Dynamic range compression is an essential tool for mixing music. Whether you're working on a personal computer or in a million dollar recording studio, using an audio compressor can help you craft mixes that sound more balanced and professional.The function of a compressor is to compress the dynamic range of the affected audio. In other words, it makes the difference between the loudest and softest sounds less drastic. This results in a more even sound with a volume that fluctuates less. This is useful in mixing, because it ensures that the different instruments remain at the volumes in which they can be heard clearly and stay in balance with each other.
A compressor can be a software plugin or a hardware unit. If you just beginning to learn about compression, it is likely that you will be using a software compressor. Nearly every digital audio workstation (DAW) has their own compressor built into the program. To begin using it, you must understand the different parameters and functions of a compressor and be able to locate the controls on the specific compressor you are using. While every compressor is different, most of them will have the same basic parameters. Two of the most important of these are the threshold and the ratio. The threshold designates the volume at which the compressor effects the audio running through it. If the threshold is set at -7dB (decibels), every sound with an amplitude louder than -7dB will have compression applied to it, and no sound below -7dB will be compressed. The ratio determines the amount of volume reduction that will be applied to sound surpassing the threshold. If a compressor is set to have a 2:1 ratio, it means that every 2 decibels surpassing the threshold will be reduced to 1. If it is set to 8:1, every 8 decibels of sound surpassing the threshold will be reduced to 1. When a sound with a volume of -3dB enters a compressor with -7dB threshold and a 4:1 ratio, it is compressed to -6db. This is because the original sound is 4dB louder than the threshold, but the 4:1 ratio designates that only 1dB of sound is allowed to leave the compressor for every 4dB that enters. If the original had a sound of 1dB (8dB over the threshold), it would be reduced to -5db (2dB over the threshold).
The other parameters you are likely to find on any given compressor are output gain, attack, and release. The output gain, which might also be referred to as make up gain, does not affect the compression but increases or decreases the volume of the audio after the compression is applied. The attack controls how long it takes for the compression to start after the threshold is passed. It is measured in milliseconds. By raising the attack time on a compressor, you can allow the beginning of the sound to be uncompressed. This is useful when compressing vocals because it allows the consonant sound to circumvent compression, bring clarity and definition to lyrics, without sacrificing the benefits of compressing the rest of the vocals. The release determines how long it takes for the compression to stop after the volume of the sound passes under the threshold.
While there are countless varieties of compressors with more capabilities than explained above, these are the fundamental parameters available on most compressors. A wide variety of different effects can be achieved by changing the configuration of only these. This guide should serve you well in understanding the fundamentals of dynamic range compression, but only you can decide how to use it. With experimentation, practice and research you can learn to apply compression to best suit your musical needs.