As we have seen the main job of a compressor is to reduce the dynamic range of the audio material it is applied to. Peaks are reducedand quiet sounds are more audible. The main effect is to give a more stable overall levelbut an added density or thickness to the sound and enhanced character or 'warmth" can be welcome side effects. There can be trade-offs though. Over-compression can increase unwanted noise that is not part of the musical signal such as breaths, rustling paper and fret noise or it can increase room ambience. If you have recorded in a great sounding room this is not necessarily a problem but if you're recording in a home environment you'll probably want as little of the room sound as possible so as to add a nice convolution reverb later. Badly applied compression can also introduce audible 'pumping' which is not usually desirable unless you want it for a special effect.
On the plus side again though you can affect the amplitude envelope of a sound by adjusting the attack and release times of a compressor. This means you can enhance (or for that matter, reduce) the front end crack of a snare or the click part of a bass drum, or you could bring out the sustain part of notes on acoustic, or clean electric guitar. If your compressor is quite transparent and you want to hear more effect as a general rule use a faster attack, a shorter release and a higher ratio. If you're after a smooth compression sound use a soft knee approach but if you need tighter control and a more obvious effect go for the hard-knee option.
The impact of drums can be changed dramatically through the use of carefully applied compression. By nature drums have a large transient peak followed by a quieter "body" sound. Compressing the transients will make the kit seem louder but it is easy to squash the life out of it too by eliminating the crack in the sound completely! A slowish attack time will let though this initial crack so that you retain some life in the sound (possibly peak Limiting will be needed to prevent overload). The decay of a drum will be altered by the release control: with a slow release you will get a bit less of the "body" part of each drum sound since the gain will still be at a reduced level. A fast release will seem to extend the sustain, the natural decay being in effect faded up. A possible use for a slow release is to subdue an over bright and rattly snare. On the other hand if you have a snare that needs to be fattened up a bit and perhaps you'd like to hear more of those rattly snares then a fast release will bring up those sounds (and the noise too unfortunately) after the initial compression of the snare's attack. Compressing the drum overheads will tend to make the room character more noticeable. If your recording environment did not flatter the sound then it might be better to use less compression on the overheads and use a good room ambience on the close-miced dry kit sounds. If you have made a stereo audio subgroup for your drum kit it often makes sense to apply some overall compression (or even limiting) at this stage to help gel the sounds together and present the kit as a coherent instrument in its own right rather than as a set of individually played disparate elements.
This is one area where you can be quite aggressive with your compression, as bass tends to fluctuate in level by considerable amounts. It is important for your whole mix for the energy levels in the low frequencies to be as smoothly controlled as possible especially if you're using a mix compressor and you want to avoid pumping artifacts on the mix bus. Bass compression will fatten up the tone and prevent more quietly played notes from getting lost. High ratios of up to 10:1 are not uncommon for bass depending on the playing style and quality of the performance. You can also adjust the attack and the release times, of course, to enhance or reduce front-end click on the bass as desirable.
Acoustic and clean electric guitars also tend to benefit from compression. They often sound smoother and fatter and, if the release is fairly short with a high ratio, it will bring out the sustain, If you want to enhance the initial transient, just as for drums, you can use a slower attack time to let it through. For slighty distorted guitars, compression can help to increase sustain. With a heavily distorted sound there is unlikely to be much dynamic range anyway since the process of overdriving an amp or using a distortion pedal limits the output in itself. So there's not a lot of point in using compression here. It is more likely to bring up the level of unwanted noise or hiss during any pauses.
Compression is widely used on vocals in rock and pop mixes. Rock and pop backing tracks tend to be quite powerful with up front drums, guitars and bass. The voice is a comparatively subtle instrument prone to large changes in dynamics as a result of, the singer's expressiveness. Therefore, in order for the voice to sit comfortably in this kind of mix, the engineer will generally use some compression to bring down the peaks, even out the sound and thereby raise the average signal level. This results in a tighter, punchier and more confident sound for the vocalist. Compression can also help with unwanted level changes due to poor microphone technique.
As always the settings you choose will depend on the vocal performance and musical style, as a rule of thumb try adjusting the threshold so that the quiet parts are not compressed. This should help to avoid completely flattening all the expression in a performance through over compression. Ratio's of between 1.5:1 and 6:1 are usually sufficient with attack times of around 1ms to 10ms and a releases of around 80 to 150ms as starting points. These figures are just a rough guide — you can always try out some of the presets that come with your software compressor and adjust from there. Try to work out why the programmers have chosen particular settings for a particular vocal style. Remember your ears are always the best judge.
Remember too that different parts of the song may have elicited different vocal styles from the performer. So you may need to use different settings, or indeed different compressors, in the chorus as compared to the verse. This can be done through automation or by chopping up the audio file and putting the verse vocal on one track and the chorus vocal on another.
If you have a selection of compressors, either internal, outboard or both it's a good idea to try them out on vocals, as this is where differences between models can come to light. For example the UREI1176 - which is widely available in software form - tends to be quite transparent — evening out the levels without making the process obvious even with quite heavy compression- whereas certain units, such as the Joe Meek ones, deliberately introduce 'character' to the sound. Neither one is best. Different material will require different approaches - it's a matter of personal taste. Ideally it's good to have both 'character' and 'transparent' styles of compressor in your armoury.
As we noted earlier backing vocals are often grouped together for level control and processing in a stereo audio subgroup. Very often compression, EQ, and possibly limiting are applied at this stage. If computer-processing power is an issue then grouping multi-tracked BVs and adding compression at this stage can be a valuable way to save resources. Even if you are