How to use an audio compressor

A compressor is a piece of equipment that you’ll find in every studio across the world, so it’s important to understand what it is and when to use it.

The primary function of a compressor is to reduce dynamic range. Dynamic range is the difference in level between the quietest and loudest parts of a track or mix.

In certain genres like classical or jazz, these dynamics are usually unchanged; whereas in Rock ‘n’ Roll, Pop, and Hip Hop, compression is typically used more liberally on the individual tracks as well as the entire mix.

Before we dive in, a general rule of thumb with compression is to apply as much as you think you need and then turn it down a bit.

Also, ideally you mix on a decent pair of studio monitors and also check on a pair of headphones for best results.

Audio compression basics

Threshold

The Threshold is the point at which the compressor will start reducing gain. If the level of a signal is above the threshold, then the compressor will begin attenuating the signal.

A threshold of -10 dB means any signal above -10dB will be attenuated. Any audio quieter than the threshold is not attenuated by the compressor.

Once the audio gets louder than the set threshold, the compressor starts reducing the gain by a factor known as the “ratio.”

Ratio

The ratio is the intensity of the compressor or the amount of attenuation that is actually occurring. This is expressed as a ratio of input vs. output.

A ratio of 2:1 means that for every 2 dB of input that is above the threshold, the compressor will only allow 1 dB of the signal in output.

Similarly, a ratio of 10:1 means that for every 10 dB of input above the threshold, the compressor will output, you guessed it, 1 dB of the signal above the threshold.

Attack

The attack is the time it takes for the compressor to reach its full gain reduction after the threshold is crossed.

For example, if you have an attack setting of 10 milliseconds, once the audio is higher than the threshold, it will take 10 milliseconds for the compressor to reach its full amount reduction.

This doesn’t mean that there is a delay before it starts compressing, it is simply how long it takes to get all the way there.

Release

The release is the other side of the coin. When the audio drops back below the threshold, the compressor is still attenuating for a short time.

The release setting is how long it takes for the compressor to completely stop attenuating the signal.

For example, with a release setting of 100 milliseconds, once the audio drops below the threshold, it will take 100 milliseconds for the compressor to reach 0 dB of gain reduction.

Makeup gain

The last piece of the compression puzzle is called “makeup gain.” Since a compressor reduces the level of the signal running through it, the compressed signal is quieter.

Makeup gain is simply a control that brings the overall output level back up. Adjusting the makeup gain will allow you to maintain the volume of the track while keeping the dynamic control and other benefits of using compression in place.

For example, if the compressor is on average hitting about 8 dB of gain reduction, try boosting the makeup gain (sometimes referred to as “output”) by 8 dB to bring the overall volume back up.

Compression in action

Now that we’ve gone over the common controls of a compressor and you have an idea of how a compressor works, let’s go over some uses cases.

Compressing vocals

Perhaps the most common use of a compressor is on vocals. Singers are naturally dynamic; one word may be whisper quiet while the next could be a blood-curdling scream.

The exact settings of your vocal dynamic compression will depend entirely on that particular singer.

As always, experimentation and experience will help to find the settings that best suit your mixing objectives. For a very dynamic vocalist, try a higher ratio with a lower threshold.

If your talent is pretty consistent with his/her dynamics, you may prefer a lower ratio with a higher threshold.

Compressing drums

A snare drum is another common application of a compressor. Using a compressor on a snare drum (or any drum, for that matter) can be used to control the dynamics.

You can also use a compressor in a more artistic way to modify the sound of a drum, or even just to add some punch to a track.

A compressor with a fast attack on a snare will limit the peaks so that the overall level of the higher transients (peaks) are brought down.

A slower attack will let some of the initial transients through and can help to add some punch.

Compressing bass and guitar

Bass guitar is another naturally dynamic instrument, so compression is important.

Try a relatively low ratio (2:1 to 4:1) with a quick attack and long release.

Acoustic guitar can be dynamic or consistent, depending on the player. A low ratio (like 2:1) is often preferred for acoustic guitar, but a higher ratio (like 10:1) can definitely be used if desired.

An electric guitar is similar to an acoustic guitar. Start with a lower ratio and go from there.

It’s easy to make guitar and bass compression sound like it’s pumping, so tweak the attack and release times to get the sound you’re looking for.

Compressing everything together

Another common use of compression is on your entire mix bus, or your final stereo (or surround) master output.

Adding subtle compression on your entire mix bus can add a lot of punch, or “glue,” to your mix. Mix bus compression typically uses a very low ratio, often 2:1 or even lower, maybe 1.5:1.

Be careful; mix bus compression is very easy to overdo.

To hear some of the artifacts, or negative effects of overly-aggressive compression, try putting a compressor with a low threshold and high ratio on your mix bus.

Compressing vs limiting

We now know that a compressor controls the dynamics of a source. A limiter does the same but in a more extreme manner by not allowing any signal past the threshold.

This is why you will often hear the term “brick wall limiting”. It acts like a brick wall in that it lets absolutely nothing pass.

So, now, what is the difference between a compressor and a limiter?

Since limiters do not have an attack control, the instant the signal crosses the threshold the limiter immediately prevents any additional level increase.

Just like a compressor, many limiters will have a release control to set the time it takes after the signal goes below the threshold for the limiter to completely stop attenuating.

You will find limiters in use in mastering, broadcast, live sound and other situations where level control is required to prevent clipping, hefty fines, or blown loudspeakers.

Advanced compression techniques

While you can do a lot with just simply using compression over individual tracks that may need it and maybe a bit on the overall mix you can also use more advanced techniques for various effects.

Parallel compression

Parallel compression is the process of duplicating a track, compressing one of them, then playing it back at the same time as the uncompressed track.

But what is the value of parallel compression?

One common reason is to increase the perceived volume of a track without compromising the dynamics with a limiter.

Even though you can accomplish this in part by just using a compressor on the individual track, sometimes you can’t get it quite loud and punchy enough without distortion and unwanted side effects.

Parallel compression gives us the best of both worlds: the clean, transparent sound of the original track plus the richness and presence of the compressed track.

Many of the popular compressor plugins actually have a “mix” control, which essentially accomplishes the same result. This controls the mix between the dry and compressed signal within the plugin.

When using parallel compression, try more extreme settings since your dry track will still be there and you aren’t as concerned with the transient detail.

For drums, start with a higher ratio with a fast attack and fast release. This gives precise control over the transients and brings out the decay (or ambiance) of your track. To control the transients without bringing out as much of the rooms sound, slow down the release time and keep the attack and ratio the same.

Parallel compression doesn’t just have to be used on drums, try it on other tracks to see what you like. It can even be used on your entire mix bus!

As always, there are many common approaches and best practices but there are no right or wrong settings when using parallel compression. Experiment and see what sounds best to you!

Multi-band compression

The term “multi-band” refers to the fact that multiple bands of frequencies can be compressed independently of each other.

You compress the low, mid, and high frequencies all with different parameters or leave some alone while focusing on those that need attention.

Most multi-band compressors will have 4 or 5 bands. Each band will have completely independent controls, meaning that you can have a different ratio or attack speed for your mid and low frequencies, and so on.

Since the dynamics of a track aren’t always broadband (across all frequencies) using a broadband compressor isn’t always the right tool. If you want to compress the low frequencies of a kick drum without affecting the cymbals, you can use a multi-band compressor.

If you focus on the low frequencies of a drum bus, you can isolate the kick drum. Then, if you want to control the snare drum, you can use a mid-range frequency. And as you might have guessed, to control cymbals, we use the high-frequency band.

Make sure to use make-up gain appropriately with multi-band compression. Compressing a certain band with no makeup gain will result in that band being quieter, which you probably don’t want.

Another cool application for multi-band compression is vocals. Just like any other instrument, a singer’s voice may be more or less dynamic in a different frequency range.

For example, the sibilance (the high frequency “sss” sounds) might be somewhat consistent at all volumes, but when the singer gets loud, his/her mid-range might get louder than other frequencies.

Using a multi-band compressor, you can control that dynamic mid range without touching the sibilance. Or for the opposite effect, a singer that is too sibilant, more compression on the high frequencies can act as a de-esser.

In conclusion

A compressor is something that controls the dynamic range of your audio. It will typically have 5 main parameters, which are all related to each other.

“Threshold” is the level at which the compressor will start reducing gain. Once your audio gets louder than this threshold, the compressor starts reducing the signal.

“Ratio” is the intensity of the compressor, or how much it is actually attenuating.

“Attack” is how long it takes for the compressor to reach it’s full attenuation after the threshold is crossed.

“Release” is how long it takes for the compressor to completely stop attenuating after the signal drops below the threshold.

Lastly, “Makeup gain” is the level control that allows you to make up for reduced level when compressing.

You probably already have a setup with a compressor available. Play around and exaggerate things to make the changes super easy to spot and understand.

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