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Compression is an effect that lets you automatically control volume levels. However, compression doesn’t affect the actual sound of what it is being applied to. It only adjusts the volume level. Compression is a “dynamic” effect, which means that the effect depends on time. So that means that when compression is applied to a signal, there may be times when the compressor has no effect on the input, and other times when the effect is very strong.

What A Compressor Does

Imagine watching an action movie. There may be parts of the movie where characters are whispering to each other, and then a scene where there is gunfire and explosions. Compression is what allows you to to be able to hear the whispering parts clearly, and at another point in the movie, hear the the explosions and other loud sounds without going deaf. For music production, a compressor can be used to even out the levels of an instrument that has a wide dynamic range. For example, a complicated clean guitar part can be difficult to record because it will be either too quiet during the articulated picking parts, or too loud during the hard strummed chord parts. A compressor can reduce the volume of the loud parts, while not reducing the volume of the quiet parts too much. Then, output gain can be added to make the entire performance loud enough to hear clearly but also not be extremely loud.

How A Compressor Works

Compression evens out the volume so that the quiet and loud parts are transformed to similar levels. It reduce the volume of your input, so compressors usually have an output gain parameter, which allows for increasing the volume of the compressed signal. The result is a signal that has an even volume level, with less dynamic range. If you were to adjust the volume level of a track by hand, you would have to decide when to turn the volume down, how much to turn it down by, and more options. These things are essentially the parameters you set for a compressor. However, some compressors may not have all of these parameters available for adjustment.

The Parameters

• Threshold – When the input volume exceeds this parameter, the compression effect turns on. This parameter is measured in decibels (db).
• Ratio – The ratio of input to output volume when the compressor is active. Essentially, it is the amount that the volume will be turned down. For example, if your compression ratio is 3:1, when the input signal exceeds the threshold by 3db, the output signal will only exceed the threshold by 1db.
• Attack – The speed at which apply the compression effect (reducing the volume) when the signal exceeds the threshold. Measured in seconds (s) or sometimes milliseconds (ms).
• Release – The speed at which to turn the compression effect off (increasing the volume back to it’s original level) when the input no longer exceeds the threshold. Also measured in s or ms.
• Output Gain – The amount to increase the output volume. This is measured in db.

Example

Here’s an example of compression, made with Audacity. This sample is just a quiet sine wave, and then a loud sine wave, back to back. This first sample is completely uncompressed.

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Next, we will apply some compression. The threshold (-15db) is lower than both sine waves, so it will reduce the volume of both. The ratio (2:1) is a pretty light amount of volume reduction. The attack time (0.8s) is set nice and slow so the compression effect is noticeable. Note that the checkbox to normalize is not selected. This would increase the resulting volume to 0db, but for this demonstration we don’t want any post processing volume change.

The resulting compressed output is a bit quieter. Because we set the attack time to 0.8s, the louder sample doesn’t compress immediately. That’s the little bump at 1.o.

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