What Does A Compressor Do?

Compression effects the audio content’s dynamic range. What do I mean buy dynamic range…well it’s the distance between the “louds and “softs”. A compressor changes that ratio. Lets say you have a bass track where the playing is very uneven- LOUD soft LOUD soft…etc. A compressor could be used to to help even up the track. (At this point I would be remiss if I did not mention EQ would be the first tool to try in this situation…but since I’m talking about compression I won’t mention anymore other than that both compression and EQ would probably be wise in such a case.)

The main controls on most compressors are: Ratio, Attack, Release, Threshold, Input, and Output.

First the Ratio. It controls exactly what the name suggests…the ratio of compression. A ratio of 1:1 is equal and therefore no change. Higher ratios such as 10:1 effect significatively more dynamic reduction. The greater you increase the ratio, the greater the chance for audible artifacts…sometimes desirable, sometimes not. A good starting point to when you start using compression is 3:1

Next, the Attack and Release. Attack is nothing more than how quickly or slowly the compressor starts working or “grabs the note.” Release is the same thing except on the other end. It is how fast/slow the compressor “gets off the note.” What does that mean practically? Lets say we have a singer who blasts the front of each word much more than the middle or end of note. In that case you might want to set the attack as fast as possible to “knock down” the first of his words. Conversely, it may be desirable for a more even singer to have a slower attack to keep the “bite.” Release is the same. If the note “swells” too much set the release longer as to get all the note. If the release is set too quick than the note could “swell” back up. The thing to remember is their is no hard and fast rule…use your ears and your mind. Listen to the track and analyze it…ask your self: “What does it need?”- Then make it happen.

Now comes the Threshold. To me the most important controls on a compressor are the ratio and threshold. Before I tell you what the threshold does, I want to take a moment to tell you how it is measured (on most plug-ins). Ratio is measured in uh…ratio!!! (i.e. 1:3), Attack and Release are measured in milliseconds (most of the time), and threshold is measured in dBs (decibels). The threshold scale is goes from the no/softest sound (i.e. ∞ -x) up to zero (digitally) Note: the analog compressors sometimes measure above zero So an example of an average threshold of a vocal track recorded at a reasonable level might be -15dB.

Now that you know how the Threshold is measured it’s time to understand it’s use. Ratio determines how “extreme” the compression is while the Threshold determines how much of the dynamic range is processed. Example time! – You have a track that’s top peak is -10dB. If you set your ration 1:10 and the threshold at -5db…nothing will be compressed because the threshold is above peak of the track. Take the same example and set the threshold to -15db…now 5db of the track is “touched” by the compressor. If you adjust the ratio control, it will only effect that 5dB difference between -10dB and -15dB. Say you want more of the overall track compressed lightly- set the threshold to -25db and the ratio to 2:1

Finally the Input and Output. On a standard compressor their only job is gain staging. If the signal is too weak coming into the compressor, turn up the input. When the audio is processed by the compressor it suppresses xdB of compression so the peak of your track is lower so you use make-up gain or the Output to “make-up” for what is compressed down.

I’ve talked about the controls a modern standard style compressor but every unit or plugin isn’t exactly the same but the above describes the basic essence of what they do. An example of a vintage compressor (MY FAVORITE !!!) is the UA1176ln. It has 4 preset ratio switches instead of a variable knob. Also it’s threshold, input, and output are combined into two knobs (two knobs but lots of tonal variety).

Now that you understand more about compression, don’t be afraid of it. It can “make” a recording or…uh..break it if overly used. So use your new tool with responsibility. Enjoy !!!