Dynamics Compressor
A compressor reduces differences in volume between quieter and louder sections of audio. It is applied to audio which goes above the threshold level by turning the volume down by an amount determined by the compression ratio. The attack time is how fast it responds to levels above the threshold, and the release time is how fast it reacts when the levels drop below the threeshold. The post gain increases the overall volume to compensate for the lowered volume in the formerly loud sections.
Another way to describe the dynamics compressor is to imagine someone watching the peak level meter while the audio is playing, and whenever the level goes above a certain threshold, he turns down the volume. This is what a compressor does. The compression ratio determines how far to turn down the volume when the level goes above the threshold, with larger ratios resulting in the volume being turned down more. The attack time determines how fast the compressor reacts to the level going above the threshold. The release time determines how long after the level goes below the threshold the volume is returned to normal.
The "Post Gain" control compensates for how the compressor turns down the volume on the loudest passages. It does this by amplifying the audio after it goes through the compressor. Turning on the "Auto" option causes it to calculate the optimum post-gain, so that the volume of the loudest passages stays about the same but the quieter passages are increased in volume. The effect of this is to bring up the level of quieter passages without increasing the volume of louder passages, essentially compressing the dynamic range of the audio.
Compression is useful for taking music with a wide dynamic range, where the quiet passages are very quiet and the loud passages are very loud, and making all the passages about the same volume so that they can all be heard in a noisy environment such as in a moving car. Compression is also used when someone is speaking into a microphone, so that if that someone moves closer to or farther from the microphone, or speaks softer sometimes and louder sometimes, the compression can compensate for these differences and output a consistent level of audio.
The Peak/RMS options under Threshold determine what kind of level meter to use. A peak level meter measures the very tops of the waveform, and is triggered whenever the wave goes above the threshold. A RMS level meter measures the average power of the waveform, and closely follows what our ears perceive to be the level of the audio.