What is a limiter?

Limiting in plain English

A 16 or 24-bit file has a top: 0 dBFS is the highest level it can store, and anything that tries to go above it is cut off when you export. A limiter keeps your audio under a line of your choosing, called the ceiling, by turning the sound down for the few milliseconds when a peak would cross it.

It does this the way a compressor does, by changing the volume. What makes it a limiter is how far it goes. A compressor at 4:1 lets a sound that goes 8 dB over its threshold through at 2 dB over. A limiter lets it through at 0 dB over, every time, so the ceiling works like a wall the waveform can't get past. That's why mastering limiters are often called brickwall limiters.

Most of the time a limiter is used for loudness. If the loudest snare hit in your mix peaks at −0.5 dBFS, you can't turn the mix up without clipping it. Put a limiter on, turn the mix up 4 dB into it, and the limiter takes those few loud hits down by 4 dB while most of the quieter music in between gets the full 4 dB louder. The peaks stay where they were and the track sounds louder.

Reading the lab

The lab is a mastering limiter on a loop, with a loudness meter after it. The Limiter knob turns the loop up into a ceiling at −1 dBTP, so it works like the input gain or threshold on a real limiter. Gain is plain volume, and it's there so you can hear what happens when you turn a mix up with nothing to stop the peaks.

The big screen shows the loudness of the loop over time in LUFS, with the original as a pale line behind your master. Next to it are the meters: momentary, short-term and integrated loudness, then TP for true peak, the level the waveform reaches between the samples. Below them, the waveform of the whole loop fills out towards the dashed 0 dBFS lines as you push the limiter, with an amber lane underneath that draws the gain reduction moment by moment. The crest factor screen shows the gap between the peaks and the average level of each version.

The Right now panel tells you how far the limiter is turning the loudest moments down, which is its gain reduction, measured on the audio you hear. The Release knob sets how quickly it lets go, and the amber lane shows what that does. The streaming buttons apply each service's loudness normalisation, and A/B switches between your master and the untouched loop.

The limiter in the lab looks 5 ms ahead, lets go with an 80 ms release until you move the Release knob, and listens to both channels together, so the stereo image doesn't shift when it works. Those are typical settings for a mastering limiter.

How does a limiter work?

A limiter measures how high the next stretch of audio will go and works out how far it has to turn the sound down to keep that peak at the ceiling. Then it moves its gain there quickly, and back up slowly once the peak has passed, and it does this for every sample.

Say the ceiling is −1 dB and you've pushed the mix 6 dB into the limiter. A snare that peaked at −2 dBFS now arrives at +4 dBFS, 5 dB over the ceiling, so the limiter turns the sound down by 5 dB for that moment. A hi-hat that peaked at −12 dBFS arrives at −6, still under the ceiling, and passes through 6 dB louder with nothing taken off.

Lookahead

A drum transient can go from silence to its peak in under a millisecond. A limiter that only reacts once the peak arrives has to slam the gain down within that millisecond, and a gain change that fast distorts the sound. So most limiters use lookahead: they hold the audio back by a few milliseconds and measure it as it comes in, which means the gain can start moving down smoothly before the peak reaches the output. The lab's 5 ms of lookahead is 220 samples of warning at 44.1 kHz.

The cost is delay. A limiter with lookahead makes the track a few milliseconds late, and your DAW moves everything else to match without you having to do anything. It's only a problem when you're recording through a limiter and hearing yourself back, where a delay of a few milliseconds can be enough to feel.

Release

Release is how quickly the limiter lets go after a peak has passed. With a short release, say 10 ms, the gain springs back almost straight away, so very little of the music around the peak gets turned down and the track ends up louder. Push that too far and the gain starts moving within a single cycle of a bass note. A 55 Hz note takes about 18 ms per cycle, so a release shorter than that reshapes the wave itself, and you hear it as a fuzzy, distorted low end.

A longer release, 100 ms or more, is smoother but holds the level down for longer after each hit. Push a limiter with a long release hard and you can hear the whole mix dip after every kick and swell back up, which is called pumping. Many limiters have an auto release, which shortens the release for short peaks and lengthens it when the limiter has been working for a while. Waves calls theirs ARC on the L2, and each of the styles on FabFilter Pro-L 2 handles the release in its own way.

The controls on a limiter

Limiters have fewer controls than compressors, and the names change from one to the next.

Input gain or threshold

This sets how hard you push the sound into the ceiling. Some limiters have an input gain knob that turns the sound up, like Logic's Adaptive Limiter and FabFilter Pro-L 2, which both call it Gain. Others have a threshold you pull down, like the Waves L1 and L2. Pulling a threshold down by 4 dB pushes the loudest peaks 4 dB harder into the ceiling, the same as 4 dB of input gain, and either number tells you roughly how much louder the quiet parts will get.

Ceiling or output

The ceiling is the highest level that comes out, and it stays fixed however hard you push. Waves calls it Out Ceiling and Ableton Live's Limiter calls it Ceiling. Set it to −1 dB on a master that's going to streaming, which leaves room for the small overshoots that happen when a file is turned into MP3 or AAC.

True peak

A plain limiter watches the samples, but the waveform your speakers play is a smooth curve drawn through them, and that curve can rise above the highest sample. A limiter with true peak mode works out the curve between the samples and holds that under the ceiling too. Switch it on for anything you're about to release.

Release, lookahead and style

Release and lookahead work as described above. Many limiters also have a style or mode switch that changes how the release reacts, from clean and slow to fast and aggressive. Try a couple on the same material at the same loudness and keep the one where the drums still sound like drums.

Stereo link

A linked limiter turns both channels down by the same amount, even when the peak is only on one side. Unlinked, a loud hit panned left would turn the left channel down on its own and pull the whole mix to the right for a moment. Leave it linked on a master.

Why does a limiter make a track louder?

How loud a track sounds depends mostly on its average level, while how far you can turn it up depends on its peaks. The gap between the two is the crest factor. The full mix in the lab, with no limiter on it, peaks 16.2 dB above its average level, and a few of those peaks, each lasting a couple of milliseconds, decide how loud the whole song can go.

A limiter shrinks that gap. The peaks are held at the ceiling while the music under them comes up, so the average rises and the peaks don't. In the lab, 6 dB of push on the full mix takes its crest factor from 16.2 dB to 12.2 dB, and it plays 3.2 LU louder at a lower peak level than it started with.

How much loudness you get depends on where the loudness is. The lab's vocal has a few loud syllables with plenty of space between them, so 6 dB of push leaves most of it untouched and makes it 5.0 LU louder. In the drum loop most of the loudness is in the kicks and snares themselves, which are what the limiter turns down: it has the loop more than 1 dB down for over a third of the time, and the same push buys 2.1 LU.

Each extra decibel buys less than the one before, because the harder you push, the more of the music sits over the ceiling, and the limiter takes a larger share of each new decibel straight back off. On the full mix in the lab, 2 dB of push buys 1.2 LU and 6 dB buys 3.2 LU, a little over half the push each time. The next 8 dB adds only 1.5 LU more, while the gain reduction on the loudest hits climbs from 6.9 to 14.9 dB.

How much limiting is too much?

Watch the gain reduction on the loudest part of the song, usually the biggest chorus or drop. These ranges are a rough guide for a full mix on a mastering limiter.

Genre moves these lines a lot. Dense electronic music with soft-clipped drums can take more than an acoustic folk song can, because there's less sharp detail left to lose. If you need more than about 6 dB, split the job: a clipper before the limiter can shave the fastest drum peaks with less audible change, and a bus compressor earlier in the chain can do some of the levelling.

The source matters as much as the number. Drums lose their snap first, because the transients are what the limiter turns down; a vocal or a pad can take the same gain reduction on its few loud moments and sound almost the same. Try both in the lab at the same push.

Limiting for Spotify, Apple Music and YouTube

Streaming services measure the loudness of every track and turn it up or down to their own level. Spotify and YouTube play tracks at about −14 LUFS and Apple Music at −16. So when you push a master to −8 LUFS, Spotify turns it down 6 dB and plays it at the same level as a −14 LUFS master of the same song. The loudness the limiter bought is gone, and the smaller peaks are left.

Loud masters still have their place. Plenty of dance, hip-hop and rock records are limited hard because the dense sound is part of the style. For anything going to streaming, though, judge your limiter at the level people will hear it, which the streaming buttons in the lab do for you. The LUFS lesson covers the targets in detail.

Where does a limiter go?

On the master, last in the chain, it sets the final peak level and the loudness of the file you export. Nothing goes after it apart from a meter, and dither if you're exporting at 16-bit.

On a drum bus or a group, a limiter can stop the odd hit that's far louder than the rest, so you can mix the group a little louder without it spiking. Use a light touch here and leave the loudness to the master.

On a single track it's useful when one part has a few peaks you can't tame any other way, like a sampled vocal with one shouted word. For general control on a vocal or a bass, a compressor sounds smoother, because it works over a wider range instead of stopping everything at one line.

Limiters also work as safety nets, for example on a live mixing desk or after a synth patch with a lot of feedback, where one runaway moment could hurt someone's ears. Set the ceiling a few decibels under the maximum and the limiter only ever acts on accidents.

Hardware units with limiter in the name, like the Universal Audio 1176LN Limiting Amplifier or the LA-2A with its Compress/Limit switch, are compressors that can be set to clamp down hard on peaks. Without lookahead they can't stop every sample at a ceiling the way a mastering limiter plug-in can.

How to set a limiter on a master, step by step

1. Put it lastPlace the limiter at the end of the master chain, with a loudness meter after it so you see exactly what you'll export.
2. Set the ceiling first−1 dB, with true peak mode on. If your limiter has no true peak option, use −1.5 dB.
3. Leave the release on auto, or start around 100 msYou can come back to it once you hear what the limiter is doing.
4. Play the loudest section and add input gainWatch the gain reduction while the biggest chorus plays. Stop at 2 to 4 dB on a first pass, or where the drums start to lose their edge.
5. Listen for the two ways it failsFuzz on the bass means the release is too short. The mix dipping after each kick means it's too long, or you're pushing too hard.
6. Compare at the same loudnessBypass the limiter and turn the bypassed version up with plain gain until the two match on the LUFS meter. If you can't hear what the limiter improved, back the input off.

Limiter, compressor or clipper?

All three can lower peaks. A compressor turns loud moments down by a ratio, so it reshapes the level of a sound over a wide range and changes how the hits feel; the compression lesson covers it. A limiter only acts at the ceiling and stops everything there. A clipper doesn't change the volume at all: it cuts the top off the wave instantly, which adds distortion but can keep drum hits punchier at the same loudness. The compression vs limiting and limiting vs clipping comparisons put them side by side.

Common limiting mistakes

Judging it while it's louder

Switch a limiter off and the track drops in level, and quieter nearly always sounds worse for the first few seconds. That's how a limiter pushed far too hard wins a quick comparison. Match the loudness of the two versions before you decide.

A 0 dB ceiling

A ceiling at 0.0 or −0.1 dB with sample-peak detection can let true peaks through above 0 dBTP, and those can distort when the file is encoded for streaming. Use −1 dBTP.

Fixing the mix with the limiter

If one snare hit is 6 dB louder than the rest, the limiter turns the whole mix down for that hit. Fix the snare level or compress the snare track, and the limiter has less to do.

Mixing into it

With a limiter on the master from the start, every fader move you make is heard after 5 or 6 dB of limiting. The balance you set only works with that limiter on, and it shifts as soon as someone masters the track differently. Mix with headroom and add the limiter at the end.

Adding plug-ins after it

An EQ boost or a stereo widener placed after the limiter can push the peaks back over the ceiling. Keep the limiter last.

Training your ears to hear limiting

A limiter's job is to change loudness without changing the peak level, so the first skill is telling loudness apart from peaks. The quiz under the lab plays two versions of the loop with the meters covered, some of them limited, and asks which one is louder. Often the louder one is the limited version with the lower peaks. There are eight rounds, and your best score is kept in your browser.

Questions people ask

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