What is Compression?Compression is used to alter the dynamics (contrast between loud and quiet) of a sound. Downward compression in it’s simplest form makes the louder parts of a signal quieter. This levels out the signal and can give a more balanced result.
Why would I need to use compression?
Compression has a number of uses, the first being that if the level of a particular sound fluctuates too much during a song (e.g a vocal that is too loud in some bits while too quiet in others). Compression can be used here so that the vocal can be heard clearly in the softer bits while not being too overpowering in the louder sections (helping it ‘sit’ in the mix).
Compression can also be used to shape a sound in a much more obvious way such as using a slow attack time on a snare to emphasise the transient (essentially making it sound more ‘snappy’) Alternatively it can also be used to soften drum hits (with a fast attack time).
There are a few different controls that change how a compressor affects the signal. The ‘threshold’ controls how much of the signal will actually be compressed. In downwards compression when the volume goes over this level the signal will be reduced in volume. (The lower the threshold the more the sound will be compressed.)
‘Ratio’ determines how much the volume will be reduced by, it is given in the format 1:1, 4:1, 10:1 etc, if the ratio is 2:1 then for every 2dB over the threshold the input is the output will be 1dB over the threshold. (the sound has been reduced by 1dB)
The ‘attack’ time determines how soon the volume is lowered after the threshold is crossed. The release time is how long it takes for the signal to stop begin compressed after it is no longer above the threshold.
A good place to start with when applying compression to a signal is the ratio / threshold balance. A low ratio (e.g 2:1) is useful for adding a subtle amount of compression over a sound to help ‘thicken’ it. Stronger ratios (e.g 5:1, 8:1) are useful for more dynamic audio (e.g drum tracks) that needs to be tamed.
When setting threshold it can be seen as how much compression is being added. If you hear compression in an obvious way (usually called ‘pumping’) then it is probably being overdone. The idea is to get close to the point of it being audible and then back it off to avoid over compression.
A trick when setting attack / release is to bring the threshold right down to the point where the signal is clearly being over compressed. At this point it makes it much more obvious how the attack / release is affecting the sound, once these are set the threshold can be brought back up to a normal level.
Types of compression
There are a number of different compressors, each with their own unique flavour. Some tube based hardware compressors ‘colour’ the signal which can be useful in making things feel ‘bigger’. Other types such as optical compressors are designed to be ‘transparent’ in that they don’t change the sound as much. This can be useful for things such as getting the overall level of a vocal track to sit in the mix.
Even though it is hardware technologies that make outboard compressors sound so ‘musical’ there are many software based compressors that emulate these imperfections to achieve similar sounding results.
Tricks for using compression in a mix
- As well as using compression on individual elements (e.g snare, kick etc) it can be used on a buss to help ‘glue’ the mix.
- ‘Parallel compression’ is the process of duplicating a channel, adding a compressor with a strong ratio and layering it just under the original sound. This can help ‘thicken’ a sound without losing the ‘punch’. Be careful with this as adding the parallel compressed signal will bring up the volume so you need to be careful that parallel compressing is actually making things sound better and not just louder!
- As mentioned when it comes to drum hits a slow attack time can help bring out the transient of a sound. With a faster attack time on things such as hi-hats and cymbals it can help bring out the ‘tail’ of the sound which can be useful if things are sounding a little too ‘clicky’ or harsh.
- If some sounds are particularly dynamic it may be an idea to use more than one compressor adding a little compression each to give a more natural sound. (rather than using one compressor doing a lot)
Compression although tricky at first is an incredibly powerful and versatile tool that when mastered will give any mix a professional and balanced sound. Make sure to experiment with different types of compression and settings to get a feel for what they can add to your mixes and remember not to overdo it!