Compressors can be confusing little buggers. When I was a recording novice, I was often baffled by them and was never really sure if they were doing anything. Even now, with more experienced ears, it’s still sometimes hard to tell exactly whether a compressor is adding anything useful to a track.
A compressor’s effect on a track can be subtle—and that Makeup Gain knob doesn’t help, since we usually perceive “louder” as “better.” I’ve often discovered, after thinking I’d improved my audio by adding a compressor, that all I’d done was make it louder. A few years ago I learned this handy trick for setting compressors: go way too far with the compressor’s settings so you can really tell what you’re doing, and then back them off:
- To start, set the Threshold at its maximum. On most compressors, this is all the way down, since you’re lowering the Threshold so it affects more of the signal. On some compressors however, like the Fairchild 670, the Threshold control works the opposite way—turning it up reduces the threshold. And a few compressors don’t have a Threshold control at all. On these models, you need to raise the Input Gain (or Peak Reduction, depending on the unit) to control how much compression you’re getting. But whatever the operable knob is, turn it all the way up so you can really hear the compressor working.
- Now, set the Ratio control at its maximum. Some compressors (like the 670 mentioned above) don’t have a Ratio knob. In this case you’re stuck with a predefined ratio. You may need to adjust the Gain (or Output Level) to compensate for any decrease in volume.
- By now, the compressor should be crushing your signal pretty heavily. This allows you to hear exactly what the all-important Attack and Release controls are doing to the sound. Play with the two knobs:
- Fast Attack and Release settings will clamp down on the transient—the initial part of the sound—and make the tail sound louder. On drums, this will make the ambience louder and emphasize the room sound.
- Increasing the Attack lets some of the transient through and can help accentuate the attack of drums, pianos and guitars.
- A slower Release with a medium Attack reduces the level of the ambience even more and can make drums sound drier and more in-your-face.
For more details on compressor basics, Barry Rudolph had an excellent article in Mix magazine called Understanding Audio Compressors and Audio Compression. Sound On Sound magazine also published a couple of great articles: Compressors and Compressors Made Easy. If you don’t have at least a Web subscription to Sound On Sound, I highly recommend getting one. Finally, AudioTuts+ has a series of posts about compression.