Introduction: Understanding Compression
Audio compression is a dynamic processing technique that reduces the dynamic range of an audio signal by attenuating loud parts while leaving quieter parts untouched. At its core, compression is about controlling volume fluctuations to create more consistent, polished, and professional-sounding audio. Mastering compression parameters allows you to shape the dynamics, tone, and character of your audio with precision.
Core Compression Parameters Explained
Threshold
Definition: The volume level (measured in dB) above which compression begins to take effect.
| Threshold Setting | Effect on Sound | Typical Applications |
|---|---|---|
| High threshold (-10dB to 0dB) | Affects only the loudest peaks | Subtle dynamic control, peak limiting |
| Medium threshold (-25dB to -10dB) | Affects medium-loud portions | General purpose compression, vocals, instruments |
| Low threshold (-40dB to -25dB) | Affects most of the signal | Heavy compression, sustain enhancement, special effects |
Visual Representation:
^
│ Uncompressed
│ Signal
│ ╱╲
│ ╱ ╲ ╱╲
│ ╱ ╲ ╱ ╲ ╱╲
───┼─────────────────────────────── Threshold
│ ╲ ╱ ╲ ╱ ╲
│ ╲ ╱ ╲ ╱ ╲
│
│
Practical Tips:
- Start with threshold at -20dB and adjust until gain reduction meter shows 3-6dB of reduction on average
- Lower threshold for more aggressive compression
- For transparent compression, set threshold just below the loudest peaks
- When using multiple compressors in series, use progressively lower thresholds
Ratio
Definition: Determines the amount of attenuation applied to signals exceeding the threshold, expressed as input:output.
| Ratio | Compression Type | Effect on Sound | Typical Applications |
|---|---|---|---|
| 1.5:1 to 3:1 | Light compression | Subtle level control | General purpose, acoustic instruments, gentle vocal control |
| 4:1 to 8:1 | Medium compression | Noticeable dynamic control | Drums, bass, vocals, guitars |
| 10:1+ | Heavy compression | Dramatic dynamic control | Drums, special effects, aggressive sounds |
| ∞:1 | Limiting | Prevents signal from exceeding threshold | Peak limiting, brick wall limiting, protecting from clipping |
Visual Representation:
^
│ Output 4:1 Ratio
│ Level ╱
│ 2:1 ╱
│ ╱ ╱
│ ╱ ╱
│ ╱ ╱
│ ╱ ╱ 10:1
│ ╱ ╱ ╱
│ ╱ ╱ ╱
│ ╱ ╱ ╱
───┼─────────────────────────────────────►
│ Threshold Input Level
Practical Tips:
- For transparent compression, start with 2:1 or 3:1
- Bass guitar typically responds well to 4:1 ratio
- Drums may need 4:1 to 8:1 for proper control
- For de-essing vocals, try higher ratios (8:1+) with very specific frequency targeting
- Multiple gentle compressors (2:1 to 3:1) in series often sound more natural than one heavy compressor
Attack Time
Definition: The time (measured in milliseconds) it takes for the compressor to reach full gain reduction after the signal exceeds the threshold.
| Attack Setting | Time Range | Effect on Sound | Typical Applications |
|---|---|---|---|
| Fast | 0.5-10ms | Catches transients, reduces punch | Taming peaks, controlling spiky sounds, de-essing |
| Medium | 10-50ms | Balanced control, preserves some attack | Vocals, guitars, bass, general purpose |
| Slow | 50ms+ | Preserves transients, enhances punch | Drum room mics, adding punch to percussive sounds |
Visual Representation:
^
│ Level
│ ╱╲
│ ╱ ╲
│ ╱ ╲
───┼─────────────────────── Threshold
│ ╲
│ ╲
│ ╲ - Original signal
│ ╲
│ ╲
│ ╱ ╲
│ _______╱ ╲___
│ Fast attack ↑ ↑ Slow attack
│
Practical Tips:
- For percussive sounds (drums, piano): longer attack times (10-50ms) preserve initial impact
- For sustained sounds (vocals, bass): shorter attack times (1-10ms) provide more consistent control
- “Pumping” effects are often caused by attack times that are too short
- On drum bus or mix bus, attack times of 30ms+ often preserve more punch and groove
- Use very fast attack times (<1ms) for technical/corrective work, not creative compression
Release Time
Definition: The time (measured in milliseconds) it takes for the compressor to return to no gain reduction after the signal falls below the threshold.
| Release Setting | Time Range | Effect on Sound | Typical Applications |
|---|---|---|---|
| Fast | 5-100ms | Quick recovery, can cause distortion | Percussion, fast rhythmic material, special effects |
| Medium | 100-500ms | Balanced control, natural sound | Vocals, guitars, general purpose |
| Slow | 500ms+ | Smooth compression, maintains consistency | Bass, sustained instruments, gentle control |
| Auto | Adaptive | Program-dependent release time | General purpose mixing, when unsure |
Visual Representation:
^
│ Level
│
│
│ ╱╲ ╱╲
───┼─────────────────────── Threshold
│ ╲ ╱
│ ╲ ╱
│ ╲ ╱
│ ╲╱ - Original signal
│
│ ╱╲
│ ╱ ╲______
│ ↑
│ Fast release
│
│ ╱╲
│ ╱ ╲
│ ╱ ╲___________________
│ ↑
│ Slow release
Practical Tips:
- Match release time to tempo: faster tempos need faster release times
- As a starting point, try setting release to match the time between transients (e.g., between kick drum hits)
- For bass instruments, longer release times (200ms+) prevent distortion of low frequencies
- For vocals, medium release times (100-300ms) often work well
- Pumping effects are often caused by release times that are too short
- Auto release works well for program material with varying dynamics
Parameter Interaction: The Compression Matrix
Understanding how these parameters interact is crucial for effective compression:
| Parameter Combination | Effect | Musical Example |
|---|---|---|
| Low threshold + High ratio | Heavy compression, squashed sound | Drum room mics for aggressive rock |
| Low threshold + Low ratio | Gentle compression across most of signal | Overall mix bus glue |
| High threshold + High ratio | Peak limiting, transient control | Taming occasional loud notes |
| High threshold + Low ratio | Subtle dynamic smoothing | Acoustic guitar, gentle vocal compression |
| Fast attack + Fast release | Aggressive, potentially distorted | EDM sidechained bass |
| Fast attack + Slow release | Controlled, smooth | Vocal leveling |
| Slow attack + Fast release | Punchy, maintains transients | Drum enhancement |
| Slow attack + Slow release | Transparent, natural | Subtle mix bus compression |
Common Compression Scenarios with Recommended Settings
Vocals
| Parameter | Ballad/Gentle | Pop/Rock | Aggressive |
|---|---|---|---|
| Threshold | -20dB to -15dB | -25dB to -20dB | -30dB to -25dB |
| Ratio | 2:1 to 3:1 | 3:1 to 4:1 | 4:1 to 6:1 |
| Attack | 10-30ms | 5-15ms | 1-10ms |
| Release | 150-300ms | 100-200ms | 50-150ms |
| Notes | Preserve dynamics | Balance consistency | Create intensity |
Drums
| Parameter | Kick | Snare | Overheads | Room Mics |
|---|---|---|---|---|
| Threshold | -15dB to -10dB | -20dB to -15dB | -25dB to -20dB | -30dB to -20dB |
| Ratio | 4:1 to 6:1 | 3:1 to 5:1 | 2:1 to 3:1 | 4:1 to 10:1 |
| Attack | 10-30ms | 5-10ms | 15-30ms | 30-50ms+ |
| Release | 100-200ms | 80-150ms | 200-300ms | 300-500ms |
| Notes | Preserve attack | Control sustain | Gentle control | Room sound control |
Bass
| Parameter | Clean Bass | Rock Bass | Aggressive Bass |
|---|---|---|---|
| Threshold | -20dB to -15dB | -25dB to -20dB | -30dB to -25dB |
| Ratio | 3:1 to 4:1 | 4:1 to 6:1 | 6:1 to 8:1 |
| Attack | 20-40ms | 10-30ms | 5-15ms |
| Release | 200-400ms | 150-300ms | 100-200ms |
| Notes | Preserve dynamics | Control sustain | Even level |
Mix Bus / Master Bus
| Parameter | Subtle Glue | Standard | Aggressive |
|---|---|---|---|
| Threshold | -10dB to -5dB | -15dB to -10dB | -20dB to -15dB |
| Ratio | 1.5:1 to 2:1 | 2:1 to 3:1 | 3:1 to 4:1 |
| Attack | 30-50ms | 20-40ms | 10-30ms |
| Release | 200-400ms | 150-300ms | 100-200ms |
| Gain Reduction | 1-2dB | 2-4dB | 4-8dB |
| Notes | Transparent | Cohesive | Energetic |
Advanced Compression Techniques and Parameters
Knee
Definition: Determines how gradually or abruptly compression is applied as the signal approaches the threshold.
| Knee Setting | Effect | Best For |
|---|---|---|
| Hard Knee | Immediate compression at threshold | Precise control, creative effects |
| Soft Knee | Gradual transition into compression | Transparent compression, vocals, mix bus |
Visual Representation:
Output Level
^
│ Hard Knee
│ ╱│
│ ╱ │
│ ╱ │
│ ╱ │
│ ╱ │
│ ╱ │
│ │ │
│ │ Soft │
│ │ Knee │
│ │ ╱ │
│ │ ╱ │
│ / │
───┼───────────────────────────►
│ Threshold Input Level
Practical Tips:
- Use soft knee (10-30dB) for transparent compression on vocals, acoustic guitars, and mix bus
- Use hard knee for more aggressive, obvious compression effects or precise technical control
- Softer knees often sound more musical on complex material
Makeup Gain
Definition: Increases the output level to compensate for gain reduction, maintaining consistent perceived loudness.
Practical Tips:
- Typically set to match the amount of gain reduction
- Auto-gain features often provide good starting points
- For parallel compression, may be set higher than typical gain reduction
- When gain reduction varies widely, err on slightly less makeup gain to prevent clipping
Look-ahead
Definition: Allows the compressor to analyze the signal before processing it, helping catch fast transients.
Practical Tips:
- Use 1-10ms for catching transients in percussive material
- Helpful for brick wall limiting (0.5-2ms)
- Causes latency, so avoid during tracking or when latency is critical
- Particularly useful for de-essing or de-popping vocals
Common Compression Issues and Solutions
| Issue | Possible Cause | Solution |
|---|---|---|
| Pumping/Breathing | Release too fast | Increase release time |
| Distortion | Attack too fast on low frequencies | Increase attack time |
| Loss of punch | Attack too fast | Increase attack time to 20ms+ |
| Lifeless sound | Too much compression | Raise threshold, lower ratio |
| Still sounds inconsistent | Not enough compression | Lower threshold or raise ratio |
| Compression affecting wrong parts | Threshold not set optimally | Adjust threshold to target specific peaks |
| Unnatural artifacts | Too extreme settings | Use gentler settings or multiple compressors in series |
| Sounds “squashed” | Too much compression | Try parallel compression instead |
Types of Compressors and Their Characteristics
| Compressor Type | Characteristics | Best For | Famous Examples |
|---|---|---|---|
| VCA | Clean, precise, versatile | Drums, bus compression | SSL G-Comp, dbx 160 |
| Optical | Smooth, program-dependent release | Vocals, bass | LA-2A, LA-3A |
| FET | Fast, aggressive, colorful | Drums, percussion, guitars | 1176, Distressor |
| Variable-Mu | Warm, vintage, gentle | Mix bus, mastering, groups | Fairchild 670, Manley Vari-Mu |
| Digital | Clean, precise, flexible | Surgical control, multi-band | Many plugin compressors |
Advanced Compression Techniques
Parallel Compression
- Blend compressed and uncompressed signals
- Threshold: Low (-30 to -40dB)
- Ratio: High (4:1 to 10:1)
- Attack: Medium-fast (5-10ms)
- Release: Medium (100-200ms)
- Mix: 20-50% (depending on desired effect)
Multi-band Compression
- Apply different compression settings to different frequency bands
- Low band: Higher ratio, slower attack/release
- Mid band: Moderate settings
- High band: Lower ratio, faster release
- Crossover points: Typically around 100-250Hz and 2-5kHz
Sidechain Compression
- Trigger compression based on another signal
- Popular uses: Ducking bass when kick hits, vocal clarity, EDM pumping effects
- Threshold: Adjust based on trigger signal level
- Ratio: 4:1 for subtle, 10:1+ for obvious effect
- Attack: Very fast (0-5ms) for immediate ducking
- Release: Tempo-synchronized (e.g., 1/4 or 1/8 note)
Resources for Further Learning
Recommended Reading
- “Mixing with Your Mind” by Michael Stavrou
- “The Mixing Engineer’s Handbook” by Bobby Owsinski
- “Mastering Audio: The Art and the Science” by Bob Katz
Notable Compressor Plugins
- Universal Audio LA-2A and 1176 (analog emulations)
- FabFilter Pro-C2 (digital precision)
- Waves SSL G-Master Buss Compressor (console emulation)
- Softube CL 1B (optical emulation)
- iZotope Ozone Dynamics (digital multiband)
Learning Resources
- Mix engineer websites: Michael Brauer, Andrew Scheps, Chris Lord-Alge
- YouTube channels: Recording Revolution, Produce Like a Pro, MixbusTV
- Forums: GearSlutz/Gearspace, KVR Audio, r/audioengineering
Remember that compression is both a technical and creative tool. While these guidelines provide excellent starting points, trust your ears and don’t be afraid to experiment. The perfect compression setting is the one that serves the music best.