's efficient filtering and interpolation to handle sample rate differences automatically, ensuring smooth operation even when downsampling is used for performance.

Quick start

In Praat, select exactly one Sound object (mono or stereo).
Run script… → Fast_Envelope_Shaper.praat.
Choose Reverse_envelope mode (unchecked for expander, checked for reverse).
Enable Use_downsampling for faster processing (recommended).
Set Processing_sample_rate (16000 Hz works for most material).
Adjust Envelope_smoothness_Hz (10-50 Hz typical range).
Set Scale_peak output level (0.99 prevents clipping).
Enable Play_after_processing to audition results.
Click OK — processing completes almost instantly.
Output appears as "originalname_Expander" or "originalname_RevEnv".

Quick tip: Start with expander mode (Reverse_envelope unchecked) for drums and percussive material to add punch. Use reverse mode (checked) for pads and ambient sounds to create swelling effects. Always enable downsampling for fastest processing — 16000 Hz is sufficient for envelope tracking. Lower smoothness (10-20 Hz) for gradual, musical envelope following. Higher smoothness (30-50 Hz) for tight, responsive transient shaping. For stereo files, processing converts to mono for envelope calculation but applies to original stereo, preserving spatial image. The effect is most dramatic on material with strong dynamic variation.

Important: MONO ENVELOPE EXTRACTION — stereo files are converted to mono for envelope calculation, then applied to original stereo. Extreme settings may cause pumping artifacts or excessive noise amplification. Very low smoothness (<10 Hz) may cause sluggish response missing transients. Very high smoothness (>100 Hz) may introduce modulation artifacts. Downsampling uses 50Hz transition bandwidth — avoid if processing very low-frequency content. Reverse mode can significantly amplify noise in quiet sections. Output normalization to 0.99 prevents clipping but original dynamic relationships are preserved. Test on short segments first to hear the effect character.

Envelope Shaping Theory

RMS Envelope Fundamentals

Square-and-Smooth Method

Mathematical envelope extraction:

RMS (Root Mean Square) calculation: Traditional: RMS = sqrt( (1/N) * Σ x[i]² ) Continuous approximation: envelope = lowpass( sqrt( lowpass( x² ) ) ) Three-step process: 1. SQUARE: y1[n] = x[n] * x[n] (instantaneous power) 2. SMOOTH: y2[n] = lowpass(y1[n]) (average power) 3. LINEARIZE: y3[n] = sqrt(y2[n]) (RMS amplitude) Why this works: Squaring converts amplitude to power Lowpass filtering computes moving average Square root returns to amplitude domain Result tracks perceived loudness accurately Implementation advantages: No FFT required → computational efficiency Simple operations → real-time capability Natural sound → matches human perception

Why RMS Over Peak Detection?

Perceptual advantages:

Loudness accuracy: RMS correlates better with perceived volume
Transient handling: Smooth response to sharp attacks
Musical dynamics: Preserves natural amplitude relationships
Noise immunity: Less sensitive to isolated peaks
Consistent results: Predictable across different material

Dual Processing Modes

Expander/Gate Mode

Dynamic range expansion:

Operation: output = input * envelope Behavior: High amplitude → large envelope → high gain → louder output Low amplitude → small envelope → low gain → quieter output Result: emphasized dynamics, enhanced transients Applications: Drum enhancement → added punch and impact Speech intelligibility → clearer consonants Dynamic restoration → reversing compression artifacts Transient shaping → sharper attacks Example: Drum loop processing Original: consistent amplitude throughout Processed: drum hits louder, tails quieter Effect: more punchy and rhythmic

Reverse Envelope Mode

Inverse amplitude relationship:

Operation: output = input * (1 - envelope) [after normalization] Behavior: High amplitude → large envelope → low gain → quieter output Low amplitude → small envelope → high gain → louder output Result: swelling effects, background emphasis Applications: Ambient creation → ghostly, swelling textures Noise reduction → attenuating loud sections Sound design → unconventional dynamic effects Background revelation → bringing out quiet content Example: Pad processing Original: sustained chords with some movement Processed: loud sections fade, quiet sections swell Effect: breathing, evolving texture

📊 RMS Envelope Intuition

For speech "Hello World":

"H" consonant → high frequency energy → medium envelope

"e" vowel → sustained tone → high envelope

"llo" → lower energy → medium envelope

Silence between words → near-zero envelope

Expander mode: "e" vowel gets louder, silences get quieter

Reverse mode: "e" vowel gets quieter, silences get louder

Smoothness Control System

Envelope Response Time

Lowpass filter characteristics:

Smoothness parameter: envelope_smoothness_Hz (25 Hz default) Lower values → slower response → smoother envelope Higher values → faster response → tighter envelope Filter implementation: Hann bandpass 0-smoothness_Hz Effectively a lowpass filter at smoothness_Hz 20Hz transition bandwidth for smooth rolloff Temporal interpretation: Smoothness = 10 Hz → ~100ms response time Smoothness = 25 Hz → ~40ms response time Smoothness = 50 Hz → ~20ms response time Perceptual effects: Slow response (10-20 Hz): musical, gradual dynamics Medium response (25-35 Hz): natural, balanced Fast response (40-60 Hz): tight, transient-focused

Why Adjustable Smoothness?

Application-specific tuning:

Music: 20-30 Hz for natural dynamics
Speech: 25-40 Hz for clear articulation
Drums: 30-50 Hz for punchy transients
Ambient: 10-20 Hz for gradual evolution

Performance Optimization

Intelligent Downsampling

Speed vs quality tradeoff:

Downsampling logic: Original SR → 44100 Hz (typical) Target SR → processing_sample_rate (16000 Hz default) Ratio → ~2.76x speed increase Why it works for envelopes: Envelope bandwidth << audio bandwidth Nyquist for envelope: ~100 Hz maximum 16000 Hz SR provides 8000 Hz Nyquist → ample margin Praat interpolation handles SR conversion smoothly Quality preservation: Envelope calculation at lower SR Application to original at full SR No loss in final output quality Significant processing speed improvement Implementation: Resample: processing_sample_rate, 50 (50Hz transition) Process envelope at lower SR Praat automatically interpolates when applying to original

When to Avoid Downsampling

Special cases:

Very low-frequency content: Below 100 Hz may be affected
Extreme smoothness: Very low cutoff frequencies
Academic analysis: Maximum precision required
Already low SR: Original near processing_sample_rate

Processing Pipeline

🔧 Eight-Stage Envelope Processing

Complete signal path from input to shaped output:

Stage 1: Signal Preparation

Step	Operation	Purpose
1.1	Check single sound selection	Input validation
1.2	Extract original properties	SR, channels, duration
1.3	Stereo to mono conversion	Envelope calculation

Stage 2: Performance Optimization

Step	Operation	Purpose
2.1	Check downsampling option	User preference
2.2	Resample to processing SR	Speed improvement
2.3	Update working copy	Efficient processing

Stage 3: RMS Envelope Extraction

Step	Operation	Purpose
3.1	Square samples (x²)	Instantaneous power
3.2	Lowpass filter	Smooth power average
3.3	Square root (√)	RMS amplitude

Stage 4: Mode Selection

Step	Operation	Purpose
4.1	Check reverse_envelope	User mode selection
4.2	Apply reverse if selected	Effect direction
4.3	Set output suffix	Result identification

Stage 5: Modulation Application

Step	Operation	Purpose
5.1	Copy original sound	Preserve source
5.2	Apply envelope multiplication	Amplitude modulation
5.3	Automatic interpolation	Handle SR differences

Stage 6: Output Finalization

Step	Operation	Purpose
6.1	Peak normalization	Prevent clipping
6.2	Rename output	Clear identification
6.3	Optional playback	Result audition

Stage 7: Resource Cleanup

Step	Operation	Purpose
7.1	Remove envelope object	Memory management
7.2	Remove working copies	Clean object list
7.3	Select final output	User convenience

Complete Signal Flow

INPUT SOUND (mono/stereo) ↓ MONO CONVERSION: Stereo → mono for envelope ↓ OPTIONAL DOWNSAMPLING: SR reduction for speed ↓ ENVELOPE EXTRACTION: Square → Lowpass → Root ↓ MODE SELECTION: Direct or Reverse envelope ↓ MODULATION: Original × Envelope (auto-interpolated) ↓ PEAK NORMALIZATION: Scale to prevent clipping ↓ OUTPUT: "originalname_Expander" or "originalname_RevEnv"

Parameters Guide

⚙️ Complete Parameter Reference

Detailed explanation of all user-controllable parameters:

Direction Parameters

Parameter	Default	Options	Description
Reverse_envelope	0	0/1	Expander mode (0) or Reverse mode (1)

Performance Parameters

Parameter	Default	Range	Description
Use_downsampling	1	0/1	Enable processing speed optimization
Processing_sample_rate	16000	8000-44100	Target SR for envelope calculation

Envelope Parameters

Parameter	Default	Range	Description
Envelope_smoothness_Hz	25	5-100	Envelope response speed in Hz

Output Parameters

Parameter	Default	Range	Description
Scale_peak	0.99	0.1-1.0	Output peak normalization level
Play_after_processing	1	0/1	Auto-play result after processing

Parameter Interactions

Key relationships:

Reverse_envelope defines effect character: Expander vs Reverse
Smoothness affects temporal response: Lower = more gradual
Downsampling improves speed: Enabled for most applications
Processing SR balances speed/quality: 16000 Hz optimal

Simple parameter set enables wide range of effects

Recommended Settings

🥁 Drum Expander

Goal: Add punch and impact to drums

Settings:

Reverse_envelope: 0 (Expander)
Use_downsampling: 1
Processing_sample_rate: 16000
Envelope_smoothness_Hz: 40

🌊 Ambient Swell

Goal: Create breathing, evolving textures

Settings:

Reverse_envelope: 1 (Reverse)
Use_downsampling: 1
Processing_sample_rate: 16000
Envelope_smoothness_Hz: 15

🎙️ Speech Clarifier

Goal: Improve speech intelligibility

Settings:

Reverse_envelope: 0 (Expander)
Use_downsampling: 1
Processing_sample_rate: 16000
Envelope_smoothness_Hz: 30

Applications

Music Production

Use case: Enhance dynamics and transients

Technique: Expander mode on drums and percussion

Example: Add punch to drum loops without compression artifacts

Sound Design

Use case: Create evolving textures and effects

Technique: Reverse mode on pads and ambient material

Example: Transform static pads into breathing, swelling textures

Audio Restoration

Use case: Improve intelligibility of recorded material

Technique: Expander mode on speech and dialogue

Example: Enhance consonant clarity in podcast recordings

Live Processing

Use case: Real-time dynamic effects

Technique: Fast processing enables live use

Example: Live vocal processing for performance enhancement

💡 Creative Techniques

Advanced applications:

Serial processing: Apply both modes sequentially
Frequency splitting: Process different bands separately
Sidechain effects: Use one sound's envelope on another
Automation: Vary parameters over time for evolving effects

Troubleshooting Common Issues

Problem: Pumping artifacts
Cause: Smoothness too high for material
Solution: Reduce envelope_smoothness_Hz

Problem: Noise amplification in quiet sections
Cause: Reverse mode with noisy source
Solution: Use expander mode or pre-clean source

Problem: Sluggish transient response
Cause: Smoothness too low
Solution: Increase envelope_smoothness_Hz

Problem: Stereo image issues
Cause: Mono envelope on complex stereo
Solution: Process channels separately if needed