Apple Soundtrack - Manual (Page 127)

Chapter 8 Working with Audio Effects 127
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.
Limiters
Limiters (also called peak limiters) prevent the audio signal from exceeding a maximum
volume level. A compressor gradually attenuates levels above the threshold, but a
limiter puts a hard limit on any signal louder than the threshold. You use a limiter
mainly to prevent clipping.
Noise Gates
A noise gate alters the signal in the opposite direction from a compressor. While a
compressor lowers the volume of sounds above the threshold, a noise gate lowers the
sounds below the threshold. Loud signals pass through unchanged, but softer signals,
such as the decay of a loud instrument, are cut off. Noise gates can be used to
eliminate low-level noise or hum from an audio signal.