If the loud parts jump out too far, you want a compressor. If the gaps between notes are full of noise, spill or room sound, you want an expander or a gate.
Between those two, pick the expander for sounds with long, natural endings such as vocals and acoustic guitar, and the gate when you want silence between short, sharp hits like toms and snare.
At a glance
| Property | Compressor | Expander | Gate |
|---|---|---|---|
| Works on | Sound over the threshold | Sound under the threshold | Sound under the threshold |
| Dynamic range | Shrinks it | Stretches it gradually | Stretches it in one step |
| Ratio | 1.5:1 – 10:1 | 1:1.5 – 1:4 | None; the range sets the drop |
| Range control | No | Usually | Yes |
| Hold control | No | Sometimes | Yes |
| Raise the threshold and it | Does less | Does more | Does more |
| Typical job | Evening out a vocal, controlling drum peaks | Pushing back spill and noise between notes | Silencing the gaps between drum hits |
| Pushed too far | Flat, pumping, noise brought up | Tails cut short, quiet notes thinned | Notes chopped off, chattering |
What do they have in common?
All three are gain controls that listen to the sound passing through them. Each one measures the level from moment to moment, compares it with a threshold you set, and turns the sound down when the comparison says so. How quickly it reacts is set by the attack and release, and how far it's turning the sound down at any moment is the gain reduction, the amber lane in the lab.
So they differ in one decision: which side of the threshold gets turned down. A compressor turns down the sound above it, while an expander and a gate turn down the sound below it.
The other way to describe the difference is through dynamic range, the distance between the loudest and quietest moments. A compressor shrinks it, which is where the name comes from. An expander stretches it, and a gate stretches it as far as it will go. The drum loop in the lab has a gap of about 41 dB between its loudest and quietest tenths; at the settings in step 1 the compressor brings that down to 35 dB, and the expander in step 2 takes it out to 57 dB.
Compressor: turning the loud parts down
A compressor leaves everything under its threshold alone and turns down whatever goes over it. The ratio says by how much. At 4:1, every 4 dB a sound goes over the threshold comes out as 1 dB over.
Say the threshold is −24 dB and a snare hit peaks at −4 dB. It's 20 dB over, so at 4:1 it comes out 5 dB over, at −19 dB, and the compressor has taken 15 dB off it. A ghost note at −30 dB is under the threshold, so it comes through exactly as it went in. The loud hit and the quiet one started 26 dB apart and now they're 11 dB apart.
That smaller gap is the point. You then turn the whole track back up with makeup gain, and because the peaks are lower you can turn it up further than before. Everything quiet comes up with it: the ghost notes, the room sound, and any hiss or hum in the recording. So a compressor never cleans a track up. It makes the noise louder compared with the notes.
The lab matches every mode to the loudness of the original, so in step 1 you can hear exactly this. The amber lane dips on the hits, the makeup gain adds about 6 dB, and the tails between the hits come forward.
Expander: pushing the quiet parts further down
An expander is a compressor pointed the other way. It ignores everything over the threshold and turns down whatever falls under it, and the further under it falls the more it gets turned down.
Its ratio is often written the other way round, as 1:2 instead of 2:1. At 1:2 every decibel under the threshold comes out as two. With the threshold at −24 dB, a snare tail at −34 dB is 10 dB under, so it comes out 20 dB under, at −44 dB. The snare hit at −4 dB is over the threshold and passes straight through. The two started 30 dB apart and now they're 40 dB apart.
Most expanders also have a range control, the most they'll ever turn anything down. With a 40 dB range, nothing comes out more than 40 dB quieter than it went in, however far under the threshold it was. That stops the expander pulling a quiet sound into nothing and keeps a little of the room around each note.
Because the turning down happens gradually, an expander is hard to hear working. As a note fades past the threshold it just fades a little faster. That's why it's the usual choice on vocals and acoustic instruments, where you want the spaces between phrases quieter without hearing anything switch.
Gate: shutting off the gaps
A noise gate makes the same choice as an expander with no in-between. While the sound is over the threshold the gate is open and the sound passes through. When it drops under, the gate shuts and turns it down by the full range, whether it's 1 dB under or 40 dB under.
With the threshold at −24 dB and a 60 dB range, a note at −23 dB comes through untouched and a note at −25 dB comes out at −85 dB. A 2 dB difference going in becomes a 62 dB difference coming out. On the transfer curves in the lab that's the violet line falling straight down at the threshold, where the expander's teal line slopes.
Gates have a few controls an expander usually doesn't. Hold keeps the gate open for a set time after the level drops, so a note that wobbles around the threshold as it fades doesn't flick the gate open and shut. Many gates also have hysteresis, a second, lower threshold for closing; Logic Pro's Noise Gate calls it Hysteresis and Ableton Live's Gate calls it Return. And because a gate is an on-off switch, its attack is usually set under a millisecond on drums, so it's fully open before the front of the hit has gone by.
The range control turns a gate into something gentler. Set it to 60 dB or more and a shut gate is close to silence. Set it to 10 or 12 dB and the gaps are tucked back instead of cut out, which keeps a gated drum kit sounding like one kit in one room.
Same input, three outputs
This table shows what each of the three does to sounds at different levels with the same −24 dB threshold: the compressor at 4:1, the expander at 1:2 with a 40 dB range, and the gate with a 60 dB range. The figures are static, meaning the level each one settles at once its attack and release have caught up.
| Level in | Compressor 4:1 | Expander 1:2 | Gate, 60 dB range |
|---|---|---|---|
| −4 dB | −19 dB | −4 dB | −4 dB |
| −14 dB | −21.5 dB | −14 dB | −14 dB |
| −28 dB | −28 dB | −32 dB | −88 dB |
| −40 dB | −40 dB | −56 dB | −100 dB |
| −60 dB | −60 dB | −96 dB | −120 dB |
Read it down each column. The compressor changes the top two rows and leaves the bottom three alone. The expander and the gate do the reverse, and what separates those two is how hard they hit the first row under the threshold: 4 dB for the expander, 60 dB for the gate. Hover over the transfer curves in the lab to read any other level for all three at once.
The threshold works in opposite directions
This is easy to get backwards if you learned compression first. On a compressor, lowering the threshold puts more of the sound over it, so the compressor works harder. On an expander or a gate the work happens under the threshold, so it's raising it that makes them work harder.
Steps 6 and 7 in the lab show it on the drum loop. With the threshold at −12 dB the compressor only catches the tops of the loudest hits and takes off about 3 dB. Switch to the gate without touching the threshold and the gate is shut for about half the loop, cutting the softer hits along with the tails.
So the starting point differs too. With a compressor, start with the threshold high and bring it down until the gain reduction meter moves on the loudest notes. With an expander or a gate, start with the threshold all the way down and bring it up until the gaps get quieter, then stop before the quietest notes you want to keep start to suffer.
Expander or gate?
The two do the same job, and the line between them is blurry. A gate is an expander with an extremely steep ratio, and in step 5 of the lab an expander at 1:6 already bends down nearly as sharply as the gate does. A gate with a small range sounds a lot like an expander, as step 4 shows. That's why some plug-ins are both at once, such as Dyn3 Expander/Gate in Pro Tools.
Where they differ is at the end of each note. As a note fades through the threshold, a gate shuts and the tail stops; with a long release it fades out, but it's still one movement from open to shut. An expander keeps following the level down, so the tail gets shorter without a point where you hear it switch. On a vocal, an acoustic guitar or a piano, where every note has a long natural ending, that makes the expander the safer choice.
The gate wins when you want the gaps silent and the notes are short and loud compared with what's between them. Toms are the usual example. A tom mic picks up the whole kit, but the toms only play a few times a song, and a gate shuts that mic for everything else. Gated reverb, the big snare sound on a lot of 1980s records, is the same tool used on purpose: a gate on the reverb return cuts the tail off short.
When to use each
| Job | Compressor | Expander | Gate |
|---|---|---|---|
| A vocal that jumps between loud and soft | Best | No | No |
| Headphone spill or room noise between vocal phrases | Makes it louder | Best | With a small range |
| A tom or snare mic full of the rest of the kit | Makes it louder | Good | Best |
| Amp hum between guitar riffs | Makes it louder | Good | Good on high-gain guitars |
| Drum peaks that are too spiky | Best | No | No |
| A drum loop with too much room sound | Brings the room up | Good, shortens the tails | Tight, can sound cut off |
Step 8 in the lab is the second row of that table, played on an acoustic guitar with a pause in the middle. The compressor turns the playing down by up to 22 dB and its makeup gain lifts the room noise in the pause by about 13 dB. Switch to the expander and most of the pause drops about 16 dB below where it started. The gate makes it silent.
Using a gate and a compressor on the same track
Often a track needs both: an expander or gate to clear the gaps and a compressor to even out the notes. The gate goes first.
- Clean up the gaps firstPut the expander or gate at the top of the chain. Start with the threshold at its lowest and raise it until the noise between notes drops away, then back it off a couple of decibels so the quietest notes you want to keep get through.
- Choose how much to take offSet the range. Start around 10 to 15 dB, which sounds natural, and only go to full silence if you can't hear anything odd at the ends of notes.
- Compress after itPut the compressor next and set it for the notes. Its makeup gain lifts whatever the gate left in the gaps, which is much less than it would have been.
- Listen to the gaps againSolo the track and play the quiet sections. If the compressor has brought the leftover noise back up, raise the gate's range a little more.
The reason for that order is in the numbers. Say the notes on a track peak at −6 dB and the noise between them sits at −50 dB, a 44 dB gap that a gate can find easily. Compress first and that gap might shrink to 30 dB, and any note that dips towards the noise is now harder for the gate to tell apart from it.
Common mistakes
Compressing to get rid of noise
A compressor turns the loud parts down and its makeup gain turns everything up, so the noise between notes ends up louder than before. If hiss or spill is the problem, it needs an expander or a gate.
Turning the threshold the wrong way
On a compressor, a lower threshold means more processing. On an expander or a gate it means less. If you move the threshold and nothing happens, check which side of it the processor works on.
Gating a sound with long endings
A full-range gate on a vocal cuts off the ends of words and turns breaths on and off. Use an expander, or give the gate a range of 6 to 15 dB so the quiet parts are turned down instead of removed.
Letting the gate chatter
When a note fades slowly through the threshold, the gate can open and shut many times a second, which sounds like a buzz on the end of every note. Add hold, use hysteresis if the gate has it, or lengthen the release.
Judging by loudness
A compressor with makeup gain usually comes out louder than the original, and louder tends to sound better at first. Match the levels before deciding, which the lab does for you in every mode.
Training your ears to tell them apart
The quiz under the lab plays the current sound through one of the three at random, with the screens covered and the loudness matched, and asks which it is. Start with the hits: if they sound flatter and the space between them fuller, it's the compressor. If the hits sound untouched, the gaps tell you the rest. Tails that sink back smoothly mean an expander, and tails that stop short mean a gate.
Questions people ask
Is a noise gate a type of compressor?
They're close relatives, and both are filed under dynamics, but they work on opposite ends. A compressor turns down what goes over its threshold. A gate turns down what stays under it. Both measure the level, compare it with a threshold and change the gain, which is why one plug-in often holds both.
What is the difference between an expander and a gate?
An expander turns quiet sounds down gradually, more the further under the threshold they fall. A gate drops them by its full range the moment they cross it. Set an expander's ratio high enough and it behaves like a gate, and a gate with a small range sounds a lot like an expander.
Should a gate go before or after a compressor?
Before, in most cases. A compressor brings loud and quiet closer together, and its makeup gain lifts the noise between notes. Put the gate after it and there is less difference between the notes and the noise, so the threshold is harder to set.
Can a compressor reduce background noise?
No. It makes background noise louder compared with the notes, because it turns the notes down and the makeup gain then turns everything up, noise included. To push noise down between notes you need an expander or a gate.
What is a downward expander?
It's the ordinary kind of expander, the one that turns sounds down when they fall below the threshold. Every expander on this page is a downward one. An upward expander turns sounds up when they go over the threshold instead, and you'll see it far less often.
Why does my gate open and shut rapidly on the end of a note?
A note that fades slowly hovers around the threshold and crosses it many times. Add hold, so the gate stays open for 50 to 200 ms after the level drops, or use hysteresis if your gate has it: a close threshold a few decibels below the open threshold. Logic Pro's Noise Gate calls it Hysteresis and Ableton Live's Gate calls it Return.