What is audio compression, limiting, leveling and normalization? Learn how make your sound clear and professional!

Anders Eklöv
Dec 24, 2017 · 11 min read

Ever recorded an audio part, like a guitar recording, a vocal track or a voice over for a podcast or YouTube video? Well it may have happened to you, as to most of us, that the volume varied a bit too much throughout the recording. There were parts that were too low, perhaps someone was too far away from the microphone, the guitar guy swayed a bit during some part or the people in the studio were in different distances from the mic. Then someone raised their voice or slammed on the keys or the strings a bit too much making that part very strong. Basically, the lowest volume level is way too low to meet the highest volume with a nice finish. This is the essence of what we in the audio industry call dynamic range. The dynamic range of volume.

Dynamic range of an audio recording is just that, how dynamic, or big, the differences are from the lowest volume level to the highest. And now we want to minimize that range to suit our production, be it a podcast or a musical masterpiece. There are a couple of ways to accomplish this, but the most common is to use audio compression. An audio compressor is the tool of the trade.

Compressors vs. Limiters

There are many forms of compressors. Some stop the sound very gently as the sound level reaches too high values, some stop it faster and harder. Most compressors can do both. The ones that stop it very fast and harshly (instantly) are called limiters. These are just different types of compressors that are usually used for drums, on mix buses or on outputs of complete projects or songs. This is to stop the sound peaks from reaching too high values which would otherwise result in digital clipping. Limiting is heavily used in mixing and mastering, as it can help raise volumes of entire productions. Through the use of compressors and limiters we can make our song sound as things do on the radio, or just sound more steadier and even. There is always a trade off, using compressors and limiters, as they tend to squash the life out of a natural sound very easily. We must therefore use them in moderation and with the appropriate settings for the sound in question.

Read more about the history and prior usage of compressors in this article, or watch a video right here:

Groove3 video “A Brief History (Compression Explained)”

“Basically, the lowest volume level is way too low to meet the highest volume with a nice finish. This is the essence of what we in the audio industry call dynamic range. The dynamic range of volume.”

Basic functionality of an audio compressor

The compressor has an input for the raw signal, a processor of the sound and an output of the processed signal. Some compressors also have the option to mix of the processed and unprocessed signals at the output stage. This functionality is called parallel compression and is a very often used in audio mastering (the last stage in a music or broadcast production) as it maintains some of the audio´s original dynamics.

The “Fat Channel XT” compressor in Presonus Studio One 3.

Taking a look at a standard compressor, it has a couple of controls that are most commonly present on the control surface, whether it be a hardware unit or a software plugin:

Waves´s Renaissance, classic compressor plugin. This one has basic controls along with a “warm” setting for more overtones and a switch between opto/electro.

Threshold — this is the lowest volume level (usually in dBFs, peak values) at which the compressor will start working on lowering the volume, compressing it.

Ratio — this is the ratio, or strength, with which it will decrease the volume, above the threshold value. Values range from 1:1–1:20 usually.

Attack — the time, in ms, until compression starts after the volume reaches the threshold. For example, a low attack time would compress more volume peaks, resulting in less explosive transients.

Release — the time it takes for the compressor to stop working from the moment the volume dips below the threshold value. The higher this value is set, the smoother the transition.

Make-up gain — the volume increase or decrease that you want want to add after compression. Some compressors do not have this. They may have standard volume input & output knobs, which act as the same thing.

Gain Reduction meter (GR) — This is usually a meter showing the amount of volume / gain reduced by the compressor. Important to keep an eye on. If compression is more than 3–5db you will definitely hear the compressor work.

Logic Pro X´s compressor in “Studio VCA” mode, mimicking a Focusrite Red.

Auto-gain / make-up gain — Some compressors, like the built in compressor in Logic Pro X, have an automatic make-up gain function, that controls the gain based on the amount of compression applied. This is generally a function that adds too much gain and often makes you feel like you added something, even though you did not. An increase in volume is very powerful in making us feel like the sound was improved. It´s a seductive thing, as we hear more treble and bass at higher volumes. That´s a psycho acoustic effect of our ears.

Garageband, iMovie & Podcasting apps

Garageband´s dynamics instance, with simple compressor/limiter controls.

Where the music production applications and DAWs (Digital Audio Workstations) shines in terms of complexity these easier entry-level applications usually try to simplify the controls, with preset functions for ease of use. You might be lucky enough to be handed a great compressor, or you might not. Luckily, you can usually get a third party plugin program that will be recognized by your application. There are many options to choose from. See further down for some common models.

Audacity´s compressor plugin. This one has an option for auto make-up gain.

Recording audio for podcasts can be done with almost any software, but for the editing and enhancements you´ll want to have a good audio editor. You don´t have to do both the recording and editing in one single application, but many do prefer this. An audio production platform or multi-track recording software such as Garageband or Logic Pro X (both on mac osx only) or Adobe Audition (both PC and Mac) or any wave editor around all have the necessary tools. A free tool many prefer is Audacity, both for PC and Mac and even Linux.

Video podcasting and screen-casting applications usually have fewer settings for compression and limiting. But some, like ScreenFlow app for mac, do have settings for adding compression via adding an audio units plugin. Adding a plugin gives you a simple but full view of the settings and values. The image shows Apple´s AUDynamicsProcessor, a simple plug-in included with MacOS.

Other controls frequently found on compressors

Knee — a curve that controls the compressors initial reduction curve. Every compressor has a knee setting built in, and it is tied to the ratio functionality, but some have the option of adjusting it.

Sidechain filter (input filter) — a filter, or an equalizer that changes the sound that the compressor looks at or reacts to, so you can make it react to specific sounds or frequencies, such as the range of only the snare drum, or transients in speech, like the consonants or treble-rich sounds.

Input sidechain — Sidechain in this regard, pertains to the input of the whole unit. Some compressors have the option of “reading from” or using the source of another sound, that the unit reacts to, while compressing the sound channel it is on. For example, having a compressor on a bass, that only listens to the channel/output of the bass drum track. This is heavily used in dance music. You might have heard it in Eric Prytz´s “Call on me”, where the entire sound of the track is bouncing as the bass drum hits.

A little side note on Ratio. As you see in the picture above, of the Waves Renaissance Compressor, it has a big control in the middle of the plugin window that is controlling the Ratio of the compression. It is turned downwards, at the level of 3.89, which is usually expressed as “a ratio of 1:3.89”. This whole use of compression is called downward compression and does the job of compressing volume higher than the threshold level. The opposite of downward compression is called upward compression. This refers to turning the Ratio control up, above the 0 line, to instead boost any signal that is lower than the value of the Threshold. This is often used with voice and I personally use it with bass, as it can boost the lowest parts of the tones. This makes for a professionally sounding recording and can defeat the purpose of compressing certain peaks. Upwards compression also boosts the ugly parts, unfortunately, if used too much. A bit of both can save too dynamic parts and clips. To counter act the effects of boosting also the lowest rumbles in the recording, you can use a noise gate as a first plugin effect, to eliminate the worst noise in the background.

Watch Winksounds´s entry-level video on compression:

Here´s a more detailed video on advanced compression from SonicSenseProAudio:

Equalizers and compressors — in which order?

There is an advantage of using an equalizer plugin / effect before putting on a compressor. Sometimes the sound needs some filtering before it can be compressed. There are parts that does not need to be so present or even present at all in the sound and that actually “steal energy” from the other frequencies. Therefore the track can benefit from some filtering beforehand, so the compressor does not have to work on things that will not be in the mix at all. However, the bigger changes to the tonality of your track, such as changes that are broader and bigger, can often be better to do after the compression. Otherwise the equalizer used for these changes (often added to blend the sound into the mix) may cause ugly effects in the compression. So as a rule of thumb — always filter unnecessary details first, then compress, then at last do the broad tonality changes with another instance of an equalizer.

Why does my compressor boost my signal?

Do compressors add or remove volume?

Some units have a make-up gain, previously mentioned, that automatically adds volume as you lower the compression threshold value. This boost is not often not proportional and boosts the signal unnecessarily much, resulting in you liking the sound better, thus liking the compressor model. But the compressor did not naturally do a good job. Beware of this fact. So when people say that compression boosts a sound, that´s a false statement, or rather, a bit skewed. It´s a result of this feature. Compression always reduces a signal. If it does anything else as a main feature, it´s not a compressor. It´s either a Maximizer (like a limiter with a gain input, like the Waves L1, L2 or L3) or an upward compressor. An upward compressor boosts a signal below a certain threshold value, instead of reducing gain above it. Useful indeed, but in complete contrast to compression, by default. It does however contribute to the evenness of your sound, which is always the main function of compression. Other uses are to set a certain sound of say, the 80s, through the use of a special unit, or to add bite to a drum set.

While we got the topic close at hand, also have a look at our post regarding the differences between audio compression, normalization and leveling ->

Some of the best compressors around

While there is an ocean of great compressors out there there are a few that I think stand the test of most musical pieces and also does the job easily, with great musicality and sonic quality.

Logic Pro Xbuilt-in compressor - Logic Pro´s own compressor is one of the most versatile and feature-rich units of any DAW. With 7 different models of compressors, modelling FET, VCA, Vintage and Opto (optical), every conceivable control and a great sound, it will handle most types of tasks.

Presonus FatChannel XT — Great all-round EQ and compression/limiting unit, new to Studio One 3.5, which has good sound quality and great features, such as being able to swap the places of the parts. If you want to put the EQ before the compression, just move it, within the plugin itself. Many plugins have this feature, but few that are built into a DAW.

FabFilter Pro C2 — Very versatile and transparent (adds no EQ character) but has almost everything a compressor can have, including variable knee setting, sidechain / input filter, parallel compression and a very usable graphical view that shows you a the gain reduction and output strength with a waveform overview.

Waves Renaissance Compressor — The best and most versatile compressor I have ever used and the compressor that gets any job done, with minimum cpu impact.

Waves -> CompressorsWaves is one of the oldest producers of plugins for music production and have a lot of compressors. Have a look in their shop.

Plugin Alliance compressors — Through the store of Plugin Alliance you can get the products of several high-end compressor producers. Brainworx, Elysia, Lindell Audio and SPL have great models based on real hardware originals.

My favorite Limiters

Waves L3 / L2 / L1 — classic limiters that do great work on most music and voice, whether it be a single track or the entire mix. L2 has dithering built in. L3 has dithering as well and also has a version called L3 Multimaximizer, a multi-band limiter that can control different ranges of frequencies separately.

FabFilter Pro L (v1 and v2) — has many features that are lacking in most alternatives, such as attack/release settings for transient /peak control, programs for different types of music and TPL/ISPL (true peak/inter-sample peak limiting). Also a multitude of visual aids and metering types for absolute control. Highly recommended for anyone looking for one single limiter. Easy to use as well.

Kuassa Kratos 2 Maximizer — Fantastic limiter for all kinds of music. Especially good in keeping your transients in shape, not squashing the life out of the top dynamics of your material. This is important in many music styles, but in particular in singer- songwriter material and less compressed material, that still needs a good deal of volume. The “texture” and “knee” settings help you maintain your top transient´s (the sharpest volume peaks) inherent shape and dynamics.

iZotope Ozone Maximizer — All-round great limiter and maximizer for mastering and mixing. Has several algorithms for different material and you are bound to find a good one for yours. Ozone is a concept plugin, a collection of mixing and mastering tools with both flavoring (like vintage tape units) and completely transparent mastering tools.

KeyPleezer is a producer of virtual studio instruments and Soundware for Samplers and DAWs on PC & Mac.

We have a free instrument awaiting your music production studio. The LivingRoom Upright Piano, free version, is ready to download and install on MacOS 10.8+ for all users of GarageBand and Logic Pro X! (EXS24) More formats coming soon.

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KeyPleezer Music Tech Blog

Our official blog with news about our instruments, music technology, recording and production, mixing & mastering and more!

Anders Eklöv

Written by

I´m a music production enthusiast, mixing and mastering engineer, creator of KeyPleezer and an all-round tech tinkerer with a big interest for music technology!

KeyPleezer Music Tech Blog

Our official blog with news about our instruments, music technology, recording and production, mixing & mastering and more!

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