Joined: 02 May 2011
Location: London UK
|Posted: Mon May 02, 2011 6:15 am Post subject: How to use compression
| How to use compression - compression can be a little daunting for those who are new to audio production and with 1,000's of audio forum threads there can be a pressure to use it because everyone and their dog is. This is a basic tutorial that will help you understand why it is used and some basic advice on how to practice much needed hand to ear co-ordination.
Audio compressors can be analogue or digital and you probably already have a software compressor built into your digital audio workstation such as Logic, Cubase, Fruity loops, Reason, Ableton Live, Reaper, Adobe Audition or Pro Tools. If you wish you can purchase an analogue compressor for practicing too. A reasonable analogue compressor can be purchased for $130.00 and this will be perfect for practicing the techniques that will be explained within the body of this article. I suggest a few compressors you could try out, this is not an endorsement but just a few ideas for budget devices which will be suitable for practicing with. A Behringer Composer Pro XL MDX2600, Samson S-Com Plus, Alesis 3630 all have the controls which will be discussed.
Audio compression can be a confusing theoretical and practical audio process. The reason is that when learning to use a compressor the results of applying compression change the sound in real time. It requires a certain level of listening experience to be able to hear the changes as they are often quite subtle. As such it is best to use fairly extreme settings when learning how to use a compressor. The processing results will then be more obvious and easier to hear. We start with the basic controls on a compressor.
Threshold - control which determines at what level compression will start.
Ratio - How much compression will occur when threshold is exceeded.
Attack - How quick compression will start when threshold exceeded.
Release - How quick the compression will stop once the audio has dropped below the threshold. (dependent on detection circuit)
Make up gain - output level trim. Increases overall output level of audio signal post (after)compression.
Gain reduction - this is not a control knob, but many compressors have a meter which show how much gain reduction (compression) is being applied in dB (decibels). When learning it is a useful meter to see.
Using a compressor and learning all the different ones there are is an art unto itself. However this does not mean it is necessarily complicated to use a single device. The reason compression appears complex is mainly because there is an interplay between the controls on the units themselves and the required goal. Firstly we will ask ourselves why do we want compress sounds? A goal needs to be determined as this makes for a solid reason to employ a compressor. (as opposed to using one because it is the "done thing" or because everyone on audio forums are using compressors)
Compressors can be used for a number of reasons, compressors can even out the differences between the levels of snare hits for example. This might make mixing easier. If you want your snare drum to be more even in level you may compress it, it is a practical reason. You could not sit there pulling the fader down for every snare drum hit. Even if you automated this it would take a very long time.
Compressors can impose their envelope (time constants of attack and release) on a sound and adjust the attack and release of a sound. For example a snare drum. You can increase the attack and adjust the "body" of the snare depending on where the release time is set. So compressors can be used to tonally shape sounds.Compressors can smoothen out sounds by reducing the transient attack detail in an instrument.On a complete music mix a high end compressor can produce a sense of "glue" or "gelling" of the sounds which may previously sound a little disparate or disconnected.
Whilst compression is not particularly complicated it will take time to practice and perserverance to get to grips with. Bear this in mind when you are practicing with compressors.
Compressors control dynamic range, in short they make louder peaks in a sound source quieter, that is it the easy explanation. (like a fader or volume knob being brought down when a loud sound is detected). How they make loud sounds quieter is dependent on some of the controls. The threshold determines when a compressor starts to reduce the level of a "peak" or loud sound, usually expressed in milliseconds, the shorter the attack the quicker compression starts. Ratio is the "amount" of compression, higher ratios mean more compression is applied once the threshold is exceeded. Ratio is an expression of input level verses output level, i.e. a ratio of 4:1 means for every 4dB the threshold is exceeded the output level will rise by only 1dB. Release is how quick the compressors gain reducing action is released once the level has dropped below the threshold again. Remember, compression reduces peaks in a sound so a make up gain is there to increase the overall output level. The result of "make up gain" is that softer sounds below the threshold are lifted in level and the peaks that are reduced by the compression action are lifted to where they were before compression started. The dynamic range of the material is thus reduced. This is quite an important concept to grasp so re-read if required.
The skill involved in using a compressor is to learn the basic controls by practicing on a known sound source like a vocal performance or snare drum.We will focus on a snare drum sample, this is an easy sound source to hear the changes on. The key to success is very careful and deep listening. It takes quite a long time to learn what the compressor is doing. So adjust the controls slowly with an ear on the sound and do not expect to learn this in 1 day. If you practice a little every day for a week you may start to hear the subtle changes to sound sources when you apply compression. I suggest initially practicing with a sole sound source and stick to using the same compressor then expanding to other sounds. Set your compressor to "Peak" as opposed to "RMS" if it has these buttons.(more on this later)
cont. in part 2
Last edited by SafeandSound Mastering on Mon May 02, 2011 6:16 am; edited 1 time in total