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By reducing the highest parts of the signal, called peaks, a compressor raises the overall
level of the signal, increasing the perceived volume. This gives the signal more focus
by making the louder (foreground) parts stand out, while keeping the softer background
parts from becoming inaudible. Compression also tends to make sounds tighter or
punchier because transients are emphasized, depending on attack and release settings,
and because the maximum volume is reached more swiftly.
In addition, compression can make a project sound better when played back in different
audio environments. For example, the speakers of a television set or in a car typically
have a narrower dynamic range than the sound system in a cinema. Compressing the
overall mix can help make the sound fuller and clearer in lower-fidelity playback
situations.
Compressors are typically used on vocal tracks to make the singing prominent in an
overall mix. They are also commonly used on music and sound effect tracks, but they
are rarely used on ambience tracks.
Some compressors—multiband compressors—can divide the incoming signal into
different frequency bands and apply different compression settings to each band. This
helps to achieve the maximum level without introducing compression artifacts.
Multiband compression is typically used on an overall mix.
• Expanders: Expanders are similar to compressors, except that they raise, rather than
lower, the signal when it exceeds the threshold. Expanders are used to add life to audio
signals.
• Limiters: Limiters—also called peak limiters—work in a similar way to compressors in
that they reduce the audio signal when it exceeds a set threshold. The difference is
that whereas a compressor gradually lowers signal levels that exceed the threshold, a
limiter quickly reduces any signal that is louder than the threshold, to the threshold
level. The main use of a limiter is to prevent clipping while preserving the maximum
overall signal level.
• Noise gates: Noise gates alter the signal in a way that is opposite to that used by
compressors or limiters. Whereas a compressor lowers the level when the signal is
louder than the threshold, a noise gate lowers the signal level whenever it falls below
the threshold. Louder sounds pass through unchanged, but softer sounds, such as
ambient noise or the decay of a sustained instrument, are cut off. Noise gates are often
used to eliminate low-level noise or hum from an audio signal.
68 Chapter 4 Dynamics Processors