What is an expander?

Expansion in plain English

An expander is a compressor working the other way round. A compressor waits for a sound to get loud and turns it down. An expander waits for it to get quiet and turns it down further, and everything over the threshold is left exactly as it was.

Say a snare mic picks up the hi-hat between snare hits. The snare peaks at −6 dBFS and the hat bleeding into the mic sits around −40 dBFS. Put the threshold at −30 dB and the expander ignores the snare, then turns the hat down whenever the snare isn't playing. The snare sounds the same and the mess around it gets quieter.

The distance between the loudest and quietest moments is the dynamic range. A compressor squeezes it smaller and an expander stretches it wider, which is where both get their names.

What does an expander do?

Mostly it cleans up the gaps. Any recording has sound in the spaces between notes that nobody played on purpose: hum from a guitar amp, preamp hiss, a fridge in the next room, a click track leaking out of the singer's headphones. On a drum kit every mic hears every drum, so the tom mics are full of snare and cymbals even when nobody touches the toms. That's called spill, or bleed.

While a note is playing it covers that low-level noise, so you don't hear it. In the pauses there's nothing to cover it, and across twenty tracks those pauses add up to a mix that sounds cloudy. An expander turns each track down in its own gaps, so the noise is pushed back only where you'd have heard it.

It can also change the shape of a sound. Put an expander on a drum loop with a long room sound and the tail after each hit gets shorter, so the loop sounds drier and tighter. The same move on a reverb return makes the reverb die away faster than it would on its own.

The idea is old. The dbx noise reduction used on tape machines compressed the sound 2:1 on the way onto the tape and expanded it 1:2 on the way back off. The expansion restored the original dynamics, and because tape hiss is easiest to hear in the quiet parts, pushing those down on playback pushed the hiss down with them.

Expander or noise gate?

A noise gate does the same job with no in-between. Under the threshold it drops the sound by its full amount almost at once, like a door closing. An expander turns the sound down gradually: a little when it's just under the threshold, a lot when it's far under.

You hear that difference at the end of a note. As a cymbal or a vocal fades past the threshold, a gate shuts and the tail stops dead, which can sound chopped. An expander keeps turning it down as it fades, so the end of the note falls away a little faster and you rarely hear anything switch. So an expander suits sounds with long, natural decays, such as vocals and acoustic guitar, and a gate suits short, tight ones.

Most plug-ins treat the two as one tool. Pro Tools ships them together as Dyn3 Expander/Gate, and Logic Pro has its own Expander next to its Noise Gate. Push an expander's ratio to the top of its range and it behaves like a gate. The full side-by-side, with a compressor included, is in compressor vs expander vs gate.

Reading the lab

The wide screen at the top shows the whole loop. The pale shape is the sound going in and the pink shape is what comes out, so wherever the pink is smaller than the pale, the expander is turning it down. The amber lane underneath shows gain reduction. With an expander it dips in the gaps between hits, the opposite of what you see on a compressor, and hovering over it gives you the figure at that moment.

The square screen is the transfer curve. Input level runs along the bottom and output level up the side, and above the threshold the pink line follows the dashed diagonal, so what goes in comes out unchanged. Below it the line drops away more steeply, and the higher the ratio, the steeper it falls. While the loop plays, a white dot rides the curve at the current input level, and you can watch it slide down into the steep part between hits.

Next to it is a close-up of the loudest hit, which is where you see the expander catch the tail. The Right now panel reads the loop as it is, with numbers measured from the sound, and its explanation changes whenever you move a knob.

Press A/B to switch the expander off. The lab matches the loudness of the two versions, so what you're comparing is how the quiet parts behave. You can also upload a file or record eight seconds with your microphone, and it loops through the lab in place of ours without leaving your device.

The controls

An expander has the same core controls as a compressor, plus range. The names shift a little between plug-ins; Ableton Live's Gate, for example, calls the range Floor.

Threshold

The threshold is the level where expansion starts. Anything above it passes straight through and anything below it gets turned down. On a compressor, lowering the threshold makes it work harder. An expander works harder as you raise the threshold, because more of the sound ends up underneath it.

Set it between the quietest part you want to keep and the loudest part of what you want gone. On the drum loop in the lab the hits peak near 0 dBFS and the tails between them fall to around −50 dB, so anywhere from −35 to −25 dB catches the tails and leaves the hits.

Ratio

The ratio decides how fast the level falls away under the threshold. At 1:2, a sound 1 dB under the threshold comes out 2 dB under, and a sound 10 dB under comes out 20 dB under. At 1:4 that same sound 10 dB under comes out 40 dB under, which is close to silence.

Plug-ins write the ratio in different ways. Some show 1:2 and some show 2:1, but on an expander both mean that every decibel under the threshold comes out as two. A ratio of 1:1 means the expander does nothing at all.

Ratios from 1:1.5 to 1:2 are gentle and sound natural, and they're where most vocal and guitar cleanup sits. From about 1:4 upwards an expander sounds more and more like a gate.

Range

Range is the most the expander will ever turn the sound down, however far under the threshold it goes. With a 1:4 ratio and no limit, a tail 15 dB under the threshold would come down by 45 dB. Set the range to 10 dB and it stops at 10.

This is the control that makes an expander sound natural on spill. A tom mic with 40 dB of reduction in its gaps sounds empty, because the room and the rest of the kit drop out every time the tom stops. With 8 to 12 dB of range the spill is pushed back far enough to stop clouding the mix, and the kit still sounds like it was recorded in one room.

Release

On an expander, release is how fast it turns the sound down once the level falls under the threshold. On a compressor it's how fast it lets go. They sound like opposites, but on both the release is what happens after the level drops.

A fast release of 20 to 80 ms clears out short gaps, like the space between drum hits at 120 BPM. A slow release of 200 ms or more lets each note fade more gently, but if the gaps are shorter than the release, the expander never gets far before the next note arrives; at 400 ms on the drum loop in the lab it takes off only about 4 dB. Go too fast on a slowly fading note and the gain flutters up and down, which you hear as a rough, chattering decay.

Attack

Attack is how fast the expander lets the sound back up when it rises over the threshold. On percussive sounds keep it fast, around 1 ms or less. The first few milliseconds of a hit are its transient, and they arrive straight after a gap while the expander still has the gain turned down, so a slow attack chops the front off every hit and makes the drums sound soft and distant.

On a vocal or a pad, an attack of 5 to 10 ms is fine and avoids a click at the start of each phrase.

Hold, knee and sidechain filter

Many expanders have more than the lab does. Hold keeps the expander open for a set time after the sound drops under the threshold, which stops it fluttering on a note that hovers around the line. A soft knee eases the expansion in across a few decibels around the threshold. A sidechain filter lets the expander listen to an EQ'd copy of the sound, so on a tom mic you can cut the high end in the detector and stop cymbal spill from setting it off.

Expansion in numbers

This table shows what a −30 dB threshold does to sounds at different levels, with a 40 dB range. A gate with the same range is in the last column.

The second row is where they part company. A note that has only just dipped under the threshold barely changes with a 1:2 expander, and with a gate it vanishes. In the last row the 1:4 expander has hit its 40 dB range and stops there, which happens to be exactly where the gate is.

How to set an expander, step by step

The same order works on any track with noise or spill in its gaps.

1. Start gentleSet a 1:2 ratio, a 10 dB range, a 1 ms attack and a release around 100 ms, with the threshold all the way down so nothing happens yet.
2. Find the noiseSolo the track and play a gap between notes. The level on the input meter there is roughly where the noise or spill sits.
3. Raise the thresholdBring it up until it's a few decibels over the noise and under the quietest note you want to keep. The gain reduction meter should move in the gaps and sit at zero during the notes.
4. Set the rangeTurn it up until the gaps sound clean enough in the mix, then stop. Ten to fifteen decibels is usually plenty; past that, the gaps start to sound empty.
5. Shape the releasePlay the ends of notes and lengthen the release until they fade instead of dropping. If the gaps don't get quiet in time, shorten it again.
6. Check it in the mixTake the solo off and switch the expander on and off. You want a cleaner mix with no notes missing.

Starting settings for common sounds

These are places to begin. Set the threshold from your own recording's meter, since it depends on how loud the noise and the notes are.

Common expander mistakes

A threshold that eats the quiet notes

Ghost notes on a snare and the soft last words of a vocal line often sit only a few decibels above the noise. Set the threshold too high and they go down with it. Find the quietest moment you want to keep and check the gain reduction meter shows nothing there.

Too much range

Silence between notes sounds strange in a mix, because a real room never goes silent. If the gaps sound like holes, pull the range back until some of the spill returns.

Expanding after the compressor

A compressor lifts the quiet parts towards the loud ones, so the noise and spill come up closer to the notes. Put the expander first, while the gap between notes and noise is still wide.

Expecting it to clean up the notes themselves

An expander only acts in the gaps, so hiss or hum you can hear under a held note stays there. That needs a noise reduction plug-in, or a better recording.

Judging it on solo

Soloed, every bit of spill sounds terrible and it's tempting to take all of it out. In the mix most of it is hidden, and the spill on drum mics is part of what makes a kit sound like one instrument. Make the final call with everything playing.

Training your ears to hear expansion

Expansion is hard to hear because it only works where little is happening. The quiz under the lab hides the settings and matches the levels, then asks whether the expander is on and, in the later rounds, whether its ratio is gentle or steep. Listen to the gaps between the hits, since that's the only place it changes anything. Your best score is saved in your browser.

Upward expansion

Everything above is downward expansion, which is what almost every expander plug-in does and what people usually mean by the word. An upward expander works on the other side of the threshold. It leaves the quiet parts alone and turns the sound up when it goes over, so the loud parts get louder and the dynamic range widens from the top.

It's much less common, partly because turning peaks up runs into the ceiling at 0 dBFS very quickly. Multiband plug-ins such as FabFilter Pro-MB can do it, and on a master it can put back a little of the punch that heavy compression took out. Don't confuse it with upward compression, which turns the quiet parts up and shrinks the range.

To make drum hits jump out regardless of their level, a transient shaper is usually the better tool, since it turns up the front of every hit whether it's loud or quiet.

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