Understanding Audio CompressionNovember 17, 2011
Compression second only to audio equalizers is the most commonly used processing effect used during
recording, mixing and making a song. It also remains one of the most
commonly misunderstood processes for producers as a whole. Why is
this you may ask? Well there are many factors to take into
consideration, but I will try to demystify the subject of compression
for you. Do not let the fact that many producers and online beat makers out
there don't really understand compression scare you off. The key to
understanding is knowing the basics first before you just dive in. What is music mixing without compression and effects? If you learn the basics of a compressor your tracks will surely improve as a result.
WHAT IS COMPRESSION?
A compressor is a special amplifier that is used to reduce dynamic range. Dynamic range is the span between the softest sounds and loudest sounds in a song.
Did you know that each of us have a built in compressor in our ears? It is true. In fact when an incoming sound is too loud there is a muscle that contracts to help reduce the sound before reaching the inner ear. This human compressor allows us to hear a quiet whisper and a loud shout equally to some degree. It really is pretty amazing when you think about it. Well an audio compressor's goal is the same. The goal is to give the listener a more consistent sound level by boosting a lower volume sound and reducing a sound that is louder.
USING A COMPRESSOR
Most audio compressors are basically designed to do the same job. Sure there are different makes and models. Different designs and interfaces. They also don't all sound or work exactly alike, but the jobs of the compressor are the same. Yes jobs! In this economy we are lucky to have one job but the trusty compressor actually has four. So an easy way to understand compression is to understand these four jobs that are being done within the compressor. The tricky part about using a compressor is that each job greatly affects the next job in line. If you get one wrong then the next one will surely be wrong also. Think of it like an old fashion assembly line at a factory. So let's take a closer look at these jobs:
FOUR MAIN WORKERS AT BIG COMPRESSOR INC.
TOM THRESHOLD – His job is to set up the work project.
ROSA RATIO – She is the accountant. She decides how much is to be done.
ANDY ATTACK – Andy's job is to tell everyone when to start working.
RICK RELEASE – His job is to tell everyone when to stop working.
THRESHOLD – Tom Threshold starts his day early before the others get to work. This is because he sets up the job to be done. See the threshold is simply the audio level at which the compressor will begin to work. Any audio that has not reached this level will not be affect by the compressor at all. Any audio that is at this level or above the level will be compressed. It is important to get the threshold correct because if set incorrectly you may end up with too much compression or not enough. If you are new to compression a good rule of thumb is to use a compressor that will show you your gain reduction visually. Compressors with a light or a meter showing the amount of reduction taking place make it a lot easier to find the proper threshold that you will need for your track.
RATIO - Ms. Ratio's job is to figure out how much compression will happen once the audio has gone over the threshold. Since she is an accountant the amount is expressed using mathematics in the form of a ratio. So a ratio of 4:1 for example means that if the input level is 4db over Tom's threshold, the output level will be 1 dB over that threshold. This means that all the audio that is compressed will be reduced 4 times with a ¼ of the audio coming out of the compressor.
ATTACK - Andy Attack is a clock watcher. That is all he does all day is watch the clock. But this is his only job and it is one that he does very well. See the attack is based on time. It is this amount of time that the compressor will go from no compression to full compression. The full amount of compression was decided by the ratio earlier. The attack time is almost always based on milliseconds. The smaller the number the less time and the larger the number the more time.
RELEASE – Rick is also a big fan of time. The difference between Andy Attack and Rick is that he is only concerned with quitting time. See Rick only wants to know when it is time to stop working. This is because the job of the release is just the opposite job of the attack. The release function is the amount of time that it takes to go from a full compression back down to no compression at all.
This may seem like an elementary way to learn compression, but it will help you to learn the basic functions of a compressor and which to use first. Keep in mind you will find plenty of compressors with a lot more settings than the basic four here. Some may have a makeup gain, a knee, side chaining effects, ect.. Before using all the other goodies that are out there when it comes to audio compression it is a good idea to master the basic four. Find a compressor that you really like using and try it on different types of sounds. Drums are a great starting point when it comes to compression because it's effects are easily heard. Once you have mastered the drums then you could move on to other instruments and then finally vocals. It takes a bit of time to train your ears to pick up on the use of compression, but with a solid foundation of the basics you will be an expert in no time.
written by Alex Butler
Alex is an audio engineer, studio producer and freelance writer based out of Seattle, WA.