In a nutshell, the job of an audio compressor is to smooth out volume levels, making them more consistent. In a vocal performance by a singer, there might be really quiet breathy moments at the beginning and end of words and phrases, with big open vowels sung in between. Compression can help the listener hear the detail and nuance in the quiet parts without being overwhelmed by the loud parts.
Or while recording something like drums or a podcast recording where people might suddenly burst out laughing, a compressor can help ‘tame’ the loudest moments, keeping things from distorting and surpassing what the mic can pick up cleanly.
In any case, the first job of a compressor is to reduce loud sounds. Once those loud sounds are reduced, the whole recording is going to sound at least a little quieter. So depending on how much quieter things sound, the last part of a compressor’s job is to compensate for that by making everything a little louder once more, back to whatever ideal listening level. That final stage is typically referred to as ‘makeup gain’.
The idea of makeup gain is sometimes confusing to people because it sounds like you’re reducing loudness just to add the same amount of loudness right back, so why bother? But to give it a goofy analogy let’s say you’re 6’4” and we need you to be under 6’ to get through a doorway. So you crouch down and now you’re 5’1”. But you’re not feeling like your old self because you’re so much shorter and bent over and all. So we give you a nice pair of 10” stilts to walk on. So now you’re still bent over and ‘compressed’ down to 5’1”. But on a pair of ‘makeup gain’ stilts. And still coming in comfortably under the overall level we needed you at. More or less confused..?
Too much compression can make things sound flat and lifeless, losing the natural dynamics that give a recording life and character. But without any compression, it may be difficult to hear the quietest parts of the performance, especially in context of a full mix.
Compression is often used on multiple levels of production. Sometimes at the recording stage, usually on individual tracks in mixing, on sub groups, and at the mastering stage. Just like EQ, compression is at the same time one of the most basic and most varied tools in audio. We’ll elaborate on different kinds of compressors, and the various parameters you might find on them, in another post.
For now, we hope to have shed some light on the general concept and application of compression.