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Audio Dynamics 101: Compressors, Limiters, Expanders, and Gates

by Griffin Brown, iZotope Contributor March 4, 2019

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I remember one of my friends talking about compressors when we first started getting into music production. “I don’t really know what a compressor does, but I put it on everything because it just makes things sound better.” This is the mentality that a lot of early producers have, and thinking this way hinders their ability to make informed decisions when producing and mixing.

In this article, we’ll demystify the compressor and other dynamics processors. We’ll cover the four main types of plug-ins used to control dynamics: limiters, compressors, expanders, and gates. We’ll discuss the mathematical processes behind these tools, how they affect the sound, and the best scenarios in which to use them.

Dynamics and dynamic range

Before we discuss the dynamics processors themselves, it’s important to understand what we’re actually doing to the sound.

Each of these processors is able to affect the dynamics of a sound, changes in volume over the course of its lifespan. Dynamics are a huge part of a sound’s identity, so having the ability to control dynamics is an invaluable tool for producers and mix engineers.

The difference between the sound’s loudest and quietest moments in the track is called the dynamic range. This dynamic range is the main aspect of a sound that we will be affecting with these processors.

For the most part, the names of these processors refer to what we are doing to the dynamics or dynamic range. 

Jump to a specific section:

Compressors | Limiters | Expanders | Gates

Compressors

iZotope Vintage Compressor

What do compressors do?

Simply, a compressor is used to compress a sound’s dynamic range. That is, to make the louder and quieter parts of the sound’s performance closer to each other in level.

It does this in one of two ways. In “downward compression,” the compressor attenuates the signal when it gets too loud. This is the most common form of compression.

In “upward compression,” the compressor boosts signal until it reaches a certain level. In either case, the dynamic range becomes narrower.

Compressor parameters

We actually won’t dive too deep into parameters here, as we’ve actually already covered compressor parameters in our Pro Audio Essentials course, which you can find here.

In short, there are six main parameters to consider for the average compressor: threshold, ratio, knee, attack time, release time, the makeup gain.

The threshold essentially activates the compressor according to the incoming signal. It’s set at a certain level (in dB), and the compressor will activate when the signal is loud enough to cross this threshold level (downward compression) or quiet enough to fall below it (upward compression).

How much compression occurs is controlled by the ratio. In a standard compressor (which is downward), a ratio of x:1 attenuates signal to a level of 1 dB above the threshold for every x dB it crosses.

To understand this, let’s look at an example. Say we had a downward compressor with a ratio of 3:1 and a threshold set at 0 dB. If incoming signal were at 3 dB (3 dB above the threshold), signal would be attenuated to 1 dB at output. If signal were at 6 dB (6 dB above the threshold), signal would be attenuated to 2 dB at output. The louder parts of the signal are now quieter.

In an upward compressor, a ratio of x:1 boosts signal to a level of 1 dB below the threshold for every x dB it drops below the threshold.

Say we had an upward compressor with a ratio of 3:1 and a threshold 0 dB. If incoming signal were at -3 dB (3 dB below the threshold), signal would be amplified to -1 dB at output. If signal were at -6 dB (6 dB below the threshold), signal would be amplified to -2 dB at output. The quieter parts of the signal are now louder.

With the threshold, ratio, and input level, we can determine the output level using the equation below. For the remainder of these parameters, we’ll be speaking in a downward compressor context, as they’re easily the most common.

Output level equation

The knee affects compression around the threshold. Think of it narrowing or widening the threshold point, affecting how signal at levels around the threshold will be compressed.

Here’s the knee parameter in Ableton Live’s native compressor. The Vintage Compressor handles knee with its “Mode” parameter.

Ableton compressor knee

Notice that, in the input level to output level graph of the compressor above, the resulting line has a corner at the threshold.

A signal below and up to the threshold goes through untouched, but a signal that crosses even 0.1 dB above the threshold causes compression to activate.

This is a “hard” knee. With a “softer” knee, this corner is rounded, and the compressor is now able to compress signal before it reaches the threshold. Considering the fact that hard compression can often be heard and distracting, a soft knee can help to make the compressor a bit more transparent.

Ableton compressor short knee

Remember, we said that the threshold activates the compressor, not compression in general. The compressor isn’t able to instantly compress the signal, and needs time to react as signal bounces above and below the threshold.

The attack time (measured in ms) determines how quickly the compressor reacts once the signal has crossed the threshold. The attack time is the amount of time the compressor will go from zero compression to full compression caused by the ratio and threshold.

The signal will eventually drop below the threshold, meaning compression has to stop. The release time determines how long it takes for the compressor to go from full compression to zero compression.

Because compression will only impact the sound while it’s crossed the threshold, the loudest parts of the resulting signal become quieter. See this in the guitar tracks below:

Uncompressed vs. compressed

To compensate for this, we can use the makeup gain parameter and amplify the output signal, which will bring those louder parts back to their previous level.

However, when we do this, the entire signal is amplified, boosting the quieter parts as well. The signal will have a narrower dynamic range, but a higher “RMS level” (average, see the PAE video here) than before.

Why use a compressor?

The main reason to use compression is that, by increasing the RMS level, the sound as a whole will sound “louder” and more present in the mix. This loudness is something that our ears naturally enjoy, which was what my friend was picking up on when he said compressors make things “sound good.”

Compression will cause the sound to be less dynamic and organic, but this added presence can help a sound stand out in the mix.

Additionally, compressors can be used to add color to a sound. Each compressor is unique, with different analog circuits and digital algorithms being used. Some compressors have a particular “sound” that engineers like for different types of instruments (e.g. the Teletronix LA-2A compressor for vocals).

Teletronix LA-2A compressor

Compressors are also important for controlling the dynamics of live-recorded instruments and vocals. These tend to vary quite widely in level over the course of a performance, so some compression can help make the level more consistent.

This will sound fuller and more polished, more like the “professional” sound of instruments in the mix. However, you can always opt to perform less compression (or none at all) if you want to preserve an organic quality to the performance.

You can also use compressors to shape transients in sounds like drums. Lower attack times can be used to attenuate that transient, making the tail of the drum hit more prominent.

By increasing the attack time, you can allow the initial transient information through before compression begins. This will make the transient pop out even more, making drums punchier.

Check out the “Compressor Attack” video to hear this in action:

Lastly, compressors are super useful for sidechain compression. This is when a sound (“Sound A”) is compressed based on the fluctuating level of a different signal (“Sound B”). As a result, Sound A will be compressed when Sound B crosses the threshold.

This is useful, as it will cause certain elements to be attenuated when others play. A classic example is to compress the bass with the kick drum. This will cause the bass to be compressed each time that the kick hits, minimizing ugly low-frequency clashes.

Limiters

iZotope Vintage Limiter

What do limiters do?

Just as a compressor “compresses” the dynamic range, a limiter limits it. The limiter serves as a ceiling which signal cannot pass. If the signal hits this ceiling, it will be harshly compressed so that it does not pass above.

You may be wondering if a limiter attenuates the loudest parts of a signal, how is it any different from a compressor? Essentially, a limiter is just a compressor with a very high ratio.

As a compressor’s ratio increases, so will the amount of compression. Eventually, that compression amounts to an impermeable ceiling.

For example, let’s say that we have a compressor with a ratio of 2:1 (not very high). We send three signals through it, at levels of 2 dB, 4 dB, and 8 dB over the threshold.

With this ratio, the compressor would output signals at levels of 1 dB, 2 dB, and 4 dB over the threshold. Closer to each other in level, but still not so consistent.

However, if we turned the ratio up to 8:1 (quite high), the compressor would output signals at levels of 0.25 dB, 0.5 dB, and 1 dB over the threshold. These signals are now much closer to each other and much closer to the threshold level itself.

Eventually, as the ratio increases, the signal will not be allowed to cross the threshold, which becomes a sort of “ceiling.”

The exact number you’ll hear changes from source to source, but any compression with a ratio of around 12:1 or higher could be considered limiting.

Limiter parameters

Every limiter will have at least one parameter: gain. This is used to boost signal until it hits the ceiling and is compressed.

Some limiters will have an adjustable threshold level, which is also often referred to as the ceiling.

If your limiter does not have this capability, you can always compensate for the added gain with a dedicated gain plug-in or at the channel fader. However, as limiters are mostly used in mastering as a means to bring the signal to unity gain, you’ll rarely need this.

Most limiters will have a release time parameter as well. This functions like a compressor’s release time, determining how long the limiter will take to return to zero compression.

Not all limiters will have an adjustable attack time, however. Some, like the Vintage Limiter in Ozone 8, have connected attack and release parameters (set with the “Character” parameter).

Now that we know a limiter is essentially a compressor with a high ratio, take a look at our compression output level equation again:

As the ratio increases, that fraction will approach 0. Therefore, the equation will eventually become this:

As expected, as the ratio increases, the output level for a signal that crosses the threshold will become closer and closer to the threshold itself. The signal cannot pass it.

Why use a limiter?

The main use, and really only use, of a limiter, is in mastering. The compression that they offer is so extreme that they’re rarely used on the channel level. Instead, limiters are often used on the master to bring the track up to a commercial level, and through compression commercial “loudness.” This final stage of compression can glue the elements of the track together and make the track louder at the same level.

Remembering that our ears naturally prefer louder music, limiters provide mastering engineers a big advantage in making a track sound professional. Just be sure not to overdo limiter settings, as the added compression and eventual distortion can suck the life out of a dynamic mix.

Another use for limiters is in a live sound setting, as a fail-safe precaution. If a loud sound occurs (one that would blow everybody’s ears out), this limiter will make sure to control it. Again, these limiters are usually placed on the master channel.

Be sure to check out our “Introduction to Limiters” article for a bunch more information on uses and parameters for different types of limiters.

Expanders

Nectar 3 gate as an expander

Note: The expander above is actually the Gate module found in Nectar 3, which has an adjustable ratio parameter. We’ll see in the Gate section that a gate is essentially a downward expander with a high ratio.

What do expanders do?

Again, like a compressor “compresses” and a limiter “limits” the dynamic range, an expander expands it. Louder and quieter parts become relatively louder and quieter respectively. As such, it’s essentially the opposite of a compressor.

“Upward expanders” amplify the level of signal that passes the threshold, rather than attenuate it like a “downward compressor.” A “downward expander” attenuates signal that drops below the threshold, rather than amplify it like an “upward compressor.”

Be sure to check this article out for more information on upward and downward expanders in mixing.

Most expanders are upward expanders (like the expansion featured in Ozone 8 when setting the compressor/limiter ratio to a negative number), but you’ll find plenty of downward expanders too. Downward expanders act similarly to gates, which we’ll get to in a second.

Expander parameters

The parameters found in an expander are and function mostly the same as those in a compressor.

The threshold once again determines the input level at which the expander will activate. This happens when the signal is loud enough to cross this threshold level (upward expansion) or quiet enough to fall below it (downward expansion).

Ratio, however, acts a bit differently. In a standard expander (which is upward), an expansion ratio of 1:x amplifies the signal to a level of x dB above the threshold for every 1 dB it crosses.

Again, let’s look at an example. Say we had an upward expander with a ratio of 1:3 and a threshold set at 0 dB. If the incoming signal were at 1 dB (1 dB above the threshold), the signal would be amplified to 3 dB at the output. If the signal were at 2 dB (2 dB above the threshold), the signal would be amplified to 6 dB at the output. The louder parts of the signal are now louder.

In a downward expander, a ratio of x:1 attenuates signal to a level of x dB below the threshold for every 1 dB it drops below the threshold.

Say we had a downward expander with a ratio of 1:3 and a threshold set at 0 dB. If the incoming signal were at -1 dB (1 dB below the threshold), the signal would be attenuated to -3 dB at the output. If the signal were at -2 dB (2 dB below the threshold), the signal would be attenuated to -6 dB at the output. The quieter parts of the signal are now quieter.

With the threshold, ratio, and input level, we can determine the output level using this equation (this works for downward and upward expansion):

Knee, attack time, and release time for expanders would all work the same as in compressors.

Makeup gain is only really necessary for upward expansion. As louder parts become louder, the signal will be louder after the expander than before, which can eventually lead to distortion of your gain-staging is off. The makeup gain can be used to attenuate the signal, returning the louder parts to their previous level.

Downward expansion does not require makeup gain, as the quiet parts will simply be quieter.

Why use an expander?

An expander can be used to achieve the opposite result of a compressor, expanding the dynamic range rather than compressing it. Therefore, expanders are best used when you want to have a wider dynamic range.

Expanders can be used to make instrumental or vocal performances a bit more varied in volume. This can be very useful if you want a more organic sound. This can, however, reduce presence in the mix. It can also potentially cause unnatural pumping, as these expansions in dynamic range are caused by mathematical processes ignorant to musical phrasing.

One of the main uses of expanders is in mixing a recorded drum kit. Each drum is individually mic'd, allowing each to have a separate channel on the mixer. However, total isolation is difficult, and bits of the other drums are bound to bleed through into other microphones.

An expander can be used, for example, to decrease the volume of the hat in the snare mic. As the hat will be further away from the mic than the snare, it will be quieter than the snare when picked up by the snare mic. Therefore, downward expansion can be used to attenuate it.

With the same logic, you can use expanders to remove reverb from drums. The reverb signal will be lower than the threshold, causing it to be attenuated in between the drum hits. Listen to the example below:

The reason this works with drums is that, after each drum hit, there is space for the signal to drop below the threshold level. With a low release time, it’s possible to mostly cut out the reverb tail. In a more sustained instrument, like a vocal or keys, this wouldn’t work because the dry instrument will still keep the overall signal above the threshold, allowing the reverb through.

Lastly, expanders can be used like compressors for sidechaining purposes. With sidechain expansion, a signal (“Sound A”) is attenuated by an expander, which is triggered by the fluctuating level of a different signal (“Sound B”). As a result, Sound A will be attenuated when Sound B drops below the expander’s threshold.

This is interesting, as it can cause an element to play more loudly while another element is playing. This has the effect of blending the two sounds like they’re one sound.

Check out this example of sidechain compression in action. The first audio file is a drum loop. The second is a sample of vinyl static. 

I’ve placed an expander on the static‘s channel and triggered it with the drum loop. Hear how the static pumps with the drum groove when it’s soloed. And hear the added layer of texture it gives to the drums when both are played together.

Gates

iZotope Nectar 3 gate

What do gates do?

Our last dynamics processor is a gate, which is essentially the extreme version of a downward expander. Gates provide a floor level which signal must cross to get through the gate. If the signal is too quiet to reach this floor, it will be attenuated to silence.

Let’s see how a gate simply acts as an expander with a high ratio.

For example, let’s say that we have an expander with a ratio of 1:2 (not very high). We send three signals through it, at levels of 2 dB, 4 dB, and 8 dB below the threshold.

With this ratio, the expander would output signals at levels of 4 dB, 8 dB, and 16 dB below the threshold. The signals’ levels are further apart but are all still relatively close.

However, if we turned the ratio up to 1:4 (very high), the expander would output signals at levels of 8 dB, 16 dB, and 32 dB below the threshold. These signals are now much further apart and much closer to being inaudible.

Eventually, as the ratio increases, any signal will be greatly attenuated, and all signal that passes through the gate will have to cross this floor level.

Gate parameters

Every gate will have at least three parameters: threshold, attack time, and release time. These all function the same as in compressors and expanders.

Some gates will also have a hold parameter, causing the gate to remain open for a period of time (in ms) after the signal has dropped below the threshold and before the release phase begins.

Some gates will also offer the ability to have the gate close at a different level than the threshold, which is only used for opening the gate. This parameter is often called the close or return level.

And just like the fact that some limiters have an adjustable ceiling, some gates will have an adjustable floor level. This is the level that signal will remain at while the gate is closed, and can be increased up from -∞ dB.

Now that we know a gate is essentially an expander with a high ratio, take a look at our expansion output level equation again:

As the ratio increases, that total fraction will become larger and larger. Therefore, the equation will eventually become this:

As expected, as the ratio increases, the output level for a signal that falls below the threshold will become quieter and quieter. Eventually, the signal will not be able to pass if it is below the threshold—a gate.

Why use a gate?

Gates are mainly used to cut out audio when it’s quiet and unneeded. This can be on a vocal to eliminate breaths, or can be used as an expander to isolate louder signals in a recording (e.g. isolating drums). Gates are equally useful in a studio recording or live sound context.

They can also be used in a similarly creative way as the expander example above. However, a gate would cause that static sample to pump harder and therefore have a more difficult time blending with the drums. Therefore, you may just want to use an expander for that.

Conclusion

Dynamic range is a major aspect of any sound’s identity. Additionally, the level balance between elements over the course of a track is super important for mixing. As a result, dynamics processors like compressors, limiters, expanders, and gates are invaluable tools for any producer or mix engineer. With the four of them, you should be able to shape a sound’s dynamics in any way you’d like.

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