Compressors and limiters are used to control the loudness of an audio
signal. This purpose of which is to control the incoming level to another audio
device. Have you ever heard a video or audio post where the background noise is
fine but when the person speaks it is distorted? That is because the level of
the persons voice is too loud for the either the next audio device in line or
the recorder itself thereby causing it to overload and distort. In the movement
of audio from voice to mic to mic preamp to eq to audio converter and then to
computer each device has an audio range it can operate cleanly in. (The practice
of maintaining an even level through all these different devices is called 'gain
stageing' and requires one to oftentimes be familiar with each device in use.)
While some devices can be driven past this level for a desired warming or
distortion effect most often engineers are shooting for a clear and clean tone
that is faithful to the original.
Proper gain staging is good, but the random world we live in is not
properly gain staged. Often when recording a singer will sing a little, or a
lot, louder than than anticipated in your initial level setting or the song
itself will vary greatly in soft and loud parts. Bass players too often vary in
their playing levels dramatically. Both of these once committed to a final
recording will vary so much in level that some parts are too loud while some are
not loud or clear enough. Although not a complete solution compressors and
limiters can help with this problem.
Simply put a compressor will grab audio that goes over a predetermined
threshold level and lessen it by a fixed ratio. This is controlled by the ratio
knob. Typical ratios are: 2:1, 4:1, 8:1. After having set a threshold level
with, yep, the 'threshold' knob a singer would have to sing 4 decibels above
your threshold level for their voice to raise 1 decibel above the same
threshold. The speed that the compressor grabs the audio is called the 'attack'.
The speed of release of the compressor effect is called the release. These are
important parameters especially when dealing with fast material such as drums or
piano. If the attack is too fast it will cut off the initial transient of the
signal and cause it to be darkened or muddied tonally. This can be heard very
clearly if you put a snare sample on loop and run it thru a compressor setting
about 15db of gain reduction and then vary the attack from slow to fast. You
will notice that as the attack gets too fast that all the bite is lost in the
first wave of sound. The release knob releases the compressor function.
Typically you set release slower for vocals, bass, or anything where the notes
are often sustained. This helps it sound natural. Fast release times are nearly
always used for any percussive sound such as drums, piano, claps etc.
Wait! My compressor doesn't have a attack and release knob! Relax. This
only means that you have what is known as an optical compressor. These have
predetermined attack and release that typically vary based on how hard the unit
is driven. The more compression, the more attack.
So, typically, we set the threshold to a natural amount (2-4db). Then set
the ratio depending on the variance of the material in level. Then we adjust the
attack and release knobs for a natural sound. The attack and release tend to
affect the overall tonality of the material the most. Driving the threshold,
sometimes labeled 'gain', can impart some edge (distortion) to the sound.
Increasing the ratio control changes the 'knee'. Knee is how thoroughly the
initial sound crossing the threshold is grabbed. A 'soft knee' is more of a
smooth response while a 'hard knee' has a tighter grab. On most compressors that
don't have a separate 'knee' simply increasing the ratio will harden the knee
and thereby creating a tighter cut off of the sound. And this brings us to what
is different about a limiter.
Where compressors are meant to control mild (and not so mild) audio peaks,
a limiter is meant to limit the audio completely. And so a compressor set to a
ratio of 20:1 or more is acting as a limiter. Limiters are very often used where
peaks could damage other equipment: radio, the mastering of albums, live
performances and their wound systems. Their effect is definitive.
Compressors are very useful tools and are indispensable in the audio
world. Many are used, especially in the studio, because of their character
(honestly, this is just their individual form of distortion). Knowing how to
push certain models creates a certain sound that most will recognize instantly.
Learning to hear the effect and 'learn' its sound is easiest if done on solo
tracks with percussion (piano is a percussion instrument) being the easiest.