The attack setting adjusts how quickly the compressor reacts to signal that exceeds the threshold you've set for compression.

The decay is the amount of time it takes the compressor to return to a null state after the signal has exceeded the threshold or 'how long it holds on and continues compressing'.

Basically the compressor will not do anything to signal level below the threshold and will reduce the output by the ratio you set, e.g. 3:1, above the threshold.

So if you set a threshold of -10dB with a ratio of 3:1 then for every 3 dB the signal exceeds the threshold you will get 1db of extra gain.

Attack adjusts how quickly this compression comes into play and, if set longer, will ignore transients that are shorter than the setting you choose.

Short attack and decay settings can result in 'pumping' which is particularly noticeable if there is an audible background noise on the track you are compressing.

Longer decay times can result in an 'AGC' effect typical of cheap recorders where loud signals cause a reduction in gain that takes some time to return to normal level.

There is no hard and fast rule for these settings it requires adjusting, listening and adjusting again until the signal sounds the way you want it.

A good place to start though is one of the presets that best describes what you are trying to achieve have a look at the attack and decay settings then adjust the parameters until you achieve your required results.

Regards
Stephen Hope
New Hope Media