Compression is designed to control dynamic range within a recording. This is achieved by making loud passages of a certain threshold quieter, therefore reducing the overall dynamic. In modern commercial music we have become accustomed to the sound of compression and whether mixing or mastering it now an essential tool.

Let's start by looking at the most common controls you will find on most compressors:

1. Threshold: measured in db (decibels) the threshold allows you to select how much dynamic range you want compressed
2. Ratio: The amount of compression applied over the threshold. 4:1 means every 4db over the threshold will be produce 1db of output (this is the actual compression).
3. Attack (ms sec): Allows you to select how much of the signals initial sound escapes the compression. A higher attack setting will postpone the compression from kicking in.
4. Release (ms sec): Allows you to select how much of the signals release escapes the compression. When set to fast or too slow this can cause pumping effects. Release times can be the hardest thing to get right in compression so if you're not sure use a compressor with an auto setting.
5. Knee: Causes the effects of compression to be applied gradually.
6. Makeup gain/output: Once gain reduction is applied you will need to compensate for the loss in level. As a general rule of thumb you will have to increase level by the same amount of gain reduction you have applied.
   

How to judge your compression:
You will need to reduce the dynamic range to make an instrument or sound sit in a 'mix' of other sounds. Generally the larger the dynamic range (the difference between the quietest and loudest signal) the more compression needed in a modern mix.

When setting your threshold decide how much of the signal needs compression applied to it. For example with overheads the cymbals may be louder in the signal then the rest of the kit, so you may just want to bring down the volume of the cymbals and keep the rest of the sounds dynamics intact. So you would set the threshold to affect the loudest part of the signal, in this case the cymbals.

Once you have chosen what to compress you need to decide how hard to compress it, this is where the ratio comes into play. Start with 3.1 and adjust until the dynamic changes sit with everything else, generally the more signal being compressed the lower the ratio. If you find that your compressed instruments lack definition or impact or you wish to keep the attack of your notes intact increase your attack settings to suit.

To avoid pumping effects adjust the release to match the average note length in the program material where the notes lengths vary (e.g. vocals often have complex dynamics) try an auto release or set an average. If the effects of the compression are too obvious and out of context with the track adjust the knee, think of the knee as a smooth tool.

Finally apply make up gain to adhere to amount of gain reduction achieved. Also remember different compressors have different sounds. Choose the correct compressors for the correct sound.

As you learn more and more about compression and how it sounds you will develop your ear and know what kind of settings work faster. There are some useful techniques with compressors that you can then start to look at. These include side chain compression, ducking, multiband compression, invisible compression and upward expansion.