Soft clipping vs hard clipping vs limiting

At a glance

What is clipping?

Digital audio has a hard upper limit called 0 dBFS. When a sound is written to a normal WAV file, any part of the waveform that tries to go above it gets chopped off, because there's simply no number left to store it with. That chopping is clipping.

By accident, it's the harsh crackle you hear when a recording was too hot or a mix was exported too loud. On purpose, it's one of the most-used tools in loud music: a clipper plug-in does the same thing in a controlled way, at a ceiling you choose, and it's how a lot of electronic and hip-hop masters get their level.

Look at the big screen in the lab with Hard clip selected. The pale shape is the loop after the drive knob has pushed it up, and the dashed lines are the ceiling. Everything shaded pink is what the clipper removes.

Hard clipping

A hard clipper leaves everything under the ceiling exactly alone and cuts everything above it to a flat line. Turn the drive up and the tops of the loudest hits become plateaus, which you can see clearly in the close-up screen.

Those flat tops are where the distortion comes from. A sharp corner in a waveform can only be made out of lots of high frequencies, so a hard clipper adds a long row of harmonics stretching up the spectrum. On a short drum transient they last a couple of milliseconds and you mostly hear the drum get louder and a bit crunchier. On a held note, like the 110 Hz sine in the lab, the same flat tops turn a smooth tone into a buzzy square-ish one.

Soft clipping

A soft clipper bends the waveform gradually as it approaches the ceiling, so the peaks come out rounded instead of flat. The curve it uses starts to flatten before the ceiling, which means it touches more of the sound than a hard clipper does, just more gently.

Because a rounded top has no sharp corners, soft clipping adds fewer high harmonics. Most of what it adds sits close to the note, which tends to sound thicker and warmer. That's the same thing tape and valve circuits do when you push them, and it's why soft clipping and saturation are close relatives; a saturation plug-in is often a soft clipper used below its ceiling.

Push a soft clipper hard and it ends up close to a hard clipper anyway. With 18 dB of drive the curve is so steep that the rounding only lasts a fraction of a millisecond, and you can watch that happen on the transfer curve as you turn the drive up.

Limiting

A limiter doesn't touch the shape of the wave. It watches the level and, whenever a peak is about to go over the ceiling, turns the whole sound down just enough to keep it under, then brings the level back up afterwards. It's a compressor with a ratio so high and an attack so fast that nothing gets through.

To catch peaks without distorting them, most limiters use lookahead: they delay the audio by a few milliseconds so they can see each peak coming and start turning down before it arrives. The one in this lab looks 5 ms ahead. The cost is a small delay, which your DAW compensates for automatically.

The way a limiter fails is different from a clipper. Push it too hard and you won't hear crunch. You'll hear the quieter sounds ducking every time a loud hit arrives, and the loud hits themselves getting softer, because the limiter is turning everything down around them. Switch to Limiter in the lab, drive it to 15 dB and listen to the quiet hits after each loud one.

Why does clipping sound louder than limiting?

Set all three to the same peak level and the clipped versions are usually louder. The reason is the crest factor, the gap between the peak level and the average (RMS) level. How loud something feels depends mostly on the average, while how loud you're allowed to make it depends on the peak.

Drum transients are tall and very short, so they set the peak level without adding much to the average. A clipper shaves them off in an instant, the crest factor shrinks, and you can turn the whole track up by the amount you shaved. A limiter turns everything down around those peaks for a few milliseconds, which costs some of the average along with them.

The lab measures this as you go. Watch the crest factor and the "louder at the same peak" figure in the panel on the right while you switch modes. Then turn Match loudness off to hear how much the louder versions fool your ears.

Harmonics, odd and even

Every clipper adds harmonics: new frequencies at whole-number multiples of each note. A 110 Hz note picks up odd harmonics at 330 Hz, 550 Hz and on up the series, and sometimes even ones at 220 Hz and 440 Hz. The graph at the bottom of the lab measures them on a pure sine wave, so every bar after the first is something the process added.

Clipping that treats the top and bottom of the wave the same, as both clippers do by default, only adds odd harmonics. They tend to sound hollow or edgy. Turn up Asymmetry and the top gets clipped harder than the bottom, which adds even harmonics, and those sit an octave and an octave-and-a-fifth above the note, so they tend to sound fuller and smoother. Valve circuits are naturally a little asymmetric, which is part of why people describe them as warm.

When to use each

On drums, a clipper is usually the better first step. Transients are so short that a few decibels of clipping is hard to hear, and the drums stay punchy because nothing is ducking around them.

Sustained sounds are the opposite. Vocals, pads and long bass notes show every bit of added distortion, so a limiter, or a compressor before it, is the safer choice there.

On a master, use both: a gentle clipper first and a limiter at the very end. That's the next section.

Using a clipper and a limiter together

Each tool sounds best when it's only doing a little. If a master needs 6 dB of peak control, asking a limiter to do all of it tends to cause pumping, and asking a clipper to do all of it tends to cause audible distortion. Splitting the job keeps both under the point where you can hear them working.

1. Clip the fastest peaksPut a soft clipper first and set it to shave 1 to 3 dB off the loudest drum hits. Watch for the point where you start to hear crunch on the snare, then back off a little.
2. Limit what's leftPut the limiter last and lower its threshold, or raise its input gain, until you reach the loudness you want. It now has much less to catch.
3. Set the ceiling for true peaksSet the limiter's ceiling to −1 dBTP with true peak detection on. Spotify recommends staying under −1 dB true peak, and under −2 dB for masters louder than −14 LUFS, so the file survives being turned into an MP3 or AAC stream.
4. Compare at matched loudnessTurn both off and use your DAW's gain to match levels before deciding whether the chain helped.

Common mistakes

Clipping the master without knowing

Most DAWs process audio in floating point, so the master meter can go red without anything distorting inside the session. The damage happens on export. Once the file is written as 16 or 24-bit, everything above 0 dBFS gets cut, so if the meter goes over, put a limiter on the master or pull the master fader down before you bounce.

Clipping sub-bass

A clipper treats a long 40 Hz wave the same way it treats a 2 ms snare transient, which means a held sub note gets flattened for its whole length and turns into a buzz. If the low end gets fuzzy as you push a clipper, try a limiter on the bass, or clip the drums on their own bus before they meet the bass.

Choosing by loudness

With Match loudness off, the clipped versions win every quick comparison simply because they're louder. Judge them at the same level.

Ignoring true peaks

A sample-peak meter can read −0.1 dB while the waveform between the samples goes above zero. Those true peaks can distort when the file is converted to MP3 or AAC, so leave a margin and turn on your limiter's true peak option.

Training your ears to tell them apart

At matched loudness, the differences come down to the edge of each hit and what happens just after it. The quiz under the lab plays the selected sound, the drums to start with, through one of the three with the screens covered and asks which it is. Most people find soft against hard the hardest pair, so listen for grit on the very front of the hits.

Questions people ask

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