Understanding Compression

A compressor is an automatic volume knob. It turns down the loud parts so the quiet parts can come up. Used well, it makes vocals steady, drums punchy and mixes glued together. Used badly, it sucks the life out of everything.

What a Compressor Does

The difference between the loudest and quietest parts of a sound is its dynamic range. A compressor reduces dynamic range: when the signal gets louder than a set level, the compressor turns it down. Then you turn the whole thing back up, so the quiet details are louder than before.

Imagine a singer who whispers the verse and belts the chorus. Without compression, you'd have to ride the fader all song. A compressor does that riding for you, thousands of times per second.

The Controls

Threshold

The threshold is the level where compression starts, in dBFS. Anything above it gets turned down; anything below it is left alone. A lower threshold means more of the signal gets compressed.

Ratio

The ratio sets how much the signal is turned down once it's over the threshold. At 4:1, for every 4 dB the input goes over the threshold, only 1 dB comes out.

Ratio in numbers (threshold at -20 dBFS, input peaks at -8 dBFS: 12 dB over)

Ratio Output above threshold Output peak Gain reduction 1:1 12 dB -8 dBFS 0 dB (no compression) 2:1 6 dB -14 dBFS 6 dB (gentle) 4:1 3 dB -17 dBFS 9 dB (medium) 10:1 1.2 dB -18.8 dBFS 10.8 dB (heavy) ∞:1 0 dB -20 dBFS 12 dB (limiting)

Output above threshold = amount over ÷ ratio. At 4:1, 12 dB over becomes 12 ÷ 4 = 3 dB over.

Attack

Attack is how quickly the compressor reacts once the signal crosses the threshold, usually from about 0.1 ms to 100 ms. It's the most important control for tone.

Release

Release is how quickly the compressor stops compressing once the signal drops below the threshold, usually 20 ms to 1 s. Too fast can cause distortion on bass or a "pumping" effect; too slow keeps the sound squashed all the time.

Knee

The knee controls how compression starts. A hard knee starts full compression exactly at the threshold, which sounds more aggressive. A soft knee eases in gradually around it, which sounds smoother and more transparent.

Make-up gain

Compression makes the signal quieter. Make-up gain (or output gain) turns it back up. Set it so the compressed sound is about as loud as the bypassed sound, so you can judge the change fairly.

Gain reduction meter

The gain reduction (GR) meter shows how many dB the compressor is turning the signal down right now. Watch it! It tells you more than your ears do at first. For most jobs, 2–6 dB of gain reduction on peaks is plenty.

Attack and Punch

The beginning of a drum hit is called the transient: a very short, loud spike that gives the sound its punch. Attack decides what the compressor does with it:

Slow attack (10–30 ms): the transient passes through before the compressor clamps down on the body. Result: more punch, because the hit stands out from its tail.
Fast attack (0.1–2 ms): the compressor grabs the transient itself. Result: softer, rounder, more controlled. Good for taming spiky sounds, bad if you wanted snap.

Release then shapes the tail. A release timed to the tempo lets the compressor "breathe" with the groove: it recovers just before the next hit. A common starting point is to set release so the GR meter returns to zero just before the next beat.

Producer tip

Can't hear what the compressor is doing? Set the ratio to 10:1, pull the threshold down for 10–15 dB of gain reduction, and adjust attack and release while listening. The effect is exaggerated, so it's easy to hear. Once you like the shape, raise the threshold and lower the ratio to a sensible amount.

Starting Settings

These are sensible starting points. Adjust the threshold for each source until you see the target gain reduction, then fine-tune by ear.

Source	Ratio	Attack	Release	Gain reduction	Goal
Lead vocal	3:1–4:1	5–15 ms	50–100 ms (or auto)	3–6 dB	Steady level, upfront
Kick / snare	4:1–6:1	10–30 ms	50–150 ms (to the tempo)	3–6 dB	Punch and snap
Drum bus	2:1–4:1	10–30 ms	100–200 ms or auto	2–4 dB	Glue the kit together
Bass	4:1	10–30 ms	100–200 ms	3–6 dB	Even notes, solid low end
Acoustic guitar	2:1–3:1	10–20 ms	100–200 ms	2–4 dB	Even strumming
Mix bus	1.5:1–2:1	10–30 ms	Auto or 100–300 ms	1–2 dB	Glue, gentle cohesion

With a ratio of 4:1, a signal that goes 8 dB over the threshold comes out dB over the threshold.

A attack lets the transient through and makes drums punchier.

Compressor Types

Analog compressors use different circuits to reduce gain. Each has a famous sound, and most DAWs and plugin makers offer emulations.

Type	Famous example	Character	Great on
VCA	SSL G-Series bus compressor, dbx 160	Clean, precise, punchy; fast and predictable	Drum bus, mix bus, snare
FET	Universal Audio 1176	Very fast attack, aggressive, adds bite and color	Vocals, drums, room mics, bass
Optical	Teletronix LA-2A	Slow, smooth, program-dependent release; very musical	Vocals, bass, smooth leveling
Variable-mu (tube)	Fairchild 660/670, Manley Variable Mu	Gentle, warm; ratio rises the harder you push it	Mix bus, mastering, vocals

A classic vocal chain uses two in series: an 1176-style FET to catch fast peaks, then an LA-2A-style optical for smooth overall leveling. Each one does a little, so neither has to work hard.

Many DAW compressors include several of these behaviors as modes. Ableton's Glue Compressor is modeled on the SSL-style bus compressor, and Logic's Compressor has models named after classic circuit types.

Parallel Compression

Parallel compression (also called New York compression) blends a heavily compressed copy of a sound with the uncompressed original. The dry signal keeps the natural transients; the crushed copy brings up the body, room and sustain. You get power without losing punch.

Drums→Dry (to master)+Send → heavy compressor (10:1, -15 dB GR)→Blend in to taste

Parallel compression: mix a squashed copy underneath the untouched original.

Many compressors have a mix (dry/wet) knob that does the same thing inside one plugin. Full details on sends and parallel processing are in Buses & Parallel FX.

Limiting vs. Compression

A limiter is a compressor with a very high ratio (10:1 up to ∞:1) and a very fast attack. Its job is to stop the signal from going above a ceiling, no matter what. Compressors shape dynamics; limiters catch peaks.

Use a compressor on tracks and buses to control and shape.
Use a limiter mostly at the end of the mastering chain, or on a track to catch rare spikes.

Learn how limiters make tracks loud in Loudness & LUFS.

Signs of Over-Compression

Watch out for

Over-compression is one of the most common beginner mistakes, because compressed audio sounds louder and louder sounds "better" at first.

Lost punch: drums sound flat and small, even though they're louder.
Pumping you didn't ask for: the level audibly ducks and swells with the beat.
Breaths and noise get loud: room sound, mouth clicks and hiss creep up between phrases.
Distortion on bass: a release that's too fast follows each wave cycle and adds grit.
Lifeless, tiring sound: everything is the same level, so nothing stands out.
GR meter never returns to zero: the compressor is working all the time.

Try it in your DAW

Put a compressor on a vocal or snare track (Ableton: Compressor, FL Studio: Fruity Limiter in COMP mode or Fruity Compressor, Logic: Compressor).
Set ratio 4:1, attack 10 ms, release 100 ms.
Lower the threshold until the GR meter shows 3–6 dB on the loudest hits.
Sweep the attack from fastest to 30 ms and listen to the transient.
Raise make-up gain until the bypassed and compressed versions are equally loud, then toggle bypass. Keep it only if it sounds better at equal volume.

1. A signal goes 12 dB over the threshold at a ratio of 3:1. How far over the threshold is the output?

12 dB
9 dB
4 dB
3 dB

12 ÷ 3 = 4 dB over the threshold. The compressor removed 8 dB.

2. You want a snare to sound punchier. Which attack setting helps most?

The fastest possible (0.1 ms)
A slower attack, around 10–30 ms
No attack control matters
Attack longer than 500 ms

A slower attack lets the transient through before the compressor reacts, so the hit stands out.

3. Which compressor type is the LA-2A?

FET
VCA
Variable-mu
Optical

The LA-2A uses an optical cell, which gives it its smooth, program-dependent response.

4. What is parallel compression?

Blending a heavily compressed copy with the uncompressed original
Using two compressors with identical settings
Compressing the left and right channels separately
Compressing only the high frequencies

The dry signal keeps the transients and the compressed copy adds body and sustain.

5. Which is a sign of over-compression?

The GR meter moves 2 dB on peaks
The drums sound punchier at the same level
The gain reduction meter never returns to zero and the drums sound flat
The vocal sits steadily in the mix

Constant gain reduction and lost punch mean the compressor is working too hard.