Apple Soundtrack Pro - Manual (Page 197)

Chapter 9 Working With Audio Effects 197
Dynamics Effects
Dynamics effects let you adjust the dynamic range (the range between the softest and
loudest sounds) of your projects. You can use dynamics effects to make sounds more
focused and to optimize the sound for specific playback situations. Dynamics effects
include compressors, limiters, and noise gates.
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.
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.
Compressors are typically used on vocal tracks to make the vocals prominent in the
overall mix. They can also be used on music and sound effects tracks, but rarely on
ambience tracks.