Compression vs limiting

At a glance

What's the difference between compression and limiting?

Both turn a sound down when it gets louder than a level you set. Both are dynamics processors, which means they change the gap between the loud and quiet parts of a sound, its dynamic range. The difference is how far they go and how much of the sound they touch.

A compressor turns sounds over its threshold down by a ratio, so loud moments come down a bit and very loud ones come down more. Its threshold usually sits in the body of the sound, so it's working on most of the notes or hits, and its attack and release are set to change how each one starts and fades. That's why a compressor can make drums punchier or a vocal smoother: it reshapes the level from moment to moment.

A limiter only cares about one line, its ceiling. Whatever tries to go above it gets turned down until it fits, as fast as possible, and everything under it is left alone. On a well-set master the limiter does nothing for most of the song and catches only the short peaks, which is what lets you turn the whole track up without anything going over 0 dBFS.

Put numbers on it. Say a snare peaks at −2 dBFS and a ghost note between the snares peaks at −12 dBFS, 10 dB quieter. A compressor at −14 dB and 4:1 brings the snare, 12 dB over, down to 3 dB over, so it comes out at −11 dBFS. The ghost note is 2 dB over and comes out at −13.5. They're now only 2.5 dB apart, and the groove sounds more even. A limiter with its ceiling at −6 dB pulls the snare down 4 dB to −6 and never touches the ghost note, which is still 6 dB quieter. The snare has lost its loudest 4 dB and the balance under it hasn't changed.

Reading the lab

The lab is one compressor. The lesson steps under it turn it into a limiter and back again, one control at a time. On the Whole loop picture the original loop is the pale shape behind and the processed loop is pink. Along the bottom runs an amber lane for gain reduction, which is how far the sound is being turned down at each moment, so the deeper it dips the harder the processor is working.

The Transfer curve tab draws the transfer curve. A compressor bends it gently above the threshold; a limiter bends it flat. One hit up close zooms in on a single drum hit, which is where the attack does its work, and the Turning down readout above the picture shows how much is being taken off. On this page, watch how much of the loop the amber lane covers, because that's the clearest way to tell a compressor from a limiter.

The Original and Compressed switch takes the processing out and matches the loudness of the original to it, so the louder version can't win just by being louder.

Is a limiter just a compressor?

Inside, yes: a limiter measures the level and turns the sound down, the same as a compressor. You can turn a compressor into something very close to a limiter by moving a few controls, and the lab's lesson does it one at a time so you can hear which ones count.

Threshold up to the peaks

A limiter's threshold, or its ceiling, sits right at the top of the sound. Move the compressor's threshold up to −8 dB, just under the drum loop's loudest hits, and it only reaches the tops of the biggest ones. At 4:1 with a 10 ms attack, the most it takes off is 1.5 dB.

Ratio as high as it goes

Next comes the ratio. At 4:1, a peak 12 dB over the threshold comes out 3 dB over; at 20:1 it's 0.6 dB over, and at ∞:1 it's held at the threshold exactly. The figure draws six ratios from 1:1 up to ∞:1 at the same threshold.

Turn the ratio to the top in the lab, though, and almost nothing changes: the compressor goes from 1.5 dB of gain reduction to 1.8. With a 10 ms attack it still hasn't reacted by the time the front of each hit has gone past, so the peaks it should be limiting get through.

The fastest attack

This is the change that makes the biggest difference. A drum transient can reach its peak in under a millisecond, so to catch it the compressor has to react in a fraction of that. Set the attack to 0.1 ms and a hard knee, so the full ratio switches on right at the threshold, and the gain reduction jumps to 5.9 dB. On the One hit up close picture, the spike at the front of the hit is held down instead of poking through.

A short release

A limiter should let go as soon as the peak has passed, so it touches as little of the music as it can. At a 50 ms release, the lab's compressor is working on 19% of the loop, down from 87% at the compressor settings it started with. It now behaves like a limiter.

The part a compressor can't copy

Watch the Loudest peak out readout in step 5. The threshold is at −8 dB but the loop still peaks at −4.3 dBFS, 3.7 dB over. Even a 0.1 ms attack needs the peak to arrive before it can start turning down, and by then the first few samples of each hit have already gone out.

A limiter fixes that with lookahead. It delays the audio by a few milliseconds and measures it as it comes in, so the gain is already moving down when the peak reaches the output. One that never lets a single sample past its ceiling is called a brickwall limiter. The lookahead lesson shows what those few milliseconds buy, and they're the reason a compressor at ∞:1 can't replace a limiter as the last plug-in on a master.

Same gain reduction, different job

Gain reduction alone doesn't tell you which one you're using. Step 6 puts the lab back on compressor settings, 4:1 with a 10 ms attack, and moves the threshold to −16 dB so the most it takes off is 5.8 dB, exactly what the limiter took off in step 5. Measured on the loop, the numbers then look like this.

The compressor is working on half the loop, more than twice as much of it as the limiter. It turns the body of every hit and the space between them down, while the 10 ms attack lets the front of each hit through. That makes the crest factor, the gap between the peak level and the average (RMS) level, bigger. The drums have more punch but no more loudness, because the peaks that decide how far you can turn them up came through almost untouched.

The limiter does the opposite. It takes 3.3 dB off the peaks and leaves most of the loop alone, so the crest factor shrinks. Turn each one back up until it peaks where the original did and the limited loop comes out about 0.8 dB louder than the original, while the compressed loop is about 1.9 dB quieter. A limiter with lookahead wouldn't let those peaks overshoot, so it would get you more.

How compression and limiting sound

Light compression is often hard to hear on its own. You notice it as a vocal that stays at the front of the mix, or drums that sound tighter, and you hear it most clearly when you switch it off. Pushed harder, a compressor changes the shape of every note: a slow attack makes the hits snappier, a fast one softens them, and a release that recovers in time with the beat makes the track breathe in and out.

Light limiting should be almost impossible to hear. A limiter taking 1 to 3 dB off the loudest moments of a master leaves the drums and vocal sounding the same, with the very tops of a few peaks gone. You start hearing it once it has to take off more than about 6 dB, when the snare loses its crack and quieter sounds dip after each loud hit.

On bass, a short release can also make a limiter distort. That happens when the gain moves within a single cycle of a low note. A 55 Hz note takes about 18 ms to complete one cycle, so a release shorter than that follows the wave itself. Pick Bass in the lab, keep the limiter settings from step 5 and pull the release under 20 ms to hear the low end turn fuzzy.

When to use a compressor and when to use a limiter

Reach for a compressor whenever you want to change how a part feels or how evenly it sits in the mix. Reach for a limiter when you need a hard stop, a level nothing gets past whatever happens.

Plenty of hardware sits between the two. Universal Audio sells its 1176LN reissue as a limiting amplifier, yet its ratio buttons run 4:1, 8:1, 12:1 and 20:1, so the same unit is a compressor at one end and close to a limiter at the other. The Teletronix LA-2A has a Compress/Limit switch. Neither has lookahead, so on a master you still want a proper limiter last, and both are usually set up as compressors with the threshold in the body of the sound.

The limiter itself is covered in detail in what is a limiter?, including ceilings, true peak and streaming loudness, and the compression lesson goes through every compressor control.

Using a compressor and a limiter together

Most finished tracks use both, at different points. Each one sounds best doing a small amount of its own job, so split the work between them instead of asking one to do everything.

1. Compress the tracks that need itVocals, bass and drums get their own compressors, set for 3 to 6 dB of gain reduction on the loudest moments. This is where the shape and evenness of each part are decided.
2. Add light bus compression if you like itA compressor across the drum bus or the whole mix at 2:1, taking 1 to 2 dB off, makes the parts move together. Skip it if you can't hear it helping at matched loudness.
3. Put the limiter last on the masterSet its ceiling to −1 dB with true peak detection on, so the file stays clean when it's turned into MP3 or AAC for streaming.
4. Push into the limiter gentlyRaise its input gain, or lower its threshold, until the loudest chorus shows 2 to 4 dB of gain reduction. If it needs much more than 6 dB to get loud enough, go back to the first two steps and do more of the levelling there.
5. Compare at the same loudnessBypass each one and match the levels before you decide. A louder version almost always sounds better for the first few seconds.

A clipper sometimes goes just before the limiter too, to shave the fastest drum peaks so the limiter has less to catch. The limiting vs clipping comparison covers that pairing, and the mastering chain lesson puts every stage in order.

Common mistakes

Levelling a vocal with a limiter

A limiter on a vocal catches the loudest syllables and leaves everything else where it was, so a verse that's 6 dB quieter than the chorus stays 6 dB quieter. Levelling takes a compressor with its threshold in the body of the voice, and the vocal chain lesson shows where each one goes.

Trusting a compressor at ∞:1 to catch everything

As step 5 shows, a compressor without lookahead lets the front of each peak overshoot its threshold, by 3.7 dB on the drum loop in the lab. That's fine for taming a part in the mix. As the last stage before export it's a problem, because any overshoot past 0 dBFS gets clipped off when you export a 16- or 24-bit file.

A limiter doing the compressor's job

If the master limiter is taking 8 dB off, it's levelling the whole mix as well as catching peaks, and that's where drums go soft and the mix starts ducking around the kick. Do some of that work with compressors earlier in the chain and the limiter can go back to catching peaks.

Losing the punch with a fast attack

A drum compressor with an attack under 1 ms and a high ratio is a limiter in all but name, and it takes the snap off every hit. If you wanted punch, slow the attack to 10 to 30 ms. The attack and release lesson goes through it.

Training your ears to hear the difference

By ear, a limiter and a compressor differ mostly in attack and ratio, and those are the two things the quiz under the lab tests. Each round sets hidden settings and asks whether the attack is fast or slow, or whether the ratio is 2:1, 4:1 or 10:1, with the controls covered and the loudness matched. A fast attack with a high ratio is the limiter sound: the front of every hit softened and the loud and quiet hits pulled close together. There are eight rounds, and your best score stays in your browser.

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