Compressor
From Audacity Manual
Compressor reduces the volume of louder sounds while leaving quieter ones untouched, so reducing the dynamic range of the audio. One of the main purposes of reducing dynamic range is to permit the audio to be amplified further without clipping than would be otherwise possible. Therefore by default Compressor amplifies audio as much as possible after compression. The resultant increase in average or RMS level can be useful for audio played in a noisy background, such as in a car stereo, or in speech, to make a distant voice sound as loud as a close one.
Graph
Controls
- Threshold: Sounds below this threshold will be left unchanged. Sounds above this threshold will cause the compressor to reduce the level of the audio. It sets the vertical position of the corner on the graph (the yellow horizontal line).??
- Ratio: The amount of compression applied to the audio once it passes threshold level. The higher the Ratio the more the loud parts of the audio will be turned down. The Ratio sets the slope of the graph above the corner (above the yellow horizontal line).
- Attack Time: How soon the compressor starts to reduce the volume level after it rises above the threshold. If volume changes are slow, you can push this to a high value. Short attack times will result in a fast response to sudden, loud sounds, but will make the changes in volume much more obvious to listeners.
- Decay Time: How soon the compressor starts to increase the volume level back to normal after the level drops below the threshold. A long time value will tend to lose quiet sounds that come after loud ones, but will avoid the volume being raised too much during short quiet sections like pauses in speech.
- Make-up gain to 0 dB after compressing: Boost the resultant audio after compression equally at all volume levels, to be as loud as possible without distortion. Because the compressor works by making loud sections quieter, you will usually want to do this. If this option is selected, the effect will be both to make the quiet sections of the audio louder, and increase the overall loudness.
Schematic example
A simple sine wave that drops off by 6 dB at half time, to demonstrate how some compressors handle signals. |
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The attack part of where the compressor is working is clearly visible at the start of the audio.The release part still affects some audio that is beneath the threshold as the compressor gain change slowly ebbs out and the material fades back to normal level. |
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