Chapter 5 - Compression

Compression is the process of shaping the dynamics of sounds. A compressor is an automated volume control. It automatically adjusts the volume of the input signal in response to changes in volume in the signal itself.

Compressors are difficult to learn to use, for several reasons. They have many different and unrelated purposes. They have complex mechanics of operation, and it is necessary to understand these mechanics in order to operate them. Their effect on the sound is not always readily audible. And finally, the specific things that one has to do to get good results out of them are routinely very different from what one would intuitively expect.

5.1 Purposes

Before diving into the operating mechanics of compression, we first need to look at why you would want to compress a signal, and what can be accomplished by doing so. As with equalization, it is important to always compress with a specific goal in mind.

5.1.1 Reducing Dynamics

The most basic use of compression in mixing is to reduce the dynamic range of the input material. This is most commonly done on recorded vocal and instrumental performances. Reducing the dynamic range of a performance can make it sit in a mix better; smoothing out volume fluctuations allows it to be more easily heard, particularly if it is being played quietly in the mix.

Furthermore, material with reduced dynamic range will have a higher average loudness relative to its peak loudness. If you apply compression to reduce the dynamic range of most of the tracks in your mixdown, then the entire mixdown will be louder. Compression is the most important tool for achieving mix loudness.

5.1.2 Shaping Percussive Sounds

Compressors can also be used to modify the amplitude characteristics of percussive sounds, such as drums and plucked string instruments. For our purposes, a percussive sound consists of two distinct parts: an attack and a body. The attack is the loud initial part of the sound, and the body is the quieter part trailing off after it. There is no sharp division between the two.

A compressor can be used to change the balance between the attack and the body of a percussive sound. It can bring up the attack, or it can bring up the body. Bringing up the attack of a percussive sound will make it punchier, but will also reduce its perceived loudness and presence in a mix. Bringing up the body of a percussive sound will increase its perceived loudness and presence in a mix, but will also make it less punchy. Your goal when compressing percussive sounds should be to achieve the ideal balance between attack and body, punchiness and presence.

5.1.3 Creating Pumping Effects

A compressor, when applied to a group of tracks or to a whole mix, can create periodic changes in volume synchronized to the rhythms of the music. Usually this effect, known as "pumping," is considered an artifact, but in certain situations it can be pleasing and desirable, because it can enhance the groove of the music. So, many producers will use compressors to intentionally create pumping effects.

5.1.4 When Not to Use Compression

Sometimes compression is not the right tool for the job. Always remember that a compressor is just an automatic volume control. If you find yourself struggling trying to get a compressor to do what you want, ask yourself if you can achieve the desired effect more easily with manual volume adjustments. For large-scale dynamics shaping, riding the levels is often more effective than compression. Furthermore, with modern DAW automation technology, even very fine-grained volume adjustments are sometimes easier to do by hand than with compression. Always be looking for the simplest and easiest way to get the job done.

5.2 How It Works

Having examined some of the situations in which one would use compression, we will now look at the theoretical principles which underlie a compressor's operation. This section is not about how to use a compressor; this section is about understanding exactly what a compressor does to your sound.

5.2.1 Threshold, Ratio, and Knee

A compressor works by reducing the volume of the loud parts of a sound; it basically brings down the peaks. It applies negative gain to all parts of the sound that rise above a certain threshold. It does not necessarily reduce the gain enough to cause the sound to fall under the threshold; rather, it reduces the difference between the threshold and the volume according to an adjustable ratio. For example, if the ratio is 2:1, then a sound that is 6dB above the threshold will have its volume reduced by 3dB, and a sound that is IdB above the threshold will have its volume reduced by 0.5dB.

Some compressors also offer the ability to adjust the "knee" of the compression curve. A compressor that operates as described above will be rather heavy-handed in its operation; it will leave sounds below the threshold completely untouched, and rapidly clamp down on sounds above the threshold. This is "hard-knee" compression. "Soft-knee" compression basically smooths out the response of the compressor. Sounds a little below the threshold are slightly compressed, and sounds that are only a bit above the threshold are compressed more gently than louder sounds. Essentially, the threshold is "blurred" out by soft-knee compression. Hard-knee compression is tighter and more controlled, while soft-knee compression is gentler and subtler.

5.2.2 Attack and Release

Compressors do not usually react instantaneously to sounds that cross the threshold; they have a certain "lead-in" time, during which the gain ramps down, and during which the sound may exceed the volume that it's "supposed" to be at.

With modern digital technology it is possible for the compressor to react so fast that the effect is essentially instantaneous. However, some amount of compression lead-in time is often a desirable characteristic, as overly fast-acting compression can cause distortion in the waveform being compressed, and can in general sound rather crass and unsubtle. Compressors allow you to set the length of the lead-in time, known as the "attack," according to the nature of your task.

Just as it is often desirable for a compressor to begin compressing with some amount of "slop," it is also usually desirable for a compressor to stop compressing with some amount of slop. When the sound falls back below the threshold of compression, a compressor will take some time to bring the gain back up to the normal level. The reason is the same; overly fast "de-compression" can distort the waveform, and so slowing down the de-compression results in a gentler effect.

The result of this lag is that when a sound that is being compressed rapidly drops in volume, rather than falling back to the normal, un-compressed level, it will fall even lower, and then gradually ramp back up to the normal level; the negative gain is still being applied, even though the sound is no longer over the threshold.

The lag time between the sound falling below threshold and the gain adjusting appropriately is called the "release" time. As with attack time, it is adjustable.

5.2.3 Compressor Parameters

Putting it all together, a typical compressor has the following parameters:

Threshold: Determines the volume level at which the compressor will begin acting.

Ratio: Determines the amount by which material above the threshold will be compressed.

Knee: Sets the sharpness of the knee, allowing for hard-knee or soft-knee compression.

Attack: Determines how quickly the compressor will react to sounds above the threshold.

Release: Determines how quickly the compressor will return to a normal state when sounds fall back below the threshold.

Finally, most compressors have one final parameter that we have not con- sidered:

Makeup Gain: Because compression is designed to reduce the volume of the peaks in the input, the output of a compressor is, unsurprisingly, usually quieter than the input. Since this is usually not desirable, most compressors feature a high-powered gain control at the end of their signal chain which will allow you to boost the signal right back up to "make up" for the compression.

5.3 Procedure for Setup

Having considered in the abstract how a compressor works, we will now move into some practical advice on how to use them.

The first thing you need to know when setting up a compressor is that, if you are also using EQ in your signal chain, the compressor typically comes after the EQ. This is because EQ, particularly extreme boosts or cuts, can change the dynamic structure of music. So, if you EQ after compressing, you may change the dynamic structure of the music, partially undoing the work that you did shaping this same dynamic structure with compressor.[1]

Once you have your compressor placed into the signal chain, the next step is to set its parameters to achieve the desired effect. The typical compressor, as we have seen, has six parameters: threshold, ratio, knee, attack, release, and makeup gain. Of these, only four are particularly troublesome to adjust, and they will account for most of the difficulty of configuring a compressor: threshold, ratio, attack, and release. Most of the remainder of this section consists of a description of a procedure for setting these four parameters.

It is not necessary to follow this procedure; if you have a good idea already of what you want to do, you can generally set things up straight away without following any special procedure. But the procedure described here is a fairly fail- safe way to get a compressor to do what you want it to do, so it is recommended if you're not wholly certain how to achieve your desired effect.

Begin by setting the ratio to the highest possible value, with a hard knee. If you have a rough idea of where you want your attack and release to be, set them there. If not, set them both to the lowest possible value. All of these settings will change later, so don't worry about them too much. Now set the threshold. With the settings set as above, you should be able to easily hear where the compressor is acting, and so you will be in a good position to set the threshold to a sensible value. Adjust your makeup gain if the result is too quiet to properly hear it.

Now set your attack and release. This is probably the subtlest part of the whole process, so spend a little bit of time on it. Experiment with different settings, and see what they do to the sound. Since you have your ratio set higher than it will ultimately be, the effect will be exaggerated, and therefore easier to hear. If the sound is distorting, you probably need to make the release slower, or possibly the attack. Small changes in attack and release can make a significant difference when compressing rhythmic and/or percussive material, so be sensitive to these differences.

Now you have your threshold, attack, and release set. Reduce the ratio until you have achieved the desired amount of compression, and set the knee to the desired value.

You should set the final makeup gain so that the compressed audio has the same perceived volume as the uncompressed audio. Do this gain adjustment with your ears, without looking at the peak meters. Toggle the bypass button on the compressor on and off while adjusting the makeup gain until you have matched the perceived levels. It should be fairly clear when this happens; they'll just "click."

There are two reasons for adjusting the makeup gain in this manner. First, so that the existing balance of the mix is preseved. But, more importantly, so that you can check your work. When you have the levels matched, then you can check your work by toggling the bypass button on and off. Does the compressed audio sound better than the uncompresed audio? You can't make this judgment if the levels are not matched, because louder sounds naturally sound better than quiet sounds. If the levels are not matched, then the version that you perceive as sounding better will be whichever version is louder.

Be sure to check your work in the context of the mix, not just by itself. You're trying to make it sound better in the mix, not by itself. In some cases, particularly with subtle compression, the effects of the compression will not be noticeable at all when playing the sound by itself, but will be quite apparent when playing it in the mix.

5.4 More Compression

In this section we will examine a variety of other topics related to compression. We will look at some specialized types of compressors and other dynamics pro- cessors, advanced techniques for using compressors, and some special applications of compressors which are unusual enough to warrant special examination.

5.4.1 Limiters

A "limiter" is a special type of compressor. Unlike a normal compressor, which may allow the input signal to exceed the threshold, a limiter will never let this happen. The input signal will always remain below the threshold no matter what. Theoretically speaking, a limiter is equivalent to a compressor with an instantaneous attack, a ratio of ∞ : 1, and a hard knee.

Many limiters are differentiated from normal compressors by the presence of a "look-ahead" feature.[2] A normal compressor can only react to audio as it arrives, which means that if it arrives at a sudden peak, it will have to clamp down on it very quickly, possibly distorting the audio signal in the process. With look-ahead, the limiter sees a few milliseconds "into the future"[3] so that, when a sudden peak is about to arrive, the limiter can begin clamping down ahead of time, resulting in a smoother and more transparent effect.

Limiters have a variety of uses. In mixing, the most important of these uses is the transparent removal of peaks. With a good-quality limiter, sufficiently brief peaks can often be reduced or removed with little or no audible effect on the sound. This increases the available headroom of the music and allows it to be made louder.

A limiter can also be used in any context where you are looking for extreme compression. In these cases, a limiter is simpler to configure than a compressor, and, due to look-ahead, can often produce a smoother result.

5.4.2 Serial Compression

One variation on the standard compressor usage paradigm is to use multiple compressors on the same sound, chained one after another. This is called "serial compression," and there are plenty of situations in which it's a good idea. Often several compressors working as a team can get the job done better than one compressor.

Generally, when using serial compression, each compressor should be doing a different job. For instance, you might have one compressor with a fast attack and a high threshold, to tame the peaks, and another compressor with a slow attack and a low threshold, to reduce the dynamic range. There's very little reason to have two compressors on a channel that have almost the same settings; just delete the second one and increase the ratio of the first one, and theoretically you'll have the same effect.[4]

5.4.3 Parallel Compression

Another variation on standard compressor usage is "parallel compression." Parallel compression involves sending a sound through a fairly heavy-handed compressor, and then mixing the dry signal together with the compressed signal.[5] Parallel compression is a gentle effect that reduces dynamic range from the "bottom up" rather than the "top down." Rather than bringing down peaks, it brings up low-level details.[6] It is often applied to drum/percussion group channels[7], but you can use it on any track where you want to bring out the details while preserving the peaks.

5.4.4 Sidechain Compression

Some compressors offer a "sidechain" input. This is a secondary audio input that allows you to use a signal other than the input signal to control the action of the compressor. This second signal is called the "sidechain signal."

A sidechained compressor behaves quite differently from a normal compressor. Rather than responding to peaks in the input signal, it responds to peaks in the sidechain signal. The dynamics of the sidechain signal are used to shape the dynamics of the input signal. If the sidechain signal goes over the threshold, then the input signal will be reduced accordingly.

Effectively, sidechain compression allows you to cause to input signal to get out of the way of the sidechain signal. It is therefore useful for creating space in a mix. For example, subtly sidechaning the background instrumentation of a song to the vocal line, causing the background instrumentation to fall back a bit when the vocals come in, can give the vocals more room to breathe while keeping the mix nice and full.

Sidechaining a bassline to a kick drum can also be very effective. It can get the bassline out of the way of the kick, so that it can really kick, without taking too much away from the power of the bassline. Furthermore, a carefully adjusted sidechain compressor can cause the kick and the bass, when they hit together, to fuse into one unified kick/bass sound, which can sound very nice.

Though sidechaning can be used subtly to create space in a mix, it can also be used as an artistic tool. With long release times and/or high ratios, sidechain compression can cause a dramatic "ducking" effect. This effect is often used in modern house and techno music, where much of the instrumentation may be sidechained to the kick drum, causing the music to rhythmicahy pulse and throb in time with the kick. A similar effect can be achieved with a single normal compressor on the master bus, as described in Section 5.4.8.

Finally, sidechain compression can be used to reduce the level of sibilance (i.e., the consonants 's' and 't') in vocal recordings, a process known as "de-essing." This is frequently desirable as these consonants can be sometimes be annoyingly loud. To de-ess your vocal, set up a sidechain compression routing where the sidechain signal is simply the input signal sent through an EQ. In the EQ, boost the sibilance range (around 2-8kHz). Now the sidechain compressor should reduce the gain of the input signal whenever there is significant sibilance.

5.4.5 Gates

A gate is not a compressor. A gate is something entirely different, but it is another device that is concerned with shaping dynamics, so it makes sense to discuss it here.

Unlike a compressor, which is concerned with reducing the volume of the loud parts of a sound, a gate is concerned with reducing the volume of the quiet parts of the sound — usually to the point that they disappear entirely.

The most important controls on a gate are threshold, attack, and release. The gate will cut all sound below the threshold. The release determines how quickly the gate will "clamp down" once the signal falls below the threshold. The attack determines how quickly the gate will relax and let the signal through when the signal rises above the threshold.

The stereotypical reason to use a gate is to reduce noise in a recording.[8] By putting a gate on a noisy track, you can cause the track to be silenced when there is no useful signal on it, thus removing the noise in those parts.

Noise is less of an issue in computer-based electronic music production than it is in traditional recording. That said, there are still reasons to use a gate that are unrelated to noise reduction.

One of the most important applications of gates is cutting off the tails of decaying sounds. For instance, if you have an acoustic kick or snare that has an undesirable tail end that's muddying up the mix, then you can use a gate to remove it. Similarly, if you have an excessively reverberant sound, you can cut off the reverb tails using a gate.[9]

Some gates also have sidechain inputs, and this opens up a variety of creative possibilities. Sidechaining a gate is conceptually analogous to sidechaining a compressor. It causes the gate to clamp down on the input signal when the sidechain signal is below the threshold, and to let the input signal through when the sidechain signal is above the threshold.

Effectively, a sidechained gate allows you to cause the input signal to follow the dynamics of the sidechain signal. Usually you will have a sustained sound as the input signal, and a rhythmic percussive sound as the sidechain signal. The final effect will be that the input signal will rhythmically pulse in time with the sidechain signal.

5.4.6 Expanders

Like a gate, an expander is not a compressor. Conceptually speaking, an expander does the same thing as a compressor, except that, rather than reducing dynamic range, it increases dynamic range. When the sound rises above the threshold, the expander amplifies it by an amount proportional to the ratio.[10]

Many compressors are also expanders.[11] To use a compressor as an expander, simply set the ratio to a value below one.

5.4.7 Shaping Percussive Sounds

One application of compression that deserves some special attention is the shaping of percussive sounds, as described in Section 5.1.2. This type of compression should be applied to single percussive sounds: a snare drum, a cymbal, a guitar, a piano, etc. It should not be applied to mixed drum kits. If you wish to apply this technique to your drums, use a separate compressor for each drum sound.

Section 5.1.2 discusses two separate cases for shaping percussive sounds: bringing out the attack, and bringing out the body. We will consider each of these cases individually.

To bring out the attack, set up a compressor with a slow attack and a moderate or fast release. Set the threshold below the level of the body of the sound. Set the ratio to taste, but fairly low is usually best. This technique works because the slow compressor attack leaves the attack of the sound intact, and the compressor then clamps down on the body (which is still above the threshold). It brings out the attack by reducing the level of the body.

You can also bring out the attack by using an expander. Set up a fast attack and a moderate or fast release, and set the threshold above the body of the sound. Set the ratio to taste, but fairly low is usually best.

To bring out the body, set up a compressor with a very fast attack. Set the release as fast as it will go without causing distortion. Set the threshold just above the highest point of the body. Set the ratio to taste. This technique works by clamping down on the attack of the sound. It brings out the body by reducing the level of the attack. You can further bring out the body of the sound by using an additional compressor to compress the body, as a serial compression technique.

Parallel compression is also very well-suited to the task of bringing out the body of a percussive sound. Simply set up the compressor to completely flatten the attack out of existence, and then use the level faders to adjust the balance between the attack and the body, turning up the compressed channel to increase the level of the body.

5.4.8 Creating Pumping Effects

One of the cool things about compression is its ability to manipulate grooves. By shaping the dynamics of the music, it shapes the patterns of emphasis and deemphasis, and by shaping said patterns, it shapes the groove of the music. We have already seen one way to manipulate grooves in Section 5.4.4. Now we will look at another method.

This method will usually be applied to a full mix, but sometimes it might also be applied to a group channel. The idea is that you have one or two loud drum parts (usually the kick drum and possibly the snare drum), which are routed, along with a bunch of other elements, to the channel being compressed. You set up a compressor on the channel, and set the threshold so that it is triggered by the drums and not much else (for this technique to work the drums must constitute the highest peaks in the music). Set a hard knee, fast attack, slow release, and moderate ratio. Now turn up the drums. They will begin to trigger the compressor more intensely, and the slow release will cause the rest of the music to pump.

The pumping, if done well, will be fairly subtle; you should hear an obvious difference when you toggle bypass on the compressor, but you probably won't be able to actually hear the pumping unless you listen very closely.

If you are going to put a pumping compressor on your master bus, or really any compressor on your master bus, it is generally best to put it there fairly early on, and then mix "into it." If you put it on after the fact, then your results will not be as good, because the compressor will have messed up a bunch of mixing decisions that you made previously. If you make those decisions with the compressor on, then you will compensate for the effects of the compressor, and get goo d results.

5.4.9 Multiband Compression

A multiband compressor is an elaboration on the basic concept of compression. A multiband compressor works by splitting the input signal into multiple frequency bands (usually three), sending each to a separate compressor, and then mixing the signals together again after compression. So, in the usual case, you have three compressors: one for bass, one for midrange, and one for treble. You can set the precise frequency range that each of these bands affects.

Multiband compressors were originally invented to be used as a mastering tool, but they do come in handy from time to time in mixing. They are useful for manipulating material that is already mixed together, such as drum loops. They can also produce interesting results when shaping percussive sounds. More generally, they can be put to a variety of creative uses; reaching inside of a sound and shapings its dynamics at that level can produce quite startling results.

Multiband compressors are also useful for evening out instrumental performances that would otherwise be difficult to correct. For instance, if you have a guitar part that has the occasionally excessively "twangy" and sharp plucked note, you can smooth it out by applying a compressor in the treble range and leaving the rest untouched.

Finally, multiband compressors provide a good method for de-essing. By setting one of the frequency bands to target the sibilance range (around 2-8kHz), you can isolate the sibilance and compress it by itself.

[1] Conversely, changing the dynamic structure of music also changes its frequency content, so compressors can often undo the work of EQs. But this generally ends up being less of a problem, and so by putting the compressor after the EQ, you choose the lesser of two evils.

[2] Some compressors also have a look-ahead feature, but it is more common in limiters.

[3] Actually, it uses a delay buffer. A look-ahead limiter will introduce latency proportional to the length of its look-ahead. Some DAWs will automatically compensate for this delay by delaying everything else by the same amount, so that the effect on the music is transparent. This feature is often referred to as "plugin delay compensation."

[4] Note that when you have multiple compressors in series, the effective overall compression ratio is the product of all of the ratios. So, for instance, if you have a compressor with a 2:1 ratio followed by a compressor with a 3:1 ratio, the overall compression ratio is 6:1.

[5] You can accomplish this by putting a compressor on an auxiliary send track and sending the track to be compressed to it.

[6] If I may insert some personal opinion here, I think that parallel compression is awesome, and you should use it a lot.

[7] When applied to drum tracks, parallel compression is called "New York compression."

[8] In fact, some people actually call gates "noise gates."

[9] This also leads to the stereotypical Phil Collins snare sound, which is based on a snare drum routed through a thick reverb and gated to cut off the reverb tail.

[10] It is interesting to note the relationship between compressors, expanders, gates, and lim- iters. Compressors and hmiters reduce dynamic range, while expanders and gates increase it. Compressors and expanders are gentle, whereas limiters and gates are merciless.

[11] In fact, it is actually fairly rare to come across an expander except as a special mode of a compressor.

Next - »« - Previous