Compressor

The Compressor effect reduces the dynamic range of 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 the 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 environment such as in a car, or in speech, to make a distant voice sound as loud as a close one. Because the gain changes relatively slowly, a compressor does not distort the signal in the way that a Limiter or clipping would do.

Note carefully that when you apply an effect to a time-stretched clip the changed speed of the clip will be automatically rendered. If you apply an effect to a selection within a time-stretched clip then Audacity will split the original clip so that the selection can be rendered as part of applying the effect.

Accessed by: Effect > Volume and Compression > Compressor

Graph

The graph shows the input level along the bottom (horizontal axis) and the output level scale on the left (vertical axis) to illustrate the dynamic range compression effect. The graph will change as you adjust the Threshold and Ratio sliders, reflecting those settings. The graph does not reflect changes in any of the other controls, although they all affect how the audio sounds after applying the effect.

Controls

The principle of how each control works is described below. In practice the effect works internally in a rather more complex manner in that, as it processes the sound, it looks ahead to determine if there are high peaks coming up. This enables the "attack" stage of the effect to work in advance of increasing level as can be seen in the example below. This feature is called look-ahead and it is enabled within the effect with no user adjustment. The benefit of look-ahead is that sudden peaks do not pass through the effect before the gain has had time to change.

• Threshold: The level above which compression is applied to the audio.

• Noise Floor: The compressor adjusts the gain on audio below this background level so as to prevent it being unduly amplified in processing. This is mainly useful when compressing speech, to prevent the gain increasing during pauses and so over-amplifying the background noise.

• Ratio: The amount of compression applied to the audio once it passes the threshold level. The higher the Ratio the more the loud parts of the audio will be compressed. The Ratio sets the slope of the blue line on the graph above the threshold.

• Attack Time: How soon the compressor starts to compress the dynamics after the threshold is exceeded. 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.

• Release Time: How soon the compressor starts to release 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 for 0 dB after compressing: Amplifies the resultant audio in all selected tracks after compression to a peak level of 0 dB. All tracks are amplified by the same amount as in the Amplify effect.

• Compress based on Peaks: Base the threshold and gain adjustment on peak values of the waveform rather than the average (RMS) value used when in default (unchecked) state.

• When using RMS, the compressor uses "downward" compression, making louder sounds above the threshold quieter while leaving quieter ones below it untouched.
• When using peak values, "upwards" compression is applied which makes the audio louder, but amplifies the louder sounds above the threshold progressively less than those below it. Where the original (input) level is 0 dB there is no amplification.

Example

Before Compression:

A simple sine wave that begins at -12 dB, jumps up to 0 dB, then drops back down to -12 dB, to demonstrate how the Audacity compressors handle signals.

When Compress based on Peaks is not selected, audio with an RMS level beyond the threshold range will be reduced. If Make up gain is enabled then the entire selection will be boosted to make up for this gain reduction.

When Compress based on Peaks is selected, audio with a peak level within the threshold range will be boosted.

After Compression:

In this example:

• Compressing based on peaks is enabled so audio with a peak level below the threshold is boosted. You can see that the audio before 1.5 seconds on the Timeline has been made louder than it was in the "Before Compression" image.
• Attack is 0.5 seconds, during which period the compressor progressively anticipates the coming peak.
• Release is 1.0 seconds, during which period the boost to the quieter audio is progressively resumed.
• Ratio is 10:1.

Further reading on compressors

There are some good (but not too technical) explanations of compression here:

• Compressors demystified (Harmony Central)
• Compression Made Easy with audio files (Sound On Sound)

More advanced:

• Advanced Compression Techniques Part 1 and Part 2 (Sound On Sound)

An alternative free compressor

Please see Chris's Dynamic Compressor for a popular alternative compressor which may be downloaded for free.

Links

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