Understanding Compression in Live Sound

TL;DR

The compressor reduces dynamic range to improve intelligibility, comfort and headroom protection. In live sound, we aim for stability without "pumping": threshold adapted to real level, moderate ratio (2:1–4:1), attack slow enough to let useful transients through and musical release that follows tempo. Finish with reasonable make-up gain and regular A/B comparison.

1) Key Concepts

1.1 Fundamental Parameters

• Threshold : level (dB) above which compression occurs
• Ratio : compression strength (e.g., 4:1 means 4 dB input = 1 dB output above threshold)
• Knee (soft/hard) : progressive or abrupt transition to compression
• Attack : time before compression acts. Too fast = dulled transients; too slow = ineffective
• Release : time to return to 0 dB reduction. Too short = "pumping"; too long = permanent compression
• Detector : peak (reacts to peaks) vs RMS (reacts to energy)
• Sidechain : filtering of detection signal (e.g., HPF at 80–150 Hz to prevent bass from triggering everything)
• Make-up gain : compensation gain after reduction

1.2 Live Sound Objectives

1. Protect headroom
2. Homogenize levels
3. Increase intelligibility
4. Calm problematic source in the mix

In live sound, compression must be transparent and musical. Avoid pumping and listening fatigue.

2) Simple Method (6 Steps)

2.1 Gain Staging

Aim for peaks at –6 / –3 dBFS without clipping.

Before compressing, ensure the input signal is clean and well calibrated. See our article on Live Gain Staging for more details.

2.2 Define Intention

Write the intention before adjusting:
- "Stabilize vocals"
- "Hold bass"
- "Thicken snare"

2.3 Detection & Sidechain

• RMS musical (more natural than peak)
• HPF 100 Hz (default) to prevent bass from triggering compression

2.4 Threshold & Ratio

Aim for 3–6 dB of gain reduction (GR).

• Vocals/instruments : start 2:1–3:1
• Percussion : start 4:1

2.5 Attack/Release

Let transients live, lengthen release until pumping is avoided.

• Attack : slow enough to let useful attack (transient) through
• Release : set to song tempo

2.6 Make-up & A/B

• Compensate lost level with make-up gain
• Compare at equal volume with and without compression

3) Recommended Starting Points

3.1 Vocals

• Lead vocals : 2.5:1–3:1 · 20–35 ms · 60–120 ms · 3–6 dB · HPF 120 Hz
• Backing vocals : 2:1–3:1 · 15–25 ms · 80–150 ms · 2–4 dB · HPF 120–150 Hz

3.2 Instruments

• Bass DI : 3:1–4:1 · 10–20 ms · 80–150 ms · 4–8 dB · RMS
• Electric guitar : 2:1–3:1 · 10–20 ms · 80–120 ms · 2–4 dB · soft knee
• Keyboards : 2:1–2.5:1 · 10–15 ms · 80–120 ms · 2–4 dB
• Brass : 3:1 · 10–20 ms · 100–180 ms · 3–6 dB · HPF 120–150 Hz

3.3 Drums

• Snare : 4:1 · 20–40 ms · 80–150 ms · 3–6 dB
• Overheads : 2:1 · 20–30 ms · 150–250 ms · 1–3 dB · HPF sidechain

3.4 Parallel Bus

• Ratio : 2:1–4:1
• Attack : medium
• Release : fast
• Dose the send to mix with direct signal

4) Pro Tips

4.1 Parallel Compression

Mix a compressed bus with direct to maintain transparency while adding density.

Create an auxiliary bus, send signal to it, compress heavily, then mix with direct signal.

4.2 De-essing via Sidechain

Use an EQ 5–8 kHz in detection to target sibilants.

Instead of a dedicated de-esser, use compressor sidechain with a filter in highs.

4.3 Keying (External Sidechain)

Kick drum can "breathe" on bass.

Use kick drum as detection source to compress bass, creating a musical "pumping" effect.

4.4 Auto-release

For sources with unstable dynamics, auto-release can help.

Some compressors offer an auto-release mode that adapts automatically to signal.

4.5 Limiter at End of Chain

Use a limiter at end of bus for gentle protection (without replacing mix).

The limiter protects against unexpected peaks without audibly affecting sound.

5) Common Mistakes

5.1 Attack Too Fast

Loss of punch : transients are crushed.

Let useful attack through before compression acts.

5.2 Release Too Short

Pumping : compression "breathes" audibly.

Lengthen release until pumping disappears.

5.3 Excessive Make-up

Latent feedback : too much compensation gain can create feedback problems.

Compensate reasonably, not beyond input level.

5.4 Compressing Without Need

Fatigue : too much compression kills dynamics.

Compress only if necessary, not out of habit.

5.5 Confusing Polarity/Phase

Masking causes : a phase problem can seem like a compression problem.

Check mic placement and phase first before compressing. See Phase vs Polarity.

6) Soundcheck Checklist (60 s)

Before the concert, quickly check:

• ✅ Peaks –6 dBFS without clipping
• ✅ 3–6 dB max GR (gain reduction)
• ✅ Attack: let useful attack through
• ✅ Release: set to tempo
• ✅ A/B at equal level
• ✅ HPF sidechain enabled by default

7) Did You Know?

• –6 dB is not perceived as half as loud (logarithmic perception)
• Look-ahead adds latency (watch out for IEM returns)
• Upward compression : raise lows instead of cutting peaks (inverse compression)

Glossary

• Headroom : margin before saturation
• Knee : smoothness of transition to compression
• Sidechain : detection signal (can be filtered)
• RMS : average energy (Root Mean Square)
• GR : Gain Reduction

Additional Resources

• Live Gain Staging - Optimize levels before compression
• Phase vs Polarity - Check phase before compressing
• Proper Microphone Placement - Good placement reduces need for compression