They use compression of all types

When it comes to mixing, few things are as important as good compression. Have you ever visited a venue and are enjoying the atmosphere when, out of nowhere, an electric guitar or vocalist pierces through the mix and hurts your ears? That my friends is the hallmark example of why you need compression on each channel. Compression can serve two primarily forms, as a safety net against spikes in signals, and as a tool to reduce peaks and thus raise low energy sounds, causing a more controlled range of volume. Despite what I just suggested and what is often thought to be true among mixers, compression does not make quiet sounds louder. It dampens loud sounds and peaks in signals. A compressor only works to increase volume in softer sounds when you, after compressing a channel, raise the gain or volume of the channel.

In compressors, there a few control terms that you must understand.

Attack

Attack is how quickly a compressor will hit the signal once it crosses a determined threshold of loudness. For a lot of pop music, the compression attack is very quick because we want all the levels to be very uniform and consistent in loudness. If you listen to pop music you will find most of it has very consistent levels in the vocals and instruments individually. They are mixing for a loud mix where the vocal is present and consistent throughout the mix. For other styles, such as rock, you might find slower attack times on instruments like the bass. Allowing around 30 ms of attack time allows low frequencies to get a full wavelength of sound through before the compressor triggers. This allows your ears to perceive a louder thump at the beginning of a loud section, but contains the loudness to minimize overwhelming signal and help maintain consistency.

Ratio

Ratio has to do with how harsh the compressor will be, how hard it will kick in for signals above the threshold. Will it reduce the signal by ½, a factor of 2, a factor of 4, and so forth. The harder you make the ratio, the less the signal output will increase as the input increases. If you turn the knob to infinite, then you basically have a limiter which will not allow any signal over the limit at all. In practice, you have to play with the ratio to find what works best. For vocals, it’s often best to use two compressors. The first has a small ratio which allows you to gently bring the harshness down on peaks. The second compressor is more severe and is set at a higher db level. It helps ensure that truly strong peaks don’t blast through the first compressor unchecked. 

Threshold 

Threshold, as mentioned above, allows you to set the level at which the compressor will start to kick in. Any signal below the threshold will be unaffected by the compressor. Once a signal crosses the threshold, it will be dealt with according to your settings. A somewhat high threshold with strong ratio can be used as a safety net. Signals will normally be unaffected by the compressor unless they hit an unusually high level, at which point the compressor will kick in and avoid catastrophic levels in the mix.

Release

Release is a hard concept to grasp with compressors. You want release to be long enough that when a signal crosses a threshold, it holds on and compresses the ensuing signal properly. If you make the compression release too long, it will not allow new peaks to form. Which depending on the method of compression you are aiming for, can leave your music feeling somewhat lifeless and unenergetic. It is possible to create a pulsing sensation with your compressor for kicks and bass signals if you time the release correctly. This can be helpful in clubs and venues that want an exciting air of energy in the low frequencies.

Output and gain

Compressors will normally have an adjustment to allow you to compensate for signal reductions you’ve made while compressing. If your overall sound level has decreased because of compression, you can adjust the level with the output or gain knob. This allows all your other settings to remain the same and keep all the adjustments related to the compressor in one location. If for some reason you were to disengage your compressor, you would not have to immediately adjust your channel gain because that tweak was part of your compressor. Keeping the entire set of compression adjustments together in one package.

Flavors of Compression

There are hundreds of different compressors and they each have a different sonic flavor, affecting sounds differently with unique tones and shapes. Playing around with available digital compressors is a great way of finding ones that work. You can also search online for popular plugins and compressors. Digital compressors often allow for much greater control over the sound and will often sound very close to analog models.

There are flavors and approaches to compression though that you will want to play with.

Parallel compression

Parallel compression is a wonderful tool to help add thickness and punch to signals. Parallel compression is very popular for mixing punchy drums and vocals. Parallel compression is where you take your channel and split it into two signals, one goes uncompressed while the other is compressed, often with a strong ratio and increased gain level. This allows you to have the dynamics of an uncompressed signal while achieving robust and thick sounds that would be left out of the mix had you not used a compressor.

Rear bus compression

Similar to Parallel compression is Andrew Scheps’ rear bus compression. Back in the day, there were often two busses for mixing, one was for the front of the room and the other was for the back of the room. Scheps discovered that if he threw all the instrumental channels together in a bus, excluding the drums and normally the bass, he could then compress the bus very hard. Like parallel compression, this separate bus would compress all the signals down, taking any peaks from instruments and lowering them all to one uniform level. By mixing this bus back into the master channel you add a robust thickness to the mix that makes it sound dramatically louder while maintaining dynamics. It also works as a volume control maintaining more consistent levels among all the instrument channels as a whole. Rear bus compression can be a vital tool for making broadcast and online mixes sound full.

Side Chain Compression

Side chain compression is unique as it uses the signal of one channel to control the compression of another channel. One popular use of this is to compress the bass with a somewhat quick release that is triggered by the kick drum channel. What this does is carve out space in the low frequency for the kick to come in thick and full and have room to make an impact without overwhelming the low frequencies. Side chain compression is a powerful tool to help make space in a mix for punchy instruments that need to take the lead and have room to breathe. 

Multiband Compression

A type of compression that was once reserved primarily for broadcast is multiband compression. Multiband compression is beautiful because it allows you to control the levels of the low, middle, and high frequencies separately without muddying the tone and dynamic of the other frequency regions. A strong signal in the higher frequencies will be pushed down while allowing the low frequencies to still breath and not be compressed down. It allows more dynamic range and life in a channel that has a broad spectrum of frequencies. It is a good safety net to use on main mix channels and broadcast feeds. It can be applied to synthesizers and troublesome vocals which require different levels of compression for different frequencies ranges.

Dynamic EQ

Dynamic EQ is a newly emerging type of equalizer that includes aspects of a compression. It allows frequencies to be dynamically controlled, raising or lowering the volume of a frequency band much like a multiband compressor would. This allows you to target a specific dynamic range and tone that you want your frequencies to fall into. In a way, it rides the volume of frequency bands to keep them inside the parameters you set. Dynamic EQ can be difficult to learn as it is very unique and powerful. It is a wonderful tool for getting the best of both worlds of EQ and compression together in one unit.

De-essers

De-essers are a type of compression that only applies to a very targeted range of mid-high frequencies. It is used to reduces the harshness of S and T sounds when people are talking. It can also be used to help address very specific frequencies prone to feedback. De-essing is very powerful in the studio but can also make a huge impact for speakers such as pastors who wear Countryman style microphones which are in close proximity to the mouth, causing explosive dynamics to be directly captured by the microphone.