Demystifying Compression: A Guide to Dynamics Processing in Music

Compressors are essential audio processing tools that audio engineers and music producers rely on to shape and control the dynamics of audio signals. From subtly evening out unpredictable vocal levels to slamming drums and adding punch to guitars, compressors allow us to manipulate transients, sustain, and the overall dynamic variation of any audio signal.

In this comprehensive guide, we'll cover everything you need to know about compressors including:

• What is a compressor and how does it work?
• Compressor fundamentals - threshold, ratio, attack, release, knee
• Types of compressors - FET, optical, VCA, variable mu
• Compression techniques - vocal riding, gluing instruments, increasing sustain, punchy drums and more
• Using compression while tracking vs mixing
• Common compressor plugins - 1176, LA-2A, API 2500, SSL, and more
• Compressing drums, bass, vocals, guitars, piano
• Buss compression on drums, mixes
• Parallel compression
• Multiband compression
• Sidechain compression
• Compressor settings cheat sheet

Understanding the power of compression is a milestone in any engineer or producer's journey. Let's dive in and explore how this essential audio tool can be used creatively to shape amazing music.

What does an audio compressor do?

A compressor reduces the volume of the loudest parts of an audio signal and levels out the overall volume variations (dynamic range) of an audio signal. By increasing the output gain (make-up gain) of the compressor, you can make the quieter parts sound louder and closer in volume to the loudest parts.

Audio Compressor Explained

When you’re using a compressor there are a few settings that you’ll need to know:

Input gain

The input gain controls the level going into the compressor.

Compressor threshold

The threshold sets the level at which the compression effect is engaged.

For example, if you set your threshold at -20 dB, everything louder than this level will be compressed. But everything below this level (-20 dB) will not be affected by the compressor.

Compressor ratio explained

What does compression ratio mean?

The ratio is the amount of compression applied after the audio signal exceeds the threshold. The higher the ratio the more compression is applied. For example:

• If the ratio is 1:1, there is no compression at all.

• If the ratio is set at 2:1, for every 2dB the audio signal goes over the threshold, you get 1dB of output above the threshold. When the audio signal exceeds the threshold by 10dB, the compressor reduces the signal 5dB over the threshold.

• If the ratio goes up to 8:1, for every 8 dB of sound over the threshold you will get 1dB of output above the threshold. So if the signal goes over the threshold by 16dB, the compressor reduces the signal to be only 2dB over the threshold.

• If the ratio is 20:1 to ∞:1 this would be considered “limiting” and can be used to ensure that a signal does not exceed the threshold level.

Attack time

Not all compressors have the attack time control feature. But this control is the time that the audio signal takes to become fully compressed after exceeding the threshold level. Most compressors measure this time in milliseconds (ms) or us (microseconds).

Release time

This is the opposite of attack time. More specifically, when the audio drops below the threshold it's the time it takes for the audio signal to get to its none compressed state. The Release times will be longer than attack times. With normal compressor operation, the release time will be to set to be as short as possible. If you set the release time too fast though, you can get some unnatural “pumping or breathing” in the audio signal. One tip would be to ask the musician to play sustained notes and then set the release time so the change of gain is smooth.

Output Level (Makeup Gain)

Because compression essentially lowers the volume of your audio, you may need to apply some makeup gain to bring the signal level back up after compression occurs. Use the output level or gain control on your compressor in conjunction with its bypass button to quickly compare and match the level of your compressed audio to the input level.

Soft & Hard Knee

The “knee” refers to the transition between the non-compressed and compressed audio signal running through the compressor. Some compressors have just one setting while others have a choice between both, a "soft knee" and a "hard knee” setting. Some compressors will even allow you to control the selection of any position between the two types of knees.

How to use an audio Compressor?

Setting up or connecting a compressor works the same way whether you’re using a plugin or a hardware compressor. First, Insert the compressor on the channel you want to compress.

Compressor Threshold Settings

You want to adjust the threshold so that the audio levels are just peaking over the threshold and you can see the compressor beginning to engage. Unless, of course, you really want to flatten an audio signal— a live bass for example—in this case, it can work to have the audio signal over the threshold all the time.

Compressor Ratio Settings

Set the Ratio. Bass guitars sound good at 4:1, drums at 2:1, vocals also at 2:1 and electric guitars anywhere from 2:1 to 6:1.

Setting compressor attack and release times

The attack and release control how fast/slow the compressor reacts. Slower attack times can work well for buss compression and mastering uses. Instruments with long sustaining notes like bass guitars will tend to need slower release times than sharp percussive instruments like percussion. Most of the newer compressors now have an auto-release button that lets the compressor set the release time automatically, and they usually work fine.

Compression make up gain/output gain Settings

Use the make-up gain/output gain to bring the audio output signal back up to match the input audio signal. Be careful though to not add any unnecessary noise.

Hard knee or soft knee

Setting the hard/soft knee: Hard knee will often work well on drums, bass, and other percussive instruments. Soft knee is more subtle and better for vocals and some guitar parts.

Compression look-ahead

Plug-in compressors sometimes have a look-ahead feature. This feature allows the compressor to look ahead at what's coming up by using a short delay on the whole recording. This enables the compressor to catch all the peaks in a smooth way before you hear it played back. It can sometimes cause the compressor to lose its ‘character’ though so don’t use it by default— only if necessary.

These are certainly not rules, so feel free to have fun and experiment. I hope these tips will help you feel more confident when using a compressor.

Types of Compressors

Not all compressors sound the same! The specific circuit components used in the design impact a compressor's sound and response. Let's explore the main types:

FET (Field Effect Transistor) Compressors

FET compressors use solid state transistor circuits to control gain reduction. The most famous FET compressor is the legendary Urei 1176, known for its fast attack time, rich harmonics and aggressive sound. It excels on vocals, drums, guitars and adding punch to a mix.

Optical Compressors

Optical compressors use a light source, LED or lamp and a photocell to control gain reduction. The LA-2A is a famous optical compressor prized for its smooth, program dependent release and intuitive controls. Perfect for vocals, bass, and gentle mix compression.

VCA (Voltage Controlled Amplifier) Compressors

In a VCA compressor, gain reduction is controlled by a VCA chip. VCA compressors are known for transparency, flexibility and clean, fast response. The SSL G-Series Buss Compressor is a prime example frequently used for mix bus compression.

Variable Mu Compressors

Variable Mu compressors use vacuum tubes to control gain reduction. The Fairchild 670 is the most coveted variable mu compressor, known for its rich tube sound and the glue it adds to vocals and mixes.

Compression Techniques

Now that we understand the basics of how compressors operate and the different types available, let's explore some of the key techniques engineers use them for when recording and mixing music:

Vocal Riding

A compressor is invaluable for evening out vocal performances by gently clamping down on louder phrases and bringing up softer passages. This allows the vocal to sit perfectly in the mix at an even level. Start with a low ratio like 2:1 and slow attack to retain dynamics.

Gluing Instruments

Using a bus compressor across a drum kit or across layered instruments like guitars can "glue" them together for a cohesive sound. Opt for soft knee compression with a low ratio and slower attack and release.

Increasing Sustain

On instruments like guitar or bass, compressors can be used to increase note sustain by boosting the lower level decay of notes. Try a medium ratio and slower release.

Punchy Drums

Fast attack compressors like the 1176 can add serious punch to drums like snares and kicks. Try a medium ratio and adjust the attack to dial in the transient smack.

Transforming Sound

Heavier compression with higher ratios can completely transform the sound of instruments. Try ratios above 8:1 to squash room mics or radically alter the tone of synths or guitars.

Mix Bus Compression

Applying gentle compression across the full mix adds cohesion. Opt for 2-4dB of gain reduction with a low ratio like 2:1 and medium attack and release.

Parallel Compression

Blending compressed and uncompressed signals creates thickness and power. Route signals to a compressor inserted on an aux track to prevent over-compression.

Pumping Sidechain Effects

Creative pumping effects can be made using sidechain compression. Feed a synth, kick drum or other trigger signal into the compressor's sidechain input to create rhythmic ducking effects.

Let's explore how experienced producers and engineers utilize compression across common instruments when producing and mixing:

Compressing Vocals

Controlling vocals is one of the most critical compression jobs. Some approaches:

• 2-4dB GR, 2:1 ratio, medium attack, fast release - Gentle riding of vocal peaks
• 4-6dB GR, 4:1 ratio, fast attack, slow release - Tame loud words and consonants
• 10+ dB GR, 10:1+ ratio, fast attack/release - Aggressive pumping effect
• Parallel compression to add thickness and power

Go-to vocal compressors: LA-2A, 1176, Distressor

Compressing Drums

Drums require multiple compressors to control each piece of the kit:

Kick Drum

• 4-8dB GR, medium attack, fast release, punchy FET compressor

Snare Drum

• 4-8dB GR, medium attack, fast release, punchy FET compressor

Overheads

• 2-5dB GR, low ratio, slow attack, medium release, for cohesion

Room Mics

• Smash room mics with 10:1, fast attack, slow release for ambiance

Drum Bus

• 2-3dB GR with slow attack and fast release to glue kit

Compressing Bass

Compression allows bass to sit uniformly in the mix:

• 3-6dB GR, medium attack, slow release
• Faster attack when bass has inconsistent transients
• Slower release to increase sustain

Compressing Guitars

Compressing electric and acoustic guitars serves different purposes:

Electric Guitars

• Medium attack and release, to increase sustain and prevent thin sound

Acoustic Guitars

• Faster attack and release to control transients and add consistency

Compressing Piano

Piano compression depends on context and desired effect:

• Low ratio and GR to add cohesion to jazz or classical piano
• Higher GR, faster attack, to help piano cut through dense rock mixes

Buss Compression

Gently compressing the final mix adds clarity and perceived loudness:

• 2-3dB GR, low ratio, medium attack fastest release
• Emulators of SSL, API, or classic bus compressors

Parallel Compression

Creates thickness and power by blending compressed and uncompressed signals:

• High ratio and GR on parallel compressor inserted on an aux track
• Blend to taste with original uncompressed signal

Multiband Compression

Multiband compressors allow compression of specific frequency bands:

• Compress only low mids on a guitar to remove boxiness
• De-essing to tame harsh high frequencies
• Tighten lows, thicken mids, and brighten highs

Mastering Compression

Mastering engineers use compression for maximizing loudness:

• Brickwall limiting with high ratio, fast attack and release
• Lookahead feature allows compressor to react before transient hits
• Usually empowered with EQ to balance frequency response

Common Plugin Compressors

Thanks to modern software, all the iconic vintage hardware compressors are faithfully recreated in plugin form. Some classics:

1176 FET Compressor - The definitive FET compressor used ubiquitously for punchy drums and vocals.

LA-2A Optical Compressor - Creamy optical compressor perfect for smooth, natural level control especially on vocals and bass.

SSL G-Series Buss Compressor - The mixer buss and mastering standard for transparent, reliable compression and limiting.

API 2500 Compressor - go-to for compression duties on drums, guitars and adding punch to a mix.

Fairchild 670 - Highly coveted variable mu tube compressor used for its magic glue and tone-shaping abilities.

Compressor Settings Cheat Sheet

Because compressor controls can seem abstract, it helps to have some baseline settings to get started. Use these as general starting points and adjust to fit your material.

Default Compression Settings

Ratio: 3:1 Threshold: -20 dB Attack: 20-50 ms Release: 100-300ms

Mild Compression (2-3 dB GR)

Good starting point for bus compression or lightly gluing instruments together.

Ratio: 2:1 or 3:1
Threshold: -10 to -20 dB Attack: 10-20 ms Release: 100-200 ms

Medium Compression (4-8 dB GR)

Works well for stronger glue between instruments like drums and guitars. Also used for more transparent control of vocal peaks.

Ratio: 3:1 or 4:1 Threshold: -10 to -15 dB Attack: 10-20 ms Release: 50-100 ms

Heavy Compression (8-12 dB GR)

Slams room mics and drum overheads. Also used for dramatic pumping vocal effects.

Ratio: 8:1 or 10:1 Threshold: -5 dB Attack: <10 ms Release: 75-150 ms

Final Thoughts on Audio Compression

Understanding compression is a lifelong journey, but learning these fundamentals will give you creative tools to start controlling dynamics in your mixes. From subtly gluing elements together to dramatic transient shaping and level management, compression is one of the most powerful mixing tools available.

The most important tip is to let your ears guide you - use the suggested settings here as starting points, but always tweak controls based on how the compression sounds. Developing your compression ear will pay unlimited dividends in your productions.

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