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Audio Compressor and how it works by Gwydion Elderwyn
Say you're sitting at a mixing desk where you've got different instruments
on different channels. At some part in the track, you notice that the
saxophone gets too loud, so you move the volume fader down. Shortly
afterwards, the sax returns to a more normal level, but now it's too quiet
with the fader down, so you move it back up. In essence, that's what a
compressor does, except of course that it can react far faster than a human
Compressors generally have four controls - threshold, ratio, attack and
release. Many also have a 'gain' control, and some have a 'knee' control as
well.
The "threshold" is the volume at which the compressor starts to work. If
the incoming sound level is below the threshold, the sound passes through
unchanged. It's usually measured in decibels.
The "ratio" is how much compression is applied to the signal above the
threshold. compression of 2:1 would make the signal *over the threshold* half as loud.
The "attack" is how fast the compressor applies the compression. It's
measured in milliseconds. In order to know what attack to use, you need to
consider the nature of the sound. If it's a drum sound, which has a very
sharp beginning followed by a more mellow sound, you don't want to set the
attack too low or you'll lose all the punch of the sound (try it on a snare
drum with a high ratio and set the attack to zero, you'll hear what I mean).
The "release" is how long the compressor waits after the incoming sound
returns to below the threshold level before stopping compression. You might
think you'd want to set it to a very low value, but you'll find if you set
it too low then the sound will start to "pump" or "breathe" where you can
hear the sound level going up and down very quickly. It's hard to describe
but it's a characteristic sound that tells you your compressor settings are
wrong.
Now, as I said, some compressors also have a "gain" and "knee" control.
Firstly, gain. Obviously, if I'm compressing my sound, it's going to get
quieter! What the gain does is increase the volume of the outgoing sound.
The net result of this is that the quiet sounds get louder. That's why it's
a common misconception that compressors are for making soft stuff louder -
they do, but that's not their purpose.
By using gain and threshold, you can do some interesting things. For
example, say you've recorded a guitar part but you're not a great guitar
player, so the volumes of the notes are not consistent (some loud, some
soft). If you set your compressor with a low threshold (when I say low, I
mean minus a lot of db, say -40db), a moderate compression level (try 6:1)
and a high gain, the end result will be that the note volumes will seem much
more consistent. Try it!
Finally, knee. Just like a human knee can bend, so can the response of a
compressor. A compressor without a knee control is generally using a "hard
knee" by default, and it responds as I've described above. A "soft knee"
setting, however, applies the compression relative to just how far over the
threshold the signal is. For example, if it's just over the threshold, the
compressor will not apply the full ratio, only part of it. At some point
over the threshold, the full ratio is applied. In between, it depends how
far over it is. Make sense?
That's basically all there is to it. As to the questions of when you use
compression, that's very much a creative decision. Some people don't use
them at all, others compress every single track and then apply compression
to the overall mix as well.
One last note: you've probably also heard of a "limiter". Whereas a
compressor will reduce the volume of a track when it goes beyond threshold,
hopefully you can see that the resulting volume coming out the other end can
still be louder than the threshold value. How much louder depends on the
ratio, of course.
A limiter, on the other hand, enforces the threshold absolutely. The sound
coming out absolutely does not go above the threshold value - that is, the
volume is "limited" to the threshold. Limiters are often called "brickwall
limiters", hopefully you now understand why.
Can a compressor act as a limiter? Some can. If your compressor's ratio
has a setting called "INF" (infinite), then yes, it can. That's right - a
limiter is basically infinite compression You can get some truly weird
(and generally, bloody awful) results by using infinite compression with
high attack and/or low release values. Try it if you dare
My advice for you to get to learn compression is just to use a track with a
single instrument. Put a compressor on that instrument, and then work with
the settings as above. Get a feel for what you can do to the sound. Listen
to how the dynamics of the sound change if you apply a lot of compression
(ie high ratio, low threshold). Get a drum track and apply high compression
with a very short attack and release, and see if you can hear the "pumping"
as the compressor wildly raises and lowers the volume. Try increasing the
attack value so you can hear how the dynamics of the initial hits changes
(zero attack = no punch at the start).
And so on, and so on. You can read all the books, articles and emails you
like, but the truth is that the best teacher is sitting there just twiddling
the knobs.
Hope that all helps.
Say you're sitting at a mixing desk where you've got different instruments
on different channels. At some part in the track, you notice that the
saxophone gets too loud, so you move the volume fader down. Shortly
afterwards, the sax returns to a more normal level, but now it's too quiet
with the fader down, so you move it back up. In essence, that's what a
compressor does, except of course that it can react far faster than a human
Compressors generally have four controls - threshold, ratio, attack and
release. Many also have a 'gain' control, and some have a 'knee' control as
well.
The "threshold" is the volume at which the compressor starts to work. If
the incoming sound level is below the threshold, the sound passes through
unchanged. It's usually measured in decibels.
The "ratio" is how much compression is applied to the signal above the
threshold. compression of 2:1 would make the signal *over the threshold* half as loud.
The "attack" is how fast the compressor applies the compression. It's
measured in milliseconds. In order to know what attack to use, you need to
consider the nature of the sound. If it's a drum sound, which has a very
sharp beginning followed by a more mellow sound, you don't want to set the
attack too low or you'll lose all the punch of the sound (try it on a snare
drum with a high ratio and set the attack to zero, you'll hear what I mean).
The "release" is how long the compressor waits after the incoming sound
returns to below the threshold level before stopping compression. You might
think you'd want to set it to a very low value, but you'll find if you set
it too low then the sound will start to "pump" or "breathe" where you can
hear the sound level going up and down very quickly. It's hard to describe
but it's a characteristic sound that tells you your compressor settings are
wrong.
Now, as I said, some compressors also have a "gain" and "knee" control.
Firstly, gain. Obviously, if I'm compressing my sound, it's going to get
quieter! What the gain does is increase the volume of the outgoing sound.
The net result of this is that the quiet sounds get louder. That's why it's
a common misconception that compressors are for making soft stuff louder -
they do, but that's not their purpose.
By using gain and threshold, you can do some interesting things. For
example, say you've recorded a guitar part but you're not a great guitar
player, so the volumes of the notes are not consistent (some loud, some
soft). If you set your compressor with a low threshold (when I say low, I
mean minus a lot of db, say -40db), a moderate compression level (try 6:1)
and a high gain, the end result will be that the note volumes will seem much
more consistent. Try it!
Finally, knee. Just like a human knee can bend, so can the response of a
compressor. A compressor without a knee control is generally using a "hard
knee" by default, and it responds as I've described above. A "soft knee"
setting, however, applies the compression relative to just how far over the
threshold the signal is. For example, if it's just over the threshold, the
compressor will not apply the full ratio, only part of it. At some point
over the threshold, the full ratio is applied. In between, it depends how
far over it is. Make sense?
That's basically all there is to it. As to the questions of when you use
compression, that's very much a creative decision. Some people don't use
them at all, others compress every single track and then apply compression
to the overall mix as well.
One last note: you've probably also heard of a "limiter". Whereas a
compressor will reduce the volume of a track when it goes beyond threshold,
hopefully you can see that the resulting volume coming out the other end can
still be louder than the threshold value. How much louder depends on the
ratio, of course.
A limiter, on the other hand, enforces the threshold absolutely. The sound
coming out absolutely does not go above the threshold value - that is, the
volume is "limited" to the threshold. Limiters are often called "brickwall
limiters", hopefully you now understand why.
Can a compressor act as a limiter? Some can. If your compressor's ratio
has a setting called "INF" (infinite), then yes, it can. That's right - a
limiter is basically infinite compression
You can get some truly weird(and generally, bloody awful) results by using infinite compression with
high attack and/or low release values. Try it if you dare
My advice for you to get to learn compression is just to use a track with a
single instrument. Put a compressor on that instrument, and then work with
the settings as above. Get a feel for what you can do to the sound. Listen
to how the dynamics of the sound change if you apply a lot of compression
(ie high ratio, low threshold). Get a drum track and apply high compression
with a very short attack and release, and see if you can hear the "pumping"
as the compressor wildly raises and lowers the volume. Try increasing the
attack value so you can hear how the dynamics of the initial hits changes
(zero attack = no punch at the start).
And so on, and so on. You can read all the books, articles and emails you
like, but the truth is that the best teacher is sitting there just twiddling
the knobs.
Hope that all helps.
