Mackie Stereo and Mono Side chain Compressor User Manual

Compression has become such an integral part 
of mixing, that we thought an appendix might 
be called for.

To use a compressor effectively, it’s important 
to understand the concept of dynamic range. 
Dynamic range is the difference between the 
loudest and quietest signal levels passing 
through the system chain. The span between 
"barely audible" and "physical discomfort" is 
about 120 dB, so we consider this to be the dy-
namic range of human hearing. Anything below 
the threshold of hearing will be lost, as will 
anything above the threshold of pain. But how 
much dynamic range do we need in our record-
ings, or, more importantly, how much can we 
really use? 

Few people listen in a totally soundproofed 
room. A very well-designed studio control room 
has an ambient noise level 10 to 15 dB above 
the threshold of hearing. Since we want to 
keep ourselves safe from hearing damage, 95 
dB or so is about all the dynamic range we can 
use. But consider this: A very quiet living room 
has an ambient noise level 25-30 dB above the 
threshold of hearing. The inside of an automo-
bile at reasonable driving speed is upwards of 
60 dB above the threshold of hearing, and with 
a window open, all bets are off. 

Average consumer audio systems aren’t capable 
of producing painful sound pressure levels (with 
the exception of those cars that cruise by with 
the bass pumping loud enough to rattle the 
windows in your house), a typical listening en-
vironment can only support a dynamic range of 
65 to 75 dB.

The electronics in our recording chain are much 
better than that, with digital recorders and ana-
log or digital consoles capable of 105 to 115 dB 
of dynamic range. This means that the listening 
environment, not the gear, sets the practical 
limit on dynamic range. If you see a number 
greater than that on a spec sheet, it’s purely 

theoretical – a 24-bit system is theoretically ca-
pable of 144 dB of dynamic range but practical 
things like electronic components stand in the 
way of actually achieving it in practice, at least 
today. 

Even in the best of control rooms, we have to 
squeeze as much as 115 dB of obtainable dy-
namic range into a 95 dB box. For the rest of 
the world, though, we have to squeeze harder so 
soft passages don’t get lost when your neighbor 
starts up his lawnmower, or when listening to 
the car radio at 65 miles per hour with the top 
down. Therefore we can’t record all the dynamic 
range that’s available if we expect people to 
hear all the music we record.

A compressor reduces dynamic range. When 
used properly, it’s hard to detect the action of a 
good compressor, but a compressor can also be 
used creatively as a sound shaping tool. A com-
pressor can be inserted into a single channel in 
the recording chain when recording or mixing a 
track, or compression can be applied to an en-
tire mix or sub-mix. 

Let’s look at applying compression to a vocal 
track. Hard consonants such as the letter ‘T’ 
create a high initial sound level before settling 
down, whereas most vowels tend to be more 
even. The average volume level of a word may 
be fairly low, but because of an initial loud con-
sonant, we can only raise the word’s volume so 
far before running out of headroom. If there’s 
music playing under the voice, even when boost-
ing the vocal level as high as possible without 
distorting the attack, a word (or a syllable) may 
be far enough below the level of the music to 
become lost or misunderstood. 

If we reduce the gain momentarily during the 
loud attack, then bring it back up when the dis-
tortion-risking blast is over, we can now boost 
the average level of the word so that it can be 
understood clearly over the music. What we’re 
doing here is reducing the dynamic range of the 
word, the difference between the loudest and 
softest parts. By doing that, plus boosting the