Continuing the series on explaining effects, this time it’s all about Compression. Compression is a form of Dynamics processing which is any device that preforms automatic gain changes. Other Dynamics processors are Noise Gates, Maximisers, and De-Essers.
Compression
Compression is an effect that can take a while to understand because the results are not always as obvious as other effects. To explain it as simply as possible, when a signal goes into a compressor, it gets turned down. That’s it. How it does this, how fast, and smoothly is what makes each one unique.
Compressor Controls
Most compressors will have the same set of controls:
The Threshold control sets what level will start the gain reduction.
The Ratio sets how much gain reduction, with a 4:1 ratio for every 4dB of signal above the threshold 1dB will be allowed through.
The Attack control sets how fast the compressor reacts to peaks.
The Release control sets how fast the compressor reacts as the signal lowers
Makeup gain is used to bring up the overall level of the compressor after the peaks have been reduced.
Sometimes there is an auto makeup gain control which will increase the output level to match the gain reduction.
Some compressors have a knee control that starts compressing at a lower ratio as the threshold is approached, this is very helpful for a more natural compression.
Compressors will usually have a few meters, an input level, gain reduction and output level. If there are only two meters there is usually a switch to change the output level to show gain reduction. Gain reduction meters go in the opposite direction of the level meters.
Setting a compressor
This is my method for setting a compressor:
I choose a ratio depending on how aggressive I want the compression to be. The type of sound I’m using it on determines this, softer sounds like voice get lower ratios, bass gets a medium ratio and drums get a higher ratio.
I turn the attack and release controls to the fastest setting, and make sure the meter is showing gain reduction.
Then I lower the threshold level until I’m getting about 1 decibel of gain reduction on the peaks.
From there I’ll fine tune the attack and release for whatever sound most natural, and use the makeup gain to match the output with the input level.
If I want more compression, I’ll lower the threshold more.
Example:
Here’s an example of some electric guitar with and without compression. I’m using more compression than I normally would on this so that the effect will be easier to hear. It should be pretty obvious that the compressor has evened out the dynamics of the performance.
Compression can bring out more details in a performance, but it will also bring up background noise especially at higher ratios, that’s not usually what you want.
A slow attack will let some of the transient through, you can use this when you want to increase the punch of drums. You want to compress the sustain of the drum, and use the make up gain to make the drums larger than life.
Example:
In this example there is an ambient room mic for a drumkit. First you will hear it without compression, then with (actually with a ton of compression), and I’ll increase (slow down) the attack time with each loop. Notice the increased bigness of the drums, and how the transients get through and keep it punchy.
Limiter?
A limiter is a compressor that’s output stays at or below a specific level regardless of the input level. It only turns down remember. The compression ratio starts at 10:1 and can go up to infinity. Limiters need very fast attack and release to be effective.
A brickwall limiter aka Maximizer, is a mastering tool used to increase the volume of a song as much as possible. These brickwall limiters have an infinite ratio and will not let anything past the threshold. This type of limiter has two main controls, one for threshold and one for the maximum output level.
With these you basically set the maximum output level, something like -0.02dB and then crank the threshold to crush everything and make it sound really loud and obnoxious (like Death Magnetic). The misuse of the brickwall limiter is often associated with the loudness war and with compression in general.
Multi-band?
Another common mastering tool is the Multi-band compressor.
A Multi-band Compressor is actually 4 compressors in one. The frequency range is split up into 4 bands like an equalizer, Low, low mid, high mid and high frequency bands. This can give you a much smoother compression with a lot more control.
De-Esser
There is one more type of dynamics processor, the de-esser. A de-esser is designed to reduce the harsh esss sounds in a voice. The compression works on a single frequency or frequency range rather than the entire input signal. These are generally used for voice processing but you might find some other uses for it.
Recommended Plugins
Simple compressor: Massey CT4
Advanced compressor: Digidesign Smack!
Master Limiter: Massey L2007
Multi-band Compressor: Wave Arts MultiDynamics 5
De-Esser: Massey De-esser
