XO Wave: How to Use
Compression is one of the most important effects available to audio
engineers. It is not uncommon for songs you hear on the radio to go
through 4 or 5 different compressors before you hear them: one while
recording, one or two while mixing, one while mastering, and another
at the radio station. Because of its ubiquity, it's important to
understand how compression works and how it can be used.
Compression works by reducing the volume of loud passages and
increasing the volume of quiet passages. This can be used to bring out
subtle details; help control a vocalist with bad mike technique; limit
distortion from loud, transient sounds; and give a track more
consistent levels, making it easier to mix. Compression can also
increase the average level of a track, effectively making it sound
louder. Just as importantly, compression is an integral part of how we
hear modern music: today's pop music sound simply would not be
what it is without compression.
As you read this tutorial, you may want to use XO Wave's compressor
to practice these techniques. We also have specific information on XO
Although there are many variations on the theme, most compressors
offer a few basic controls: threshold, ratio, attack, and release.
Don't be fooled into thinking more is better: while more controls can
offer you more flexibility, it can also take longer to find the right
settings, and there are a number of compressors out there with just
two or three controls that sound great.
One way to think of a compressor is as automatic volume adjustment.
The compressor works by making the loud passages quieter and the
quiet passages louder ("compressing" the dynamic range, so the
variation from quietest to loudest is decreased). The threshold
controls how loud the signal has to be before its volume is reduced.
Thus, setting the threshold high means that less compression will
occur, because less of the signal will exceed the threshold to activate
the compressor. Conversely, a low threshold means that the
compressor will do more, because more of the signal will exceed the
When the signal goes above the threshold, the gain of the signal is
reduced by an amount controlled by the ratio control. For example,
with the ratio set at 2:1, an increase in input of 2 dB above the
threshold will result in an increase in the output of only 1 dB. Higher
ratios yield more compression, but only to signals that pass above the
The attack and release controls work together with the threshold
control to determine when compression should begin and end. If the
signal is above the threshold for less time than the attack setting, then
little or no compression occurs. Once compression is activated, it stays
on for a length of time determined by the release control. So by
setting a long (slow) attack, you can let through the first part of a loud
sound. This is particularly useful because almost every note played on
an instrument contains a little burst of energy, called the "initial
transient". By setting the attack long enough to let the initial
transients through, you can prevent the compressor from squashing
the life out of the recording. So if your compressed tracks sound
lifeless, try a longer attack setting. On the other hand, setting the
attack too long will prevent the compressor from working.
A least half the time, you will be using compression to reduce the
dynamic range of your signal, which means that you will be reducing
the difference between the loud sections and the quiet sections. This
happens in two steps: first, the volume of each loud section is reduced
based on your threshold and ratio settings; and then the volume of the
whole track is raised based on the make-up gain settings. The net
effect is that the loud parts are still loud and the quiet parts are made
louder. Compression, therefore, can increase the average volume of
the track without increasing the maximum volume.
How to Tweak
Now that you understand the basics, we'll discuss other and more
specific uses in the next section, but let's start with reducing
dynamics. Of course, every unit and every track is different, but I
generally start by setting the ratio low, around 1.5:1, the threshold
high, and both the attack and release somewhere in the middle. I then
bring the threshold down until the gain reduction meters (or my ears)
tell me that the compressor is reducing the signal. From there, I
usually tweak the controls bit by bit until I get what I want, which may
be a particular sound, or may be a particular reduction in dynamic
Attack and release times should generally correspond to the speed of
the instrument. For example, bass tracks should normally use slow
attack and release times while drum tracks usually sound best with
fast times. When adding compression to a mix, rather than an
individual track, I generally recommend fast attack and release times,
though other engineers will insist that you use slow attack and release
times. Everyone agrees, though, to keep the ratio low: too much
compression on a mix is a sure way to wreck things!
Once you set the threshold, ratio, attack, and release settings, you
should look at the meters and increase the gain to make up for the
gain reduction imposed by the compressor. Typically, you'll set the
gain on the compressor to be equal to the gain reduction shown on the
A good thing to do is to listen to other mixes of similar music. Some
forms of music, especially acoustic music, sound best without any
compression at all. Electronic music, punk, and hip-hop often use huge
amounts of compression to help them compete with other songs on
the radio, as well as to create sounds that are both high-energy and
rich in detail. In the case of these modern forms of music,
compression is often the key ingredient that puts life into the sounds.
Folk, country and other similar genres typically benefit from moderate
compression. Country music often uses large amounts on some
instruments, such as drums, and less on others.
Obviously experimentation is crucial, not just for learning but for
everyday use, since no two tracks are ever the same. On the other
hand, many compressors, such as the one included in XO Wave, have
built-in presets that give you a nice jumping-off point. Often, you'll
only have to set threshold and make-up gain.
Other Uses of Compression
Besides reducing dynamics, compression has a variety of uses, and
sometimes a reduction in dynamics may be motivated by something in
particular, such as a poor performance or an uneven mix. Some of the
tricks of compression involve the use of the compressor's side-chain,
which is a special signal path that is only used for controlling the
compressor. For example, if you put a high-pass EQ on the side-chain,
the compressor would become more sensitive to high-frequency
content than low-frequency content, and work harder to reduce the
signal whenever the signal had a lot of high-frequency content.
Compensating for a Poor Performance: Compression can often be
used to overcome weaknesses of the performer. For example,
experienced singers move away from the mike when they sing loudly
and towards the mike when they sing softly. Less experienced singers
generally don't do this at all or, even worse, do the opposite. Even
with an experienced singer, it can be desirable to get that "close-
miked" sound even during the loud passages. Other instruments
suffer, too: it can take years of experience to play evenly and with
carefully controlled dynamics. Sometimes even great musicians will do
a great take with bad dynamics. In all these cases, compression is
essential because the dynamic range of the performance is wider than
it should be and compression can bring it back into the correct range.
De-essing: A compressor forms the basis for another effect called a
de-esser. A de-esser is used for reducing the so-called sibilant sounds
of vocalists, such as "s", "th", and "f", which often have highly
exaggerated high-frequency response in recordings. The best thing is
always to try getting rid of as much sibilance as possible by using
appropriate mike technique, but since this isn't always possible, most
engineers like to have a de-esser handy. Note that you can create your
own de-esser by using an EQ with a compressor. See the Compressor
description for details on doing this in XO Wave.
Ducking: Ducking is a little different from compression: instead of
reducing the gain based on the volume of the signal going through it,
a ducker reduces the gain based on another signal. For example, you
could use ducking in a podcast to reduce the volume of music when
someone is speaking. Note that in XO Wave, Ducking is done with the
special Envelope Generator effect instead of a compressor.
Compressing a Whole Mix: Compressing a mix is different from
compressing a single track because every instrument gets compressed
the same amount. In practice, this often means that the bass
instruments cause the whole mix to get over-compressed, which
makes it sound like the bass instruments are "punching out" the other
instruments. Many compressors, including XO Wave's, offer
compensation for this. If not, you can compensate by using the
compressor's side-chain feature with an EQ.
Compression As a Special Effect: Sometimes compression just
sounds cool and that's all there is to it. A lot of hip-hop samples, for
example, are heavily compressed. It often sounds good because tiny
details suddenly become prominent, while the loud sections are still
under control. Compression can also soften the edges of distortion,
and enhance performance nuances.
Compression to Increase Dynamic Range: After all this talk of
reducing and controlling dynamic range, it may seem strange that
compression can also be used to increase dynamic range, but it can be
a great tool for this as well. Often a track, such as a snare drum, may
sound great by itself, but may have a hard time competing with other
instruments in the mix. To help it cut through, you can add a little
compression with a relatively long attack time. This reduces the
volume of most of the snare drum sound, but keeps the initial
transient loud. This short burst of sound is often all a snare drum or
other percussion instrument needs to sound great in the mix.
Depending on your gain setting, this technique can do two things: it
can increase the volume of the transient, helping the track to stand
out, or it can reduce the volume of the rest of the note, which may
help un-clutter a dense mix.
When I was first learning to use a compressor, I over-compressed
everything, which made my early mixes sound lifeless and dull. Some
people don't use enough compression, which make the mix sound
inconsistent or "jumpy". Finding the right balance is the key, and the
only way to do that is to practice and listen, both to your own mixes
and the mixes on your favorite CDs.
-- Bjorn Roche