In this article Studio Slave aims to cover: what compression and dynamic range are, Common compressor parameters and settings, the different varieties and models of Compressors available as well as how to set a compressor up, the different types of compression and also some useful tips and tricks when applying compression.
Compression is a fine art form that when used correctly can do wonders for the mix.
Compression can serve many different purposes from colourisation, sound sculpting and shaping, dynamic control and side-chaining.
Many producers in the digital era have got into the habit of throwing compressors on every channel and not putting any thought into the settings used, or using presets which may not do the sound as much justice as dialing the settings in yourself would.
Older producers didn’t have the luxury of these kinds of resources and often only had a few hardware compressors to use within their whole mix.
Because of this, it was important that they knew their compressors, and how to use them inside out.
Did you know?
WHAT IS COMPRESSION
Compression is the process of reducing the dynamic range between the loudest and quietest parts of an audio signal. attenuating louder signals that exceed a certain threshold in order to boost the overall output signal. This results in an overall louder perceived volume.
There are many different forms of compression such as upward and downward compression, parallel compression, multi-band compression, glue compression and bus compression as well as side-chain compression. Each of these types of compression has a number of different uses and functions within a mix.
SO WHAT IS DYNAMIC RANGE
Dynamic range is the ratio of the softest sound to the loudest sound in a musical instrument or piece of electronic equipment. This ratio is measured in decibels (abbreviated as dB) units.
Dynamic range measurements are used in audio equipment to indicate a component’s maximum output signal and to rate a system’s noise floor.
As a reference point, the dynamic range of human hearing, the difference between the softest sound we can perceive and the loudest, is about 120 dB.
16-bit audio has a dynamic range of 96dB and 24-bit audio has a dynamic range of 144dB.
From this diagram, we can see that as years have gone on. The dynamic range of many releases has been reduced as producers are competing to get the loudest possible audio to make their tracks stand out, this was known as the loudness war which is still going on today. This diagram shows how compression can dramatically affect the dynamic range of a signal.
COMMON COMPRESSOR CONTROLS & PARAMETERS
Depending which compressor you’re using and whether it’s a hardware unit or a plug-in, there are some common parameters and controls that you will be using to dictate the behavior of the compression effect. Below are some of the basic parameters of compression. Your compressor may or may not include all of them, but understanding what each one does will allow you to work comfortably with a wide range of compressors.
The threshold control sets the level at which the compression effect is engaged. Only when a level passes above the threshold will it be compressed. If the threshold level is set at say -10 dB, only signal peaks that extend above that level will be compressed. The rest of the time, no compression will be taking place.
The “knee” refers to how the compressor transitions between the non-compressed and compressed states of an audio signal running through it. Typically, compressors will offer one, or in some instances a switchable choice between both, a “soft knee” and a “hard knee” setting. Some compressors will also allow you to alter the strength of the knee curve measured in decibels. As you can see in the diagram, a “soft knee” allows for a smoother and more gradual compression than a “hard knee.”
As an example if we had a soft knee set at 3db and our sample was peaking at -10dB But our compressor was set to -13db. We would still have some compression applied to our signal above the threshold. However, if we had a hard knee set to 0dBs then no compression would take place above the threshold.
This refers to the time it takes for the signal to become fully compressed after exceeding the threshold level. Faster attack times are usually between 20 and 800 microseconds depending on the type and brand of unit, while slower times generally range from 10 to 100 ms (milliseconds). Some compressors express this as slopes in dB per second rather than in time. Fast attack times may create distortion by modifying inherently slow-moving low-frequency waveforms.
This is the opposite of attack time. More specifically, it is the time it takes for the signal to go from the compressed or attenuated state back to the original non-compressed signal. Release times will be considerably longer than attack times, generally ranging anywhere from 40-60 ms to 2-5 seconds, depending which unit you’re working with. Normal compressor operation will be to set the release time to be as short as possible without producing a “pumping” effect, which is caused by cyclic activation and deactivation of compression. For example, if the release time is set too short and the compressor is cycling between active and non-active, your dominant signal — usually the bass guitar and bass drum — will also modulate your noise floor, resulting in a distinct “breathing” effect. Another thing to be wary of is to ensure your compressor’s release time is fast enough that it is set back to zero in time for the next sound to hit. This is especially important with drum loops so that they don’t lose impact. This can be set using the auto release function on many software compressors.
Auto-release tells the compressor to set the release dependent on amplitude and duration of the transients coming into the compressor.
This parameter simply specifies the amount of attenuation to be applied to the signal above the threshold. You will find a wide range of ratios available depending on the type and manufacturer of the compressor you are using. A ratio of 1:1 (one to one) is the lowest and it represents “unity gain”, or in other words, no attenuation. These compression ratios are expressed in decibels so that a ratio of 2:1 indicates that a signal exceeding the threshold by 2 dB will be attenuated down to 1 dB above the threshold, or a signal exceeding the threshold by 8 dB will be attenuated down to 4 dB above it, etc. A ratio of around 3:1 can be considered moderate compression, 5:1 would be medium compression, 8:1 starts getting into strong compression and 20:1 up to ∞:1 (infinity to one) would be considered “limiting” and can be used to ensure that a signal does not exceed the amplitude of the threshold. The diagram below shows compression ratios as they relate to the input and output signals and illustrate how setting your compression ratio will affect the overall signal.
It’s a common misconception that compression makes a signal louder. Although it is often perceived as sounding louder, it actually does the complete opposite and attenuates a portion of the signal. The reason we perceive this as louder is that of the output gain or makeup gain stage. The output gain can be used to compensate for the loss of gain caused from attenuating the signal. So the fundamental theory is to tame signals peaks in order to bring the whole sound up in volume at the end so it is perceived as louder. A word of warning is that this will also increase the volume of the noise floor as well. Make-up gain is usually on by default, which will adjust the output gain for you dependant on your gain reduction meter. It’s often wise to turn this off and set it more accurately yourself using your ears. You should always reference your compression to the original uncompressed signal to check the volume is roughly the same and to check whether it is improving the sound.
COMPRESSOR MODELS AND CIRCUITRY
Compressors come in various different flavours. These are used by engineers for different tasks and some sound far better in certain situations than others.
1) TUBE COMPRESSION
Most tube compressors are actually optical compressors that have the signal pass through a tube at the amplifier stage. The exception to this is the Variable Mu (Vari-Mu) Which replaces the standard transistor with a vacuum tube. Tube compression tends to be very slow, smooth and organic, this makes it an ideal candidate for mix busses and gluing mixes together. The transfer curve is very non-linear. This means actual ratio increases with gain reduction: The louder a transient is, the harder it will be compressed.
2) OPTICAL COMPRESSION
Optical compressors affect the dynamics of an audio signal via a light element and an optical cell. As the amplitude of an audio signal increases, the light element emits more light, which causes the optical cell (resistor) to attenuate the amplitude of the output signal.
3) FET COMPRESSION
FET or “Field Effect Transistor” compressors are similar to VCA compressors, but use transistor circuits, making them much less transparent than VCA’s. They are extremely fast and reliable, as well as being very popular due to their rich and aggressive colourisation of the signal. (which is useful for emulating any compressors containing valves in the amplification stage.) FET compressors are good all-rounders and are a go-to compressor for adding punch to sounds such as snare drums. When a FET is used, it is often because the producer wants to hear the sound of the compressor. The Urei 1176 is a very popular FET compressor.
4) VCA COMPRESSION
VCA or “Voltage Controlled Amplifier” compressors use solid state or integrated circuits. They are usually cheaper than tube or optical compressors and are extremely fast. VCA’s also tend to have less “coloration” compared to optical or tube compressors—somewhat similar to the digital vs. analog tape comparison in the recording. VCA’s are a good choice for correcting very fast transient peaks but are less useful with dealing with macro dynamics, such as over an entire vocal phrase.
HOW TO SET UP A COMPRESSOR
Whether you’re using a hardware compressor or a plug-in, setting up works the same way. Insert the compressor on the channel you want to compress.
1. Set the Ratio to 4:1 – This is a good starting point and allows us to set the threshold to a sensible level whilst hearing the compression take effect.
2. Adjust the threshold until the peaks in the signal are pushing over the threshold and triggering the compressor. Unless, of course, you really want to clamp something, for example, a live bass or vocal maybe. In this case, it can work to make it push over the threshold all the time.
3. Set the Ratio to suit bearing in mind the type of compression and sound you want to achieve. A good starting point is a medium compression setting between 3:1 and 5:1.Without threshold set correctly this can now be fine tuned to better suit our audio
4. The attack and release controls shape how the compressor reacts. A fast attack would be useful for taming attack transients or anything that has sudden peaks early in the signal. Slower attack times suit mastering uses and buss compression. A useful attack time for adding punch to drums is around 20-30ms. This allows for the attack phase of the drum to pass through the compressor whilst compressing the rest of the signal.
6. The release control can really affect the sound of the compressor. Short release times cause the compressor to sound like it’s working hard, but long release times sound more natural. The release can be used to accentuate the decay, release or reverb of a sound.
7. Use the make-up gain and output control to sit the signal back into the mix without adding any unnecessary noise.
8. Setting the hard/soft knee would depend on the material. Hard knee works well for drums, bass, and percussive elements. Soft knee is more transparent and better for vocals and guitars.
9. Look-ahead. Plug-in compressors often have this feature. It uses a slight time delay on the whole song to give the compressor a sneak preview of what’s coming. This allows it to catch all the peaks in the smoothest possible way. It can sometimes cause the compressor to lose its ‘character’ so don’t use it by default. only if necessary. Using these compressors will also mean your DAW you start factoring in a latency or delay compensation. This usually isn’t an issue but something to be aware of.
For every rule about setting up compressors, there’s someone who has broken the rules and made a great sounding record, so experiment.
TYPES OF COMPRESSION
There are many different types of compression. Here we will explain the main types and what they are used for.
This is the standard type of compression used by compressors where we reduce the dynamic range of a signal by attenuating the peaks of a signal then raise the output gain.
Upward compression is not used very often and is usually built in as a function of a multi-band compressor. This does the exact opposite of downward compression, rather than attenuate a signals peaks above a certain threshold, upward compression will Boost a signals frequencies below a certain threshold. This will bring up the quieter sounds in the mix and reduce the dynamic range of the signal in a similar way.
GLUE AND BUS COMPRESSION
Glue and bus compression can be used to gel sounds together, this can be layered samples, mix buses, stems or even the entire mix. They tend to have quite gentle characteristics and can also colourise the sounds to make them sound like they are coming from one source. This sort of compression is perfect for bringing separate samples together to form one cohesive mix.
Also, know as NY or Newtown compression is when we have a compressor set up on a bus and we send the signal to it in parallel. The compression setting is usually pretty brutal and we can then use the dry/wet or the volume fader to mix this effected signal in with the dry signal. This is a very effective method for adding punch or body to a signal.
multi-band compression is often used on the master channel or mix buses. This gives us the ability to compress different frequency bands by different amounts and settings. This is useful as often we will want to compress the low end completely different to the rest of the signal. A typical multi-band compressor such as the one in Ableton live allows us to perform downward or upward compression as well as downward or upward expansion.
Although this technically isn’t compression it’s sort mentioning here as many compressors give you the option to expand a signal. This is when signals above a certain threshold are raised in volume by a set ratio rather than attenuated resulting in more dynamic range. This can be useful for repairing problematic squashed or over-compressed mixes.
This is used for a variety of different purposes, most commonly to duck a bass-line so it doesn’t interfere with the kick drum. This works by using an external source (such as the kick drum) as the compressors trigger. Resulting in a compressor that will compress according to the side-chain input (in this case, the kick) usually you will want a fast attack setting and you can dial in the release to taste.
Side-chain compression can also be used to add rhythmical pulsing or pumping to sustained elements within a track or the entire track as a whole.
TIPS & TRICKS
Here are a few suggestions to get you up and running with compression. There are not rules that must be taken for gospel, but hopefully, these tips will help you feel more confident when using this tool within your productions. Remember experimentation is key and always A-B your compression settings.It is a common practice and recommendation to apply “gentle” compression at different stages throughout the recording/mixing/mastering process rather than applying excessive compression at just one point.
Always listen carefully while adding compression. Compression can negatively affect the timbre of an instrument. This can be simply due to the type of compressor being used, but often it’s the difference in tone between the peaks and the troughs of an instrument (if you reduce the peaks relative to the troughs, the tone will change). Fast compression on instruments with wide vibrato will demonstrate this effect.
Try starting with a moderate to medium ratio of between 2:1 and 5:1. Set your attack time to a medium-fast setting and your release time to a medium setting. Now gradually raise the threshold until you’re getting somewhere around 5 dB of gain reduction. Then set your output gain to compensate for the 5 dB attenuation. Finally, speed up your attack time gradually until it gets noticeable and then back it off slightly.
Experiment with using dramatic compression as an effect. It can sound really cool for example to use a compressor to really stomp on a snare drum can really make it stand out.
If you’re going to compress an entire mix, use caution. In many types of music, there will be a bass line with a fairly constant signal level. If you use compression to try to counter a loud peak, like a brass stab. the entire mix will drop at that point causing the bass line to dip and create the “pumping” effect mentioned above. This problem can be avoided by using a multi-band compressor. This type of compressor will split the signal into multiple frequency ranges and allow you to compress them separately.
Try to compress using your ears rather than your eyes. The different graphs on a compressor can help to give a visual representation of what’s going on however it’s always better to use your ears. Not only does this usually yield better results but also wit time you will train your ears to become acute to small differences in dynamics and tone.