Compression Basic

Compression Basic

How do you know when to compress?  The main purpose of using a compression technique is literally to compress the dynamic range of an audio signal to find the right balance.  For example, let’s say that there is a vocal track that is recorded with some mumbled words.   If you set a volume fader so that the overall vocal part is audible in your mix, the mumbled part of the vocal track would be hard to hear.  If you start to raise the volume fader to make the mumble part of the vocal audible, the overall vocal part would be stick out too much in your mix.  This happens when there is too large dynamic range in your mix, and a single volume fader would not fix this problem.  To reduce the dynamic range, we need to use compression.  Compressors will detect the peaks of the sound waves and reduce it to make the balance between the quiet signals and the loud signals.  A typical compressor is consisted with five components: threshold, ratio, attack, release, and make-up gain.

Image of an audio file before-compression (top) and after-compression (bottom) from http://www.platinumloops.com/how_to_use_a_compressor.shtml

Threshold

To make it simple, just think of a threshold as a ceiling.  It determines the trigger point in dB, which any audio signals exceed the threshold would get compression and any audio signals under the threshold would remain uncompressed.  As you lower the threshold setting, more amounts are compressed and as you move up the threshold setting, fewer amounts are compressed. 

Ratio

Ratio is the amount of compression applied to the signals above the threshold.  Usual ratio settings range from 2:1 to 8:1.  If your ratio setting is in 2:1, it means that any incoming signal of 2dB above the threshold would be reduced to 1dB on its output.  However, according to Sonic Arts Center at The City College of New York, changes in lower ratio values would have more effect than changes in higher ratio values.  For example, moving from a 2:1 ratio to a 4:1 ratio provides an extra 2 dB of gain reduction, but moving from 4:1 ratio to an 8:1 ratio only provides an extra 1 dB of gain reduction. 

Attack & Release

The attack and release controls the time parameters on your compressor by determining how quickly the compression to occur and to reset.  Faster attack time allows the compressor to quickly compress the audio signals that passed the threshold right away.  Slower attack time allows the compressor to pass initial transients of the audio signals before any compression kicks in.  The release controls the speed of time, which how fast the audio signals return to its original phase after compression.  Faster release time creates percussive characteristics by quickly releasing compression of the audio signals.  Slow release time gives a long-sustained compression to the audio signals.  According to Sound On Sound magazine, sometimes the attack & release controls may create some unwanted side effects, because it changes the waveform shapes rather than just adjusting the overall dynamic ranges.  Click here to see more information about the side effects.

Make-up Gain

The last stage of compression is called a make-up gain.  The make-up gain allows the audio signals to make up or boost its volume level on the output of the signal to compensate the levels that were lost during the compression. 

There are many other reasons of using compressions than finding the right balane in your mix, like adding crisp and flavor to the sound or sidechaining.  However, using a compressor without understanding the basic concept and knowing every components on the compressor would often lead engineers to misuse the compressor in their mix.