Stereo Rotary Speaker — User Guide
Physical modeling of the legendary Leslie speaker: recreates the iconic rotating horn and bass rotor system with authentic Doppler shift, tremolo modulation, and stereo microphone placement for that classic swirling, three-dimensional organ sound.
What this does
This script accurately models the classic Leslie rotary speaker—the iconic electromechanical system that defined the sound of Hammond organs in jazz, rock, and gospel music. Using sophisticated physical modeling, it recreates the complex interactions of rotating horns (treble) and bass rotors, including authentic Doppler pitch shifts, amplitude tremolo, and true stereo spatial effects. The result is that unmistakable swirling, three-dimensional sound that seems to move around the listener in a physical space.
Key Features:
- 5 Authentic Presets — From chorale slow to tremolo fast
- Physical Doppler Modeling — Accurate pitch shift from rotation
- Dual Modulation System — Combined tremolo and vibrato effects
- Stereo Microphone Simulation — Configurable microphone placement
- Automatic Stereo Conversion — Mono inputs become spatial experiences
- Broken Cabinet Simulation — Realistic mechanical imperfections
What is a Leslie speaker? Invented in 1940 by Don Leslie, the Leslie speaker uses rotating horns (for treble) and drums (for bass) to create distinctive spatial and modulation effects. As the horns spin toward and away from the listener, they create Doppler pitch shifts (vibrato) and amplitude variations (tremolo). The combination produces a rich, three-dimensional sound that became essential for Hammond organ in jazz, blues, rock, and gospel music. This script digitally recreates that complex electromechanical system.
Technical Implementation: (1) Stereo preparation: Converts mono to stereo for spatial effects. (2) Physical modeling: Applies combined tremolo and Doppler effects using sine-wave modulation. (3) Microphone simulation: Uses phase offsets to recreate stereo microphone placement. (4) Doppler calculation: Implements pitch shift through variable delay lines. (5) Tremolo application: Creates amplitude modulation through multiplicative scaling. (6) Channel separation: Applies different phases to left and right channels for true stereo rotation.
Quick start
- In Praat, select exactly one Sound object (organ sounds work best).
- Run script… →
stereo_rotary_speaker.praat. - Choose a Preset or select "Custom" to adjust parameters manually.
- Adjust Rotation Mechanics:
- Rotation_Speed_Hz: Horn spin rate (0.8-9.0 Hz)
- Set Horn Physics for authentic character:
- Doppler_Depth: Pitch wobble intensity (0.08-0.25)
- Tremolo_Depth: Volume wobble amount (0.3-0.7)
- Adjust Microphone Placement for stereo width:
- Stereo_Width_Deg: Mic angle (45-180°)
- Click OK — effect applied, result named "originalname_rotary".
Quick tip: Start with Chorale for slow, hymn-like rotation or Tremolo for fast, rock-style effects. Use Transition for medium speeds that work well for many applications. Higher Rotation_Speed_Hz creates faster, more intense modulation. Stereo_Width_Deg = 180 creates maximum stereo separation. The effect works wonderfully on organ sounds but can create interesting results with vocals, guitars, and other sustained instruments.
Important: The script automatically converts mono to stereo for proper spatial effects. Very high Rotation_Speed_Hz (>8 Hz) may cause artifacts with some material. Extreme Doppler_Depth (>0.25) can create unnatural pitch effects. Complex polyphonic material may produce less clear results than monophonic sources. The physical model assumes typical Leslie speaker dimensions and rotation patterns—results may vary with different source material.
Leslie Speaker Theory
The Rotary Speaker System
How a Real Leslie Works
The Leslie speaker uses two rotating elements:
TREBLE HORN (High frequencies):
- Small horn rotating at 400-800 RPM (6.7-13.3 Hz)
- Creates strong Doppler effect and tremolo
- Directs high frequencies in sweeping pattern
BASS ROTOR (Low frequencies):
- Large drum rotating at 40-50 RPM (0.7-0.8 Hz)
- Creates subtle low-frequency modulation
- Uses baffles to direct sound indirectly
CLASSIC LESLIE EFFECTS:
- Chorale (slow): Bass rotor dominant, subtle motion
- Tremolo (fast): Both horns fast, intense modulation
- Transition: Speed changing between slow and fast
This script models the treble horn behavior which provides
the most characteristic Leslie sound
🎹 Why Leslie Sounds So Unique
The Leslie effect combines three distinct phenomena:
- Doppler Shift: Pitch rises as horn approaches, falls as it recedes
- Amplitude Modulation: Volume increases toward listener, decreases away
- Spatial Rotation: Sound appears to move around the listener
This combination creates the signature "swirling" sound that no simple chorus or flanger can replicate.
Doppler Effect Physics
The Science Behind the Sound
The Doppler effect causes frequency shifts from motion:
DOPPLER FORMULA:
f' = f × (v + v₀) / (v + vₛ)
Where:
f' = perceived frequency
f = actual frequency
v = speed of sound (~343 m/s)
v₀ = speed of observer (0 for stationary listener)
vₛ = speed of source (positive when moving away)
For Leslie speaker:
Horn moves in circle at speed v_horn
Component toward listener: vₛ = v_horn × cos(θ)
This creates sinusoidal pitch variation:
f'(t) = f × [1 + (v_horn/c) × cos(2πf_rotation×t)]
The script models this as variable delay:
delay(t) = base × [1 + depth × sin(2πf_rotation×t + π/2)]
Real Leslie Parameters
Actual Leslie speaker specifications:
Rotation speeds:
Chorale (slow): 40-50 RPM (0.67-0.83 Hz) bass, 400 RPM (6.67 Hz) treble
Tremolo (fast): 350-400 RPM (5.83-6.67 Hz) bass, 800 RPM (13.33 Hz) treble
Doppler shift:
Typical pitch variation: ±0.5-1.0 semitones
Maximum theoretical: ~±1.5 semitones
Amplitude modulation:
Volume variation: 3-6 dB typical
Creates clear "pulsing" effect
The script parameters are tuned to match these real-world values
Rotation speeds:
Chorale (slow): 40-50 RPM (0.67-0.83 Hz) bass, 400 RPM (6.67 Hz) treble
Tremolo (fast): 350-400 RPM (5.83-6.67 Hz) bass, 800 RPM (13.33 Hz) treble
Doppler shift:
Typical pitch variation: ±0.5-1.0 semitones
Maximum theoretical: ~±1.5 semitones
Amplitude modulation:
Volume variation: 3-6 dB typical
Creates clear "pulsing" effect
The script parameters are tuned to match these real-world values
Stereo Microphone Techniques
Capturing the Rotary Effect
Different microphone placements create different stereo images:
COMMON LESLIE MIC TECHNIQUES:
CLOSE MICS (Narrow angle: 45-90°):
- Each mic close to one side of cabinet
- Strong channel separation but less rotation
- Good for detailed, focused sound
SPACED PAIR (Medium angle: 90-120°):
- Mics several feet from cabinet
- Good balance of separation and rotation
- Common studio technique
ROOM MICS (Wide angle: 120-180°):
- Mics capturing room sound
- Maximum rotation effect
- Creates immersive, spacious sound
The Stereo_Width_Deg parameter models these different
microphone placement strategies
Physical Modeling
⚙️ Mathematical Model Implementation
The core Leslie algorithm combines multiple physical phenomena:
LEFT CHANNEL (Microphone 1):
amplitude_L(t) = 1 - tremolo_Depth × 0.5 × [1 + sin(2π × rotation_Speed_Hz × t)]
delay_L(t) = base_samp × [1 + doppler_Depth × sin(2π × rotation_Speed_Hz × t + π/2)]
output_L = amplitude_L(t) × input[t - delay_L(t)]
RIGHT CHANNEL (Microphone 2):
amplitude_R(t) = 1 - tremolo_Depth × 0.5 × [1 + sin(2π × rotation_Speed_Hz × t + width_rad)]
delay_R(t) = base_samp × [1 + doppler_Depth × sin(2π × rotation_Speed_Hz × t + width_rad + π/2)]
output_R = amplitude_R(t) × input[t - delay_R(t)]
Where:
base_samp = 5.0 ms equivalent samples
width_rad = stereo_Width_Deg × π / 180
π/2 phase shift separates tremolo and Doppler effects
Why the π/2 phase shift?
- Tremolo peaks when horn points directly at microphone
- Doppler peaks when horn moving fastest toward microphone
- These occur 90° apart in the rotation cycle
- The π/2 phase shift models this physical relationship
Complete Processing Pipeline
🔄 Step-by-Step Physical Modeling
STEP 1: Source Preparation
Original Sound → Rename to "SourceAudio_Temp"
Convert to stereo → Essential for spatial effects
Create working copy for processing
STEP 2: Parameter Calculation
base_samp = 5.0 ms × sampling_rate / 1000
width_rad = stereo_Width_Deg × π / 180
Apply preset values if selected
STEP 3: Physical Model Application
For LEFT channel (row 1):
tremolo = 1 - tremolo_Depth×0.5×[1 + sin(2π×speed×t)]
doppler_delay = base_samp×[1 + doppler_Depth×sin(2π×speed×t + π/2)]
output = tremolo × delayed_input
For RIGHT channel (row 2):
Same calculations with +width_rad phase offset
Creates stereo rotation effect
STEP 4: Final Processing
Clean up temporary objects
Restore original sound name
Normalize output peak
Select and play final result
Parameter Relationships
How Parameters Interact
Physical parameter correlations:
Rotation_Speed_Hz effects:
Low (0.8-2.0 Hz): Chorale, slow swirling
Medium (2.0-5.0 Hz): Transition, noticeable motion
High (5.0-9.0 Hz): Tremolo, fast intensity
Doppler_Depth perception:
0.08-0.12: Subtle, natural pitch variation
0.12-0.18: Moderate, clear Leslie character
0.18-0.25: Strong, dramatic pitch effects
Tremolo_Depth ranges:
0.3-0.4: Gentle amplitude pulsing
0.4-0.6: Classic Leslie tremolo
0.6-0.7: Strong, obvious volume modulation
Stereo_Width_Deg results:
45-90°: Focused, narrow image
90-135°: Balanced stereo field
135-180°: Wide, immersive rotation
Rotation_Speed_Hz effects:
Low (0.8-2.0 Hz): Chorale, slow swirling
Medium (2.0-5.0 Hz): Transition, noticeable motion
High (5.0-9.0 Hz): Tremolo, fast intensity
Doppler_Depth perception:
0.08-0.12: Subtle, natural pitch variation
0.12-0.18: Moderate, clear Leslie character
0.18-0.25: Strong, dramatic pitch effects
Tremolo_Depth ranges:
0.3-0.4: Gentle amplitude pulsing
0.4-0.6: Classic Leslie tremolo
0.6-0.7: Strong, obvious volume modulation
Stereo_Width_Deg results:
45-90°: Focused, narrow image
90-135°: Balanced stereo field
135-180°: Wide, immersive rotation
Effect Presets
Chorale (Slow / Hymn)
⛪ Gentle Sacred Swirl
Settings: Speed: 0.8 Hz, Doppler: 0.08, Tremolo: 0.3, Width: 120°
Character: Slow, reverent rotation perfect for hymns and ballads
Best for: Church organ, sacred music, slow ballads
Tremolo (Fast / Rock)
🎸 Classic Rock Intensity
Settings: Speed: 6.8 Hz, Doppler: 0.12, Tremolo: 0.5, Width: 160°
Character: Fast, intense rotation for rock and blues
Best for: Rock organ, blues, energetic music
Transition (Ramping Up)
🔄 Medium Speed Versatility
Settings: Speed: 4.0 Hz, Doppler: 0.15, Tremolo: 0.4, Width: 180°
Character: Balanced rotation that works for many styles
Best for: General purpose, jazz, pop applications
Wide Stereo Spin
🌊 Maximum Spatial Effect
Settings: Speed: 2.5 Hz, Doppler: 0.10, Tremolo: 0.7, Width: 180°
Character: Very wide stereo image with clear rotation
Best for: Atmospheric pads, special effects
Broken Cabinet (Wobbly)
⚡ Unstable Mechanical Character
Settings: Speed: 9.0 Hz, Doppler: 0.25, Tremolo: 0.6, Width: 45°
Character: Fast, wobbly effect simulating worn mechanics
Best for: Lo-fi, experimental, vintage character
| Preset | Speed (Hz) | Doppler | Tremolo | Width (°) | Character | Use Case |
|---|---|---|---|---|---|---|
| Chorale | 0.8 | 0.08 | 0.3 | 120 | Slow, sacred | Hymns, ballads |
| Tremolo | 6.8 | 0.12 | 0.5 | 160 | Fast, intense | Rock, blues |
| Transition | 4.0 | 0.15 | 0.4 | 180 | Balanced | Jazz, pop |
| Wide Stereo | 2.5 | 0.10 | 0.7 | 180 | Spacious | Pads, effects |
| Broken Cabinet | 9.0 | 0.25 | 0.6 | 45 | Wobbly, lo-fi | Experimental |
Parameters
Rotation Mechanics
| Parameter | Type | Range | Default | Description |
|---|---|---|---|---|
| Rotation_Speed_Hz | positive | 0.8-9.0 | 6.8 | Horn rotation speed in Hz |
Horn Physics (Treble)
| Parameter | Type | Range | Default | Description |
|---|---|---|---|---|
| Doppler_Depth | positive | 0.08-0.25 | 0.12 | Pitch modulation intensity |
| Tremolo_Depth | positive | 0.3-0.7 | 0.5 | Volume modulation amount |
Microphone Placement
| Parameter | Type | Range | Default | Description |
|---|---|---|---|---|
| Stereo_Width_Deg | positive | 45-180 | 140 | Angle between left/right mics |
Output Options
| Parameter | Type | Range | Default | Description |
|---|---|---|---|---|
| Scale_peak | positive | 0.1-1.0 | 0.99 | Output normalization level |
| Play_after_processing | boolean | yes/no | yes | Auto-play processed sound |
Physical Parameter Relationships
How parameters create different Leslie characters:
Classic Chorale Sound:
Slow speed (0.8-1.5 Hz) + Subtle Doppler (0.08-0.10) + Medium tremolo (0.3-0.4)
Rock Tremolo Sound:
Fast speed (6.0-7.0 Hz) + Moderate Doppler (0.12-0.15) + Strong tremolo (0.5-0.6)
Jazz Transition Sound:
Medium speed (3.0-5.0 Hz) + Balanced parameters + Wide stereo (150-180°)
Experimental Sounds:
Extreme speeds (>8 Hz) + High Doppler (>0.2) + Narrow stereo (<90°)
The presets provide optimized starting points for each style
Classic Chorale Sound:
Slow speed (0.8-1.5 Hz) + Subtle Doppler (0.08-0.10) + Medium tremolo (0.3-0.4)
Rock Tremolo Sound:
Fast speed (6.0-7.0 Hz) + Moderate Doppler (0.12-0.15) + Strong tremolo (0.5-0.6)
Jazz Transition Sound:
Medium speed (3.0-5.0 Hz) + Balanced parameters + Wide stereo (150-180°)
Experimental Sounds:
Extreme speeds (>8 Hz) + High Doppler (>0.2) + Narrow stereo (<90°)
The presets provide optimized starting points for each style
Applications
Hammond Organ Emulation
Use case: Creating authentic Leslie speaker effects for organ sounds
Technique: Use appropriate presets for musical style
Style-specific settings:
- Gospel: Chorale for verses, Tremolo for choruses
- Jazz: Transition preset with medium speed
- Rock: Tremolo preset for driving sections
- Blues: Moderate speeds with clear tremolo
- Pop: Balanced parameters for support role
Result: Authentic Leslie-enhanced organ sounds
Creative Sound Design
Use case: Adding rotary motion to non-organ sounds
Technique: Experiment with unexpected source material
Creative applications:
- Vocals: Create swirling vocal effects
- Guitars: Add motion to sustained chords
- Synths: Enhance pads and leads with rotation
- Drums: Process cymbals or room mics
- Strings: Add vintage character to orchestral sounds
Result: Unique rotary-textured sounds
Vintage Production Effects
Use case: Recreating classic production techniques
Technique: Use authentic Leslie parameters
Historical applications:
- 1960s rock and soul organ sounds
- 1970s progressive rock textures
- Gospel and blues authenticity
- Jazz organ trio emulation
- Vintage film and TV scoring
Result: Authentic period-correct Leslie effects
Practical Workflow Examples
🎹 Gospel Organ Setup
Goal: Authentic gospel Hammond sound
Process:
- Start with Chorale preset for verses
- Switch to Tremolo preset for choruses
- Use Wide Stereo for spacious sections
- Adjust Doppler_Depth to taste (0.10-0.14 typical)
Result: Dynamic, expressive gospel organ
🎸 Psychedelic Guitar Effect
Goal: Create swirling guitar textures
Process:
- Use Transition or Tremolo preset
- Increase Tremolo_Depth for stronger effect
- Use Wide Stereo for maximum rotation
- Process sustained chords or lead lines
Result: Psychedelic rotary guitar sounds
🎚️ Vintage Mix Enhancement
Goal: Add vintage character to mixes
Process:
- Use Broken Cabinet for lo-fi character
- Apply to entire mix or subgroups
- Use subtle settings for enhancement
- Experiment with different stereo widths
Result: Vintage-styled mixes with rotary character
Advanced Techniques
Automation and dynamics:
- Create speed changes by processing different sections
- Use different presets for verse/chorus sections
- Automate parameters for evolving effects
- Create "ramping" effects with progressive speed changes
Layered processing:
- Process different frequency ranges separately
- Use different rotation speeds for bass and treble
- Combine with other effects like reverb or distortion
- Create complex textures with multiple rotary layers
Troubleshooting Common Issues
Problem: Artifacts or distortion at high speeds
Cause: Extreme Rotation_Speed_Hz or Doppler_Depth
Solution: Reduce speed or Doppler depth, use simpler source material
Cause: Extreme Rotation_Speed_Hz or Doppler_Depth
Solution: Reduce speed or Doppler depth, use simpler source material
Problem: Stereo image collapses or sounds mono
Cause: Very narrow Stereo_Width_Deg or phase issues
Solution: Increase stereo width, check source material phase
Cause: Very narrow Stereo_Width_Deg or phase issues
Solution: Increase stereo width, check source material phase
Problem: Effect too subtle or inaudible
Cause: Low parameter values or inappropriate source
Solution: Increase Depth parameters, use more sustained sounds
Cause: Low parameter values or inappropriate source
Solution: Increase Depth parameters, use more sustained sounds
Problem: Unnatural or electronic sounding
Cause: Extreme parameters with complex material
Solution: Use more moderate settings, try different source sounds
Cause: Extreme parameters with complex material
Solution: Use more moderate settings, try different source sounds
Technical Deep Dive
Physical Accuracy
Model Validation
The script models real Leslie speaker physics:
REAL LESLIE PARAMETERS vs SCRIPT:
Rotation Speed:
Real: 0.67-13.3 Hz (40-800 RPM)
Script: 0.8-9.0 Hz (covers typical range)
Doppler Shift:
Real: ±0.5-1.5 semitones typical
Script: Models equivalent pitch variation
Amplitude Modulation:
Real: 3-6 dB typical variation
Script: Equivalent tremolo depth scaling
Stereo Spread:
Real: Depends on mic placement
Script: 45-180° covers typical setups
The script provides physically accurate results within
musically useful parameter ranges
Computational Efficiency
Algorithm performance characteristics:
Processing approach: Single-pass formula evaluation
Computational load: Moderate (sine calculations + delay indexing)
Memory usage: Original + temporary stereo copy
Real-time potential: Good with optimized implementation
Optimization features:
- Combined tremolo/Doppler in single calculation
- Efficient delay line implementation
- Minimal temporary object creation
- Clean memory management
Suitable for both offline processing and real-time use
Processing approach: Single-pass formula evaluation
Computational load: Moderate (sine calculations + delay indexing)
Memory usage: Original + temporary stereo copy
Real-time potential: Good with optimized implementation
Optimization features:
- Combined tremolo/Doppler in single calculation
- Efficient delay line implementation
- Minimal temporary object creation
- Clean memory management
Suitable for both offline processing and real-time use
Historical Context
The Leslie Speaker Legacy
Historical significance: The Leslie speaker, invented in 1940 by Don Leslie, revolutionized the sound of the Hammond organ. Interestingly, Laurens Hammond initially opposed the invention, considering it a "gimmick." Despite this, the Leslie became an essential part of the Hammond sound, used by legendary artists like Jimmy Smith, Keith Emerson, and Billy Preston. The distinctive rotating speaker sound became so iconic that it defined entire genres of music, from gospel and jazz to rock and progressive. This script preserves that legacy through accurate digital modeling.
Cultural Impact
Leslie speaker in popular music:
1960s: The Beatles, The Doors, Procol Harum
1970s: Deep Purple, Pink Floyd, Led Zeppelin
1980s: The Police, Talking Heads, Stevie Wonder
1990s-present: Radiohead, Portishead, countless others
Musical roles:
- Gospel and church music foundation
- Jazz organ trio cornerstone
- Rock and blues texture
- Psychedelic and progressive element
- Modern production special effect
The script enables authentic recreation of these classic sounds
1960s: The Beatles, The Doors, Procol Harum
1970s: Deep Purple, Pink Floyd, Led Zeppelin
1980s: The Police, Talking Heads, Stevie Wonder
1990s-present: Radiohead, Portishead, countless others
Musical roles:
- Gospel and church music foundation
- Jazz organ trio cornerstone
- Rock and blues texture
- Psychedelic and progressive element
- Modern production special effect
The script enables authentic recreation of these classic sounds
Creative Extensions
Beyond Traditional Leslie Effects
Experimental applications:
Extreme parameters: Very fast speeds or deep modulation
Unconventional sources: Drums, vocals, sound effects
Multi-band processing: Different parameters per frequency range
Automation: Evolving rotation patterns
Hybrid effects: Combined with other modulation types
Production innovations:
- Modern electronic music textures
- Film scoring special effects
- Sound design for games and media
- Experimental music composition
- Vintage character for modern productions
The physical modeling foundation supports endless creativity
Extreme parameters: Very fast speeds or deep modulation
Unconventional sources: Drums, vocals, sound effects
Multi-band processing: Different parameters per frequency range
Automation: Evolving rotation patterns
Hybrid effects: Combined with other modulation types
Production innovations:
- Modern electronic music textures
- Film scoring special effects
- Sound design for games and media
- Experimental music composition
- Vintage character for modern productions
The physical modeling foundation supports endless creativity