Home Recording: The Great Compressor Debate

Stephen Wise is your guide to home recording.

Another workhorse module in the recording studio is the compressor. It can be the most essential tool or the worst evil incarnate, depending on your point of view. Debates over compression can take on a religious fervor.

“Purists” despise compressors and compression may be totally absent from classical and other acoustic recordings. Today’s pop music, on the other hand, employs multiple levels of compression. If it sounds incredibly loud, then it’s probably compressed.

The compressor has an extra level of complexity above that of the effects that we’ve used thus far. It’s a little harder to set up and sometimes you just can’t get it quite right. Compressors also introduce audible artifacts that the trained ear can identify. A compressor has two main components: a variable amplifier and a level detector.

Before compressors, humans did the compressing! You’ve probably done it yourself. You listen to some music. When you hit a passage that is a bit too loud, you turn down the volume. When the loud section has passed, you turn the volume back up again. A compressor does exactly the same thing – automatically! A compressor can also work quickly and tirelessly, never missing anything.

The Audacity® compressor can be found under the “Effect” menu. Select “Compressor…” A basic compressor has four controls that let you determine how the compressor will behave: Threshold, Compression Ratio, Attack Time, and Release Time. For reasons unknown, the basic Audacity® compressor is missing a release time setting – a rather unfortunate situation.

“Threshold” lets you set the loudness at which the compressor will start to turn down the volume. Compression Ratio tells the compressor how much you will be turning down the volume. A ration of 2:1 (two to one) means that the compressor will be turning the volume down to one half of what it should be. Some compressors will let you set the ratio all of the way to “infinity”. At this point, you have a device known as a “peak limiter.” Attack Time tells the compressor how quickly to turn down the volume. Attack Time is very important and usually takes some tweaking before you get a smooth transition. Release Time, if you have it, is usually set longer than Attack Time. You want the volume to come back up slowly in classic compression.

Now you have it. What can you do with it? Suppose that you have a recording that has occasional loud sections. If you set the volume to a comfortable level, then the loud sections really hurt or cause distortion. You can set the volume lower, but then the quiet sections are too quiet. If you’re listening in your car, then you can’t hear the quiet sections over the road noise. The compressor can fix it at the expense of giving up some of the natural dynamics of the music. Everything that you hear on the radio is compressed and peak limited to keep the sound from getting lost in the static and to keep you from moving on to a “clearer” station.

If you compress a track or a mix, then boost it up to to maximum level, it sounds louder! That’s why commercials sound so loud even though the stations insist that they’re not any louder than the programming. The speaking voice can be compressed a lot. When the parts that should be soft are loud too, it just sounds loud, plus, you can hear every word over the noises in the kitchen! The “Normalize” check box on the Audacity® compressor does it for you automatically. The compressor is the main weapon in the “loudness wars”.

You may have noticed that today’s music sounds a lot louder than your old albums from the ’60s and ’70s. In fact, the digital remixes of those old albums sound louder too. That’s because today, “louder” means “better.” If an mp3 can only hold so many bits, make them all loud ones! Compress each track, then compress the final mix – more than once if necessary!

Although I dislike this approach to remixing old albums (it changes the character in a fundamental way), I have no objection to using compression as a creative tool. It’s a part of today’s sound just as “slapback echo” was a part of the Rockabilly sound.

Compressors all introduce a “pumping effect” into the sound. “Pumping” is descriptive of the effect of the volume being turned up and down. Play some music while rapidly turning the volume control up and down and you will hear the effect. Normally, you want to hide pumping as much as possible, but it can also be used as a creative effect. As an example, if you set up the compressor just so, you can get everything to “pulsate” with the drum beats, adding extra “drive” to the sound.

Since I have completed this blog, Leah has inadvertently done me a great service by posting the article “It’s Not That You’re Older, the Music Really is Louder,” saving me a lot of wind. Here is another great link on the subject: “What Happened to Dynamic Range?”

Stephen Wise has been designing electronic musical instruments since 1975. Steve specializes in realistic recreations of traditional instruments. He became interested in the field after hearing Walter/Wendy Carlos’ “Switched On Bach” and upon being introduced to the Allen Digital Computer Organ, the world’s first all digital musical instrument. Steve is currently designing instruments for Schulmerich Bells, maker of handbells, electronic carillons, and the breakout MelodyWave® instrument.