How Does A Compressor Make An Audio Track Louder?
A compressor is probably the most misunderstood of all tools in
the recording studio. Ironically, it's also one of the most
powerful tools when recording or mixing. While there are many
aspects of a compressor that could be written about, I'm going
to explain how a compressor can make an audio track louder.
So you want to learn how to use a compressor? Well good luck. It
takes years to get even a decent feel for a compressor. I'm just
now getting where I feel that a compressor will tolerate me
playing with it's settings. In the past, it was just laughing
and mocking me because I just didn't understand how to use it to
improve my recordings.
So let's talk about how a compressor can make an audio tracker
louder. Ironically, a compressor actually knocks the volume down
on a track, but then has a makeup gain knob that boosts it back
up. To understand how a compressor can make something louder,
you need to understand the difference between peak loudness and
average loudness (also called RMS). A peak is just what it says
it is. It's a spike. The signal starts very low and goes very
high. A good example of peak loudness is a snare drum hit.
Average loudness is sound that occurs over time. Imagine hitting
a low E on a bass guitar and letting it sustain. This is an
almost constant sound.
One other concept is the volume ceiling. In other words, in
digital audio we have a volume limit. It's called zero. For
whatever reason they measure volume in negative numbers with 0dB
being the absolute loudest. If a track has a peak that jumps up
to zero, we can not push the volume up on that track even if the
other portions of the track are very low in volume (without
volume automation).
Now let's take an audio track that can be both peaky and
constant. A vocal track is a great example. A vocal can jump up
very quickly but it can also sustain. Let's say it hits 0db at
one point, but most of the track is sitting well below that.
You'll find that when the vocal is set at maximum gain before
clipping, the many of the words are unintelligible. This is
because they are simply too quite. Assuming there are no extreme
problems, the first thing I'll usually do is grab a compressor.
I'm go ahead and smash those peaks down and then I'll push the
volume back up with the make up gain on the compressor. Now the
vocal is evened up quite a bit. The vocal will sit in the track
much better and will sound fuller.
When mastering a record, compression is almost always used to
make the volume of the cd louder. Most of the time, the cd is
already hitting zero, so it's peak volume will not increase.
However, it's RMS or average volume can increase substantially.
When we put a compressor on stereo mix, we can smash the song
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down into a smaller dynamic range. It uses up less volume. While
this can be a bad thing as the dynamics are decreased, these
days overall volume seams to be more important (I'm not sure who
decided this). After the compressor does it's thing, the makeup
gain is used to boost the level of the track up the desired
amount.
When you are learning the audio mixing process, I recommend
using more compression than you think you need. Hit everything
very hard. If it sounds distorted, back off. I think that
compression is the opposite of reverb. While many beginning home
recording enthusiasts will use too much reverb, they often
times, do not use as much compression as the big boys.
Experiment. This is different for everyone.
In summary, a compressor is used to to knock off the top
(loudest parts) of an audio signal and then uses it's makeup
gain to push the volume back up. It takes lots of time to master
using a compressor. Keep in mind that you can do much more with
a compressor than make things louder. As always, don't be afraid
to experiment.
About the author:
Brandon Drury has written numerous articles for his recording studio
website.
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