Using Compressors

Compressors are amongst the most useful tools in a sound engineers toolbox and yet they are often underused, probably due to a lack of understanding as to what they can do.

A Compressor at its simplest is used to control the dynamic range of a signal. There are two main controls - Threshold and Ratio. As a signal passes through the compressor it is checked to see if it meets or exceeds the threshold value. If it does then it is attenuated (turned down) according to the ratio set. To avoid sudden, noticeable changes in the signal level a technique named 'soft-knee' compression is employed whereby the ratio is small just above the threshold and increases towards the set value the higher the signal gets above the threshold. This makes for a much more subtle effect.

As you can hopefully see, compressors are very simple in what they do and just need two controls to set them up. The more advanced models have extra controls. They often incorporate Attack and Release controls to allow you to vary the speed at which compression is applied and removed from the signal. A Peak Limiter is another common addition. This essentially sets a maximum signal level. If this is reached the signal is attenuated and cannot get any louder, unlike the soft-knee effect from the basic compressor.

Applications for the compressor include the following:

Vocals or Instruments

Individual vocals and instruments can suffer from an extremely wide dynamic range; from very quiet to extremely loud. From a sound perspective this can cause problems. For example, the mixing desk preamps need to be set high enough to cope with the minimum signal level but this does not leave sufficient headroom for the maximum signal levels. This invariably results in distorsion of the very loud signals. By adding a compressor in to the chain, either before the signal enters the mixer or by using the Insert point on the mixing channel, the loud signals can be attenuated thereby removing the distorsion and creating a more levelled sound for the instrument or voice in question.

Overall Mix

Compressing a signal can often have a purely musical effect on it resulting in a more pleasing sound than the original. It is therefore not uncommon to see a compressors inserted into the output mix of a desk. This is also used for recording purposes to once again prevent the signal getting too high and overloading the maximum recording levels of the recording media.

Protecting Loudspeakers

In a live music setup care must be taken not to overdrive the speakers or damage may occur. Most of the professional speaker manufacturers produce dedicated system controllers that will prevent their speakers from being over-driven. There are also some very good non-system-specific units (e.g. BSS Audio's Omnidrive and XTA's DP224 and DP226). The disadvantage of these units is that they are expensive. An inexpensive alternative is to use the Peak Limiter section on a standard compressor. Using this a maximum signal level can be set and this will prevent dangerously high signals from reaching the speakers. It should be noted that this method is not ideal as every amplifier/speaker is different and you would need to do some strange calculations to work out what settings on the Compressor corresponded to the appropriate maximum signal level for the amplifier / speaker combination.

We have two different compressors available for hire. The Behringer MDX2100 provides two channels of compression with the addition of a simple noise gate and peak limiter on each channel. The BSS DPR422 is more 'musical' compressor providing two channels again without the gate and limiter but with the addition of a De-esser circuit which is very useful in removing the sibilance (hissy 'S' sounds) in vocals.

Unit 6, North Leith Sands, Leith, Edinburgh, EH6 4ER, Scotland, UK Tel. 0131-555 5200