Along with EQ and Reverb, Compression is one of the main tools in the producer's arsenal. Compression works by reducing the dynamic range of an audio signal, which changes the relationship between its loudest and quietest parts and shaping its transients in the process. Compression can significantly improve a mix by bringing details into focus, giving bulk and body to sounds.

• Dynamic Range Control
• We can use Compression to control the peaks of an audio signal, reducing volume fluctuations and, after gain, producing higher average levels.
• Sound Shaping
• Compression can also be used to alter the envelope of a sound
• Sound Coloring
• Lastly, Compression can be used to change the tone of a sound.

When working with a single short sound like a kick or snare, compressors can change the shape of the sound in dramatic ways. For example, we can accentuate the click at the front of a kick, or increase the sustain on a snare. For denser material like vocals, we can use a compressor to even out the volume over time, helping to keep the part at a steady level to fit the mix.

In both cases, we can use a compressor to color the signal passing through it.

• Threshold
• Level (measured in dB) at which the compressor begins reducing signal volume.
• The signal below the threshold passes through untouched; only that which exceeds the threshold is compressed.
• Ratio
• Amount of gain reduction applied to the signal above the threshold.
• At a ratio of 1:1, there is no reduction.
• At 3:1, an input level of 3dB over the threshold is required to generate 1dB worth of level above the threshold.
• As the ratio increases to above 10:1, the compressor begins to limit the signal until the signal becomes so squashed it is said to be brick wall limited. 
• Attack
• The length of time it takes for the signal to be compressed to a specific amount of gain reduction after the input signal exceeds (and keeps exceeding) the threshold, measured in milliseconds.
• Longer attack values preserve early transients (like drum attacks) by delaying the onset of compression.
• Digital compressors have the option to 'look ahead' for instantaneous attack times, something analog units aren't capable of. 
• Release
• The length of time it takes for the compressor to return to 0 dB worth of gain reduction after the signal has returned below the threshold.
• The release setting has a major impact on rhythm and motion in a track, with a fast release time pulling the 'background' forward, sometimes resulting in a pumping sound.
• Be careful about setting the release time too long, as it can lead to a situation where the compressor isn't able to catch up and react to the input signal, resulting in an immediate squash followed by an unnaturally slow ramp back up.
• Auto Release
• Although you might not find this control on every compressor, the Auto release option sets the compressor that it constantly monitors the incoming signal and modifies the release time on the fly depending on the material.
• Auto Release really comes in handy when dealing with complex material such as vocals and bused groups where the attack and decay characteristics are constantly changing.
• Knee
• Sets the behavior of the compressor as the input signal approaches the threshold.
• On a hard knee setting, the compressor only works once the signal reaches the threshold. With a soft knee, the compressor starts acting as the signal level nears the threshold - increasing the ratio upwards until the threshold is passed and the full ratio is applied.
• Soft knee curves are more suited and subtle signals that don't need to instantly cut through a mix like vocals and synths. Hard knee settings preserve the transients.
• If in doubt, the compression knee rule of thumb says: for banging mixes, go hard!
• Make-up Gain
• Since the compressor reduces peak levels, the net result is a lower output level. You can use the make-up gain to push the volume back up. Some compressors offer auto make-up gain by default so that the new peak level matches the old one. 
• Gain Reduction
• This meter shows how much the signal is being reduced by in dB.

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