Apple Using WaveBurner Pro 1.0 (with Logic Pro) - Manual (Page 27)

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Compressor
The Compressor tightens up the dynamics of a signal by lowering the volume when it
rises above a certain level, called the Threshold. This decreases the difference between
the softest and loudest parts of the music, increasing the perceived volume. This can
give the sound more focus by making the key parts stand out while preventing the
accompanying parts from becoming lost or inaudible.
In addition to the Threshold parameter, the most important parameter for the
Compressor is the Ratio. The Ratio parameter determines how much sounds above the
threshold are lowered. The Ratio is expressed as a percentage of the original signal. For
example, if you set the Threshold to –12 dB and set 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.
Two other important parameters are Attack and Release. The Attack parameter controls
how quickly the Compressor reacts when sounds reach the threshold. At higher values,
the compressor does not fully compress the signal after the attack time ends. This
ensures that the original attack transient (for example, the sound of a pick or finger
striking a guitar string) remains intact or clearly audible.
The Release parameter determines how quickly the Compressor reacts once the sound
falls below the threshold again. If the compressor produces an undesirable “pumping”
sound, adjusting the Release parameter can eliminate or minimize the pumping sound.
Because the compressor lowers sounds above the threshold, the overall output signal is
often lower than the input signal. You can compensate for the decreased output level
by raising the Gain parameter. Turning on Auto Gain ensures that a normalized input
signal is amplified so that the output signal is also normalized, regardless of the values
for Threshold and Ratio (at least when you are working with relatively static signals).
The Compressor can use one of two methods to determine when an input signal
reaches or exceeds the threshold: either the Peak or RMS level. RMS level is a better
indication of the signal’s perceived loudness. However, turning on Auto Gain and RMS
simultaneously may cause the signal to become saturated. If the signal sounds
distorted, turn auto gain off, and adjust the gain parameter instead.