Tuesday, September 4, 2012

Compression - The Basics

What is Compression?

Basically compression is a process that helps reduce the dynamic range of a track or an element within a track. By using a compressor we help control signal input, so the level of a synth or vocal for example, stays at a constant level. Why would you want an instrument or vocal to have less dynamic range? Well if parts of a vocal or instrument were really loud and other sections were quiet, they wouldn't fit in to your mix, as the quieter parts would disappear behind other instruments and the loudest parts may cause the track to clip. Employing a compressor would capture the loudest parts and pull the volume down, so the difference between the loudest and quietest parts would be reduced. This boosts the average signal level, which allows you, as a result, to increase the volume and make the audio louder.

Our ears determine the overall volume from the average level (RMS) not peak value (top level)!

Compression Settings

To understand more on compression, it is important that you learn the affect each control will have on your sound. These basic settings found on most compressors are:

• Threshold
• Ratio
• Attack
• Release
• Hard Knee/Soft Knee
• Make-Up Gain


The threshold is the level in dB where the compressor starts to function. By lowering the threshold you will begin to hear the peaks reduce in volume, as the compressor squashes them. The amount of compression you are applying, by lowering the threshold, will also be visually apparent on the gain reduction meter.


The ratio is the amount of compression that will be applied after the signal exceeds the threshold. A ratio of 3:1 means that if the signal exceeds the threshold level by 3dB, the output signal only exceeds the threshold by 1dB. So the higher the ratio the more the exceeding signal will be squashed by the compressor.


The attack is how quickly the compressor reacts to a signal that exceeds the threshold. A fast attack will clamp onto the initial transients of a sound, so in a drum bus for example a fast attack may be useful to reduce the harshness of a kick drum and snares attack. A slower attack however, maybe more suitable on the master bus when you aim to keep all the transients in tact and focus on warming the track up.

The release is how fast the compressor will return to its normal state after the signal has moved below the threshold. The faster the release, the more you will begin to notice a 'pumping' effect. If used cautiously this can be highly effective in dance music and is commonly used on a drum bus to allow the percussion to pump around the kick drum.

Hard Knee/Soft Knee

As with a fast attack setting using the hard knee parameter will squash the incoming signal as soon as it exceeds the threshold on the compressor. Using the soft knee parameter however, will apply the compression much more gradually and naturally to producer a smoother outcome.

Make-Up Gain

The make-up gain is the final stage of any compressor. When the signal has been compressed it will, as previously explained, reduce the volume of the threshold-exceeding signal. This will boost the average signal level but in turn will reduce the peak level of the signal. By using the make-up gain you can then increase the signal to its pre-compressed level. by Andrew A Charlton

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