Essential Effects - Compression

A compressor is a tool that is crucial to the mixing process, yet can be very difficult for new recording engineers to master. Compressors work by changing the dynamic range of the signal they process. They can be used to smooth out sound with a lot of peaks, or add punch to tracks that are lacking. Compressors can also be used to control volume differences between parts of a track. Though the difference a compressor causes may be subtle to hear on an individual track, it has a large impact on a final mix, and its importance must not be overlooked.

In the past, recording engineers had limited options when using compressors. A single compressor could usually only be used on one track. It was important for recording engineers to understand how each compressor worked best to use it effectively. Now, with the use of computers and DAWs, we have access to an almost limitless number of compressors and can use them on all of our tracks if we so choose.

A compressor has many different controls that all contribute to how it affects the overall tone of a track. Understanding what each of these controls does is important to using a compressor effectively.

Input Gain

The first control on a compressor is input gain. This adjusts the level of the signal coming into the compressor that will be processed.

Threshold

Next is threshold. Threshold controls the volume level at which the compressor is activated. Signals below this level are unaffected. The signal above the threshold will be reduced by the compressor when it is active. There are two types of threshold for a compressor; they are variable threshold and fixed threshold. Variable threshold gives the user a dedicated control for the threshold level. On the other hand, fixed threshold relies on input gain to change the threshold relative to a particular signal.

Ratio

Ratio refers to the amount that a signal over the threshold will be reduced. A higher ratio means a higher reduction in signal over the threshold. For example, using a ratio of 3:1, a signal that is 6 dB over the threshold will be reduced to being only 2 dB over the threshold. Using a ratio of 1:1 will not cause any change, and a signal of ∞:1 will not allow the signal to go over the threshold.

Knee

One option a compressor may have is hard knee or soft knee. This refers to the transition point between unity gain and a set ratio of compression when the threshold is reached. Hard knee compression takes effect fully as soon as the threshold is reached, and is useful for instruments such as drums. On the other hand, soft knee changes gradually, applying a change in ratio above and below the threshold building up to the set ratio. Soft knee compression is useful when a more transparent form of compression is desired, such as when recording vocals.

Attack

An important time-based control on a compressor is attack. A compressor doesn't immediately affect the volume of sound once it passes the threshold. There is a slight delay before the compressor effectively “turns on”. This delay can be controlled and is known as attack. Setting the attack to slightly after a note is hit can make a track punchier, by reducing the volume level after the initial strike.

Release

The next important control on a compressor is release. Release refers to the amount of time it takes for the compressor to reduce its gain reduction when the signal level drops. The release time of a compressor doesn’t only have an effect when the signal level drops below the threshold, it has an effect on the amount of compression any time the signal level drops. A long release time can hold the sound at a higher level for longer, which increases sustain of the instrument.

Make-Up Gain

The final control on a compressor is make-up gain. Since a compressor will reduce the volume of a signal, it is often important to add gain to the final signal to bring it back to its original volume. Make-up gain is applied to the entire signal, not just the compressed portion. A good technique to adjust make-up gain is to match the loudness of the compressed track with the volume of the bypassed signal.

As with many things in mixing, compression is an effect that requires a delicate balance between tracks and careful listening. When using compression, it is important to remember that the goal is not to destroy a song’s dynamic range, but is more to control excessive peaks or changes that would otherwise be unpleasant. Proper use of a compressor is a skill that requires a lot of practice to perfect, but will lead to a dramatic difference in the overall quality of your mix.