Apple Soundtrack Pro 3 - Manual (Page 272)

Compressors
Compressors work like an automatic volume control, lowering the volume whenever it
rises above a certain level, called the threshold. But why would you want to reduce the
dynamic level? By cutting the peak levels, the compressor lets you raise the overall volume
of the signal. This gives the sound more focus by making the foreground parts stand out
while preventing the background parts from becoming lost in the mix. Compression also
tends to make sounds tighter or “punchier.” Because the peaks are lower, the maximum
volume is reached more quickly.
Compressor plug-in
In addition, a compressor can make a project sound better when played back in different
situations. For example, the speakers on a television set or in a car sound system typically
reproduce a narrower dynamic range than does the sound system in a theater.
Compressing the overall mix can help make the sound reproduce more clearly in
lower-fidelity situations.
Compressors have two main parameters. The threshold lets you set the amplitude above
which the compressor lowers the volume. The ratio lets you control the amount by which
sounds above the threshold will be lowered, as a percentage of the original signal.
For example, if you set the threshold to –12 dB, and the ratio to 2:1, a sound at –7 dB
(5 dB above the threshold) is reduced by 2.5 dB, and a sound at –2 dB (10 dB above the
threshold) is reduced by 5 dB.
Compressors can also include parameters for attack and release. These parameters let
you set how quickly the compressor reacts once the threshold is reached (for attack) or
once the signal falls below the threshold again (for release). Use these parameters to
make the compressor’s effect more subtle or more pronounced. Another parameter on
some compressors is the knee (or soft knee), which lets you control how gradually the
compressor transitions between no compression and the compression ratio at the
threshold.
272 Chapter 10 Working with Audio Effects