What is an Audio Compressor? – The Ultimate Guide

Martin Kristiansen

My name is Martin Kristiansen and I’m the founder and chief editor of HomeStudioIdeas.com. I’ve been playing, recording and producing music for the last 10 years.

Audio compression is commonly used in mixing and mastering applications to even out the dynamic range of individual tracks, or an entire mix. Every producer will eventually use a compressor. Why? It’s one of the most essential tools in a producer’s toolbox. 

In this blog post, we will talk about different types of compressors, common use cases, pitfalls, and much more!

We have a lot to cover so let’s dive right in!

What Does A Compressor Do?

In short, a compressor can (and should only) be used when you want to:

We will go into heavy detail below!

Dynamic Range

The term “dynamic range” is frequently used when discussing audio compression,  

So what is it?

Dynamic range refers to the difference between the loudest possible level and the quietest possible level. This range is often expressed in decibels (dB).

The noise floor is the lowest level that can be detected by an electronic system, while the clipping point is the loudest level that can be reproduced without distortion.

So why am I telling you this?

You see, one of the most important aspects of mixing is the dynamic range of each track and how it coexists with other instruments.

It’s important to learn how to control the dynamics of your mixes. We as humans can hear every subtle expression between a soft whisper to a loud scream.

And you know what? We want to be able to hear all the expressions in audio and music as well.

Compression is one of the most common ways to control dynamics. By compressing your audio, you can even out loud and soft parts. 

And why should you care about that?

The goal is to achieve a consistent volume level where each individual track in your mix can be heard clearly.

How Does A Compressor Work?

Every compressor features five to six controllable knobs. Each parameter has an essential role when it comes to shaping the dynamics of waveforms:

Let’s break them down in more detail!

Threshold

The threshold is the point at which the compressor starts working. In other words, the compressor kicks in and starts reducing the level of the signal when the signal rises above the threshold.

If the threshold is set too low, the compressor will start working on even the smallest changes in volume.

The result?

An unnatural and flat sound without any dynamics.

However, only the highest peaks will be attenuated if the threshold is set too high. When this happens you may not hear the compression at all.

Attack Time

So, the compressor will start to attenuate the signal when it exceeds the threshold.

So far, so good. 

The amount of time that it takes for the signal to go from above the threshold to be fully compressed can also be controlled.

This is called “attack time”!

You see, the attack time is an important parameter on a compressor as it determines how quickly the compressor can react to transients.

If the attack time is too long, transients will stick out and not be properly compressed.

On the other hand, transients will be indistinct and smeared out if the attack time is too fast. The result? You’ll suck the life out of the performance.

Additionally, fast attack times can cause ugly distortion and artifacts on low-frequency sounds such as kicks and sub-bass.

This could be somewhat mitigated by using a compressor with a built-in high-pass filter (HPF).

Let’s concretize!

An attack time between 10 to 100 ms counts as slow. The compressor will let through some uncompressed signal before kicking in, resulting in a punchier sound.

Furthermore, a fast attack time between 20 and 800 us (microseconds) can tighten up a sloppy performance and make it more polished.

Remember, there are no hard rules when it comes to dialing in the attack time. Trust your ears! 

Still, here are some guidelines that can get you started:

Release Time

The release time knob dictates how long the signal can remain below the threshold before the compressor lets go and allows the signal to remain uncompressed.

In other words, the amount of time it takes for a compressor to go from fully compressed to uncompressed.

The faster the release, the more aggressive the sound. A fast release can add grit and loudness to a signal, while a slow release can smooth out the sound.

Guidelines for setting the release time:

Ratio

The ratio is the amount of compression applied to an audio signal. Think of it as the relationship between the input and output levels.

The most common ratios used are 2:1, 3:1, and 8:1. So what does this mean in practice?

Let’s take an example!

Dividing your input signal level by the first number of a compressor ratio will determine the output.

A ratio of 2:1 will attenuate a signal by 4 dB if it exceeds the threshold by 8 dB. This is considered light compression.

In addition, a 5:1 ratio will attenuate the signal by 4 dB if it exceeds the threshold by 20 dB.

Furthermore, compression ratios are often described in terms of decibels (dB). For example, a compression ratio of 2:1 would be described as “2 dB of compression.

Below are some examples of different ratios and how they translate to the amount of compression:

Output Gain (Make-Up Gain)

Output gain is the last stage of controlling the level of the signal that comes out of a compressor. 

By adjusting the output gain, you can make up for any level losses that may have occurred during compression.

Output gain is also known as make-up gain.

Why?

By their nature, compressors reduce the levels of any signal going in. You need to make up for that loss by increasing the levels of the output.

Simply put, adding output gain will make the signal louder.

However, be careful not to add too much gain, or else the signal will start to sound distorted.

A good rule of thumb is to add just enough gain so that the compressed signal is just as loud as the original.

Knee

Knee-setting is how fast the compression is applied to the signal once it surpasses the threshold.

Although Knee is commonly confused with “Attack Time”, there is a distinct difference between the two parameters:

More often than not, audio engineers have to rely on the baked-in knee setting of the particular compressor they’re working with.

Below are two of my favorite compressors with a controllable knee:

In addition, there are two types of knee settings:

A hard knee compressor will start reducing the level of the signal as soon as it surpasses the threshold.

A soft knee compressor will allow some level of signal above the threshold before it starts to compress.

So, the knee setting controls how hard or soft the transition is from below threshold to above threshold levels.

Let’s take an example:

If the knee is set to zero, the compression applies as soon as the signal surpasses the threshold. The signal goes from uncompressed to fully compressed instantly.

If you want a slower transition and have your compressor compress the sound more progressively, knee-setting is your best friend!

The 4 Compression Types

Tube Compressors (Variable-Mu)

Common Applications: Vocals | Vocal Mix Busses | Instrument Mix Busses

Recommended Tube Compressors

A tube compressor uses vacuum tubes to tame the dynamics of a signal. 

The device was first used in the early days of radio to prevent distortion and maximize the output of the signal. 

Today, tube compressors are still used in a variety of applications, from recording studios to live sound reinforcement.

One advantage of using a tube compressor is that it can add character to a signal. This is because the tubes impart their own unique tonal qualities on the signal as it passes through. 

This colorization can be particularly useful for adding warmth and depth to recorded vocals.

Furthermore, tube compressors are ideal for glue compression.

So what is glue compression?

In short, you allow several separate instruments to pass through the same compressor. This “glues” them together and creates unification in the mix.

FET Compressors

Common Applications: Electric Guitars | Drums

Recommended FET Compressors

A field-effect transistor (FET) compressor is an analog compressor that uses a FET in its signal path.

Huh, FET?

A FET is a semiconductor device that utilizes an electric field to control the flow of electrons.

In the FET compressor, the source and drain are connected to the input and output of the signal, respectively.

Furthermore, the gate is connected to a control voltage that regulates the amount of gain reduction.

The FET compressor is known for its smooth and natural sound. It can be used to tame transients and add sustain to sounds. Additionally, it can also be used to create unique distortion effects.

VCA Compressors

Common Applications: Master bus | Instrument groups | Guitars | Basses | Drums | Vocals

Recommended VCA Compressors

A voltage-controlled amplifier, or VCA, is an electronic device that adjusts the strength of an input signal in response to a control voltage.

VCAs are used in audio applications to create effects like volume changes and fade-ins/fade-outs. 

The VCA does not generate the control voltage; rather, it uses an external carrier signal to create it.

This carrier signal can be generated by an oscillator or other means. The control voltage is then used to modulate the gain of the VCA, which in turn affects the amplitude of the input signal.

VCAs are used in a variety of applications, including audio equipment and sound synthesis. In general, they provide a way to dynamically control the level of an input signal without affecting its waveform.

Optical Compressors

Common Applications: Vocals | Guitars | Bass

Recommended Optical Compressors

The optical compressor uses light to control the gain reduction circuit.

Somewhat Sci-Fi, right?

Nonetheless, the light element is usually an optical photocell, placed in front of a light source.

When the signal level gets too high, the photocell becomes brighter and triggers the gain reduction circuit.

Optical compressors are known for their smooth and natural sound. Therefore, they’re often favored on vocals and acoustic instruments. 

Different Compression Techniques

Downward Compression

To begin with, downward compression is by far the most common type of compressor technique. If I would guess, it’s utilized by at least 90% of compressors.

So, what is downward compression?

This type reduces the level of loud parts of a signal when they exceed a certain threshold while boosting the softer parts.

You see, this can be a helpful tool in achieving a balanced mix. It can help to bring out the subtler elements in a track while keeping the louder parts from overwhelming them. When used judiciously, it can result in a more polished and professional-sounding mix.

One common ratio for downward compression is 4:1, which means that for every 4 dB that the signal exceeds the threshold, the output level is only increased by 1 dB.

This reduces the overall level of the louder parts of the signal, making them more manageable in relation to the softer parts.

Upward Compression

As its name suggests, upward compression is the opposite of downward compression. Furthermore, a downward compressor is always engaged while upward compression only kicks in when you need it to.

To illustrate:

Upward compression raises the amplitude of an audio signal that falls below a certain threshold, while downward compression lowers the amplitude.

Additionally, upward compression can also be used to add sustain to sounds by prolonging their decay time.

Parallel Compression

Parallel compression, also known as New York compression, is an advanced mixing technique used to make a track sound fuller and louder.

How?

By blending an unprocessed dry signal with a heavily compressed one. By doing so, you can compress more heavily without sucking the life out of the track.

The goal of parallel compression is to keep the original character of the sound intact while adding density and punch.

To do this, you adjust the levels of the uncompressed and compressed signals until they gel together.

This technique can be used on any type of instrument or vocal. However, it’s especially effective on drums and percussion. When done right, parallel compression can give your tracks more power and presence.

Side-chain Compression

Side-chain compression is an audio signal processing technique in which the level of a track is reduced when triggered by the level of another track.

You’ve probably noticed the ‘pumping’ effect in an EDM track? That’s the side-chain effect!

The most common use of side-chain compression is on kick drums and bass instruments.

As an example,

When the kick hit, the compressor process the bass. The result? It ducks and leaves headroom for the kick, making it cut through in the mix.

Multiband Compression

Multiband compression is when an audio signal is split into multiple frequency bands, where each band is compressed separately.

This can be useful for addressing different tonal inconsistencies or dynamics within an audio signal. 

In addition, multiband compression can be used to address a number of different audio problems.

For example:

If a track has too much low-end energy, a multiband compressor can be used together with a High-Pass-Filter to attenuate the low frequencies. This will help to keep the low end from becoming too muddy sounding.

Another common use for multiband compression is to help balance out the frequencies of an instrument.

Let’s say you compress the mid and high frequencies while leaving the low frequencies untouched, it can be made to sit better in the mix alongside other instruments.

Mid-Side Compression

Mid-side compression is a technique that separates the mid signal from the side signals in a stereo mix. This allows you to add different amounts of compression to the mid and side channels.

You see, most of the energy in a track is centered. As a result, the whole stereo image is affected when stereo compression is applied.

However, the mid and side channels are compressed individually with Mid-Side Compression. This can be an effective tool for adding definition and clarity to a track or mix.

When used judiciously, it can help widen the sound of a master and make transients pop.

Limiting

Audio limiting is a process that is used to control the maximum loudness level of an audio signal. When the audio signal peaks and exceeds the maximum output, it gets limited.

This can be done with extreme compression and is often used to prevent clipping. In fact, a limiter is a type of compressor with a really high ratio.

Limiting can help to avoid clipping by capping the peaks of the waveform at a certain level. 

There are two main types of audio limiting: Hard limiting and Soft limiting.

First, hard limiting is when the signal is cut off completely at the point where it reaches the maximum level.

As the name suggests, soft limiting is not that relentless. The signal is reduced before it reaches the maximum level. 

Leveling

Leveling is the process of adjusting the level of an audio signal. This is done with a leveling amplifier to automatically maintain a constant signal level, regardless of the input signal level.

They’re commonly used in audio systems to ensure that the output signal remains at a consistent volume, regardless of the input signal level.

In many ways, a leveler is a simple compressor with two controls: Gain and peak reduction. The ratio, attack, and release parameters are internally fixed.

De-Essing

A de-esser is a type of compressor that is used to reduce the harshness and sibilance of vocals.

How does it work?

The threshold of the de-esser is set to detect the sibilances that occur naturally when we speak. Next, it isolates those instances and attenuates them. The result? A smooth and pleasant vocal recording!

De-essing is an important part of the mixing process. Additionally, it can also be used on other instruments to reduce unwanted high-frequency sounds.

When Should I Use A Compressor?

Your Vocals Are Too Quiet In Some Parts And Way Louder In Others.

Ever wondered why those pop songs sound so polished and professional? A big factor is compression!

A compressor can help even out the levels of your vocals, making them sound more consistent.

Even out the levels?

Let’s say your vocals are too quiet in some parts and way louder in others. A compressor will minimize the difference between soft and loud.

You see, a lot of modern pop music features heavily compressed vocals. Besides, many mixing engineers stack multiple compressors on a vocal track to get it polished.

Here is an example:

The first compressor evens out the dynamics and increases loudness. The second compressor adds punch, pushing the vocals in front of the mix.

The compressor settings would look something like this:

First Compressor:

Second Compressor:

When Transients Are Sticking Out of Your Mix

In order to tame those wayward transients sticking out of your mix, you’ll need to employ a compressor. By compressing the signal, you can smooth out the peaks while keeping the sustain untouched.

There are a few different ways to set up your compressor in order to achieve this goal.

First, you’ll need to adjust the threshold so that compression only applies to the transient peaks.

Next, set a fast attack time so that the compressor kicks in immediately when the peak is hit.

Finally, set a release time that is long enough to let the transient decay naturally.

With these settings in place, your transients will be smoothly brought under control without adversely affecting the rest of your mix.

To effectively reduce transients, the compressor should be dialed into something like this:

When Your Track Doesn’t Cut Through The Mix

Furthermore, using a compressor can help if your track is struggling to cut through the mix. 

How?

By emphasizing transients and attenuating sustain. This will make your track sound more impactful and help it stand out against the rest of the mix.

Compressors are often used to even out the level of a signal. However, they can also be used to shape its sound.

When set properly, a compressor can emphasize the transient attacks of sounds while reducing or eliminating their sustain.

This can make percussion sounds tighter and snappier, and it can give guitars and other instruments more attack and bite.

You Want To Increase The Levels Of Your Drums But They Start Clipping

Let’s say your drum tracks are indistinct. What do you do?

You twist the volume knob to increase the overall levels. Then, you catch the level meters flashing red.

You see, when you increase the volume you’re also making the peaks louder.

And as you know by now – a compressor reduces the gain of the signal when the peaks get too high. This prevents distortion and makes the sound more even and tight.

Your Kick Drum Interferes With Your Bass

When you’re mixing a song, it’s important to make sure that all of the frequencies are in balance. Otherwise, you’ll end up with a muddy-sounding mix.

Kick drums and bass occupy a similar frequency range, which can cause them to compete with each other.

Side-chain compression can help to reduce this issue.

With side-chain compression, the bass compressor can be triggered by the kick drum. 

The level of the bass reduces every time the kick drum hits, giving the kick room to shine through.

The result? The bass and kick drum can coexist peacefully.

Common Compression Mistakes Beginners Make

Applying Compression On EVERYTHING

Compression is a powerful tool that can help control the dynamics of a track and even change its character. But with great power comes great responsibility, and if used incorrectly, compression can ruin your mix.

How?

First, you’ll lose the ability to create contrast and interest if you compress EVERYTHING in your mix. The overall loudness may increase, but the dynamics will be squashed and everything will run together.

Not good.

Secondly, tracks need room to breathe. Trust me, over-compressing will suck the life out of them.

The natural peaks and valleys of a track are what create energy and movement. Crushing them will result in a dull, lifeless sound.

Over-Compressing Individual Tracks

As previously mentioned, you are essentially squashing the dynamics of your track when you over-compress.

Remember the dynamic range?

Well, your compressor is reducing it, which means that the quietest and loudest parts of the signal are being brought closer together in volume. This can make your tracks sound less exciting and dynamic.

Another danger of over-compressing is that you can create an undesirable pumping effect.

This happens when the compressor is triggered again before it has ceased to compress. The result? The level of a track fluctuates.

This can be distracting for listeners and take away from the overall quality of your music.

So how do you know if you’re over-compressing?

Check the gain reduction meter on your compressor. You’re reaching the limit if you’re seeing more than 6 dB of gain reduction.

Tips for avoiding over-compression

Adding Too Much Make-Up Gain

Adding too much make-up gain is a common mistake that many engineers make. They think that by cranking up the make-up gain, they’ll make their tracks sound louder and “beefier”.

This is usually not the case.

More often than not, our brain tricks us into thinking loud sounds are better.

To prevent this, use the bypass button and do a proper A/B-test when the levels are equally set between the dry and compressed track.

Setting A Too Fast Attack Time

When the attack time is too fast, the initial transient of the signal is not allowed to “hit” before being compressed.

This causes the sound to become smeared and less defined. In addition, if you’re mixing snares or kicks you’ll lose the desired punch.

A fast attack also tends to reduce the overall level of the signal, because the initial transient is attenuated more than it would be with a slower attack time. This can make the sound seem thinner and less impactful.

Frequently Asked Questions

Should I Send Or Insert My Compressor?

It’s better to insert the compressor into the signal path since you want to process the signal directly. The only time you would want to use a “send” is during parallel compression.