[h2]Introduction[/h2]In audio compressors are used to make the dynamic range of a signal smaller. Often the goal of decreasing the dynamic range of a signal is to increase the overall loudness of the signal by lowering peak levels, then increasing all levels. Increasing the volume of audio has become an important part of the mastering process for albums. Compressors are further used to decrease the dynamic range to achieve a smoother sounding mix from very dynamic sources. It is often thought that ideally a compressor can not be heard.
Visit here to see a before and after compression illustration of the same file. Take note overall the volume is louder, but the spikes are not as pronounced.
[h2]Adjustments[/h2]Common controls on compressors include:
an input control to adjust the level before processing expressed in dB
a threshold control to determine at what dB level the compression becomes active expressed in dB
a ratio control to determine how much compression happens above the threshold expressed as a ratio, 1:x
an attack control is expressed in ms and is to establish how long a signal must be above the threshold before compression begins
a release control is expressed in ms and is to establish how long compression continues, and decreases, after audio falls below the threshold
a knee control will select how abruptly the compression starts, a soft knee would make a curve on a graph when compression begins, a hard knee makes an angle
a limiter control for 1:∞ compression expressed in dB
an output control to adjust the level after processing expressed in dB
[h2]Metering[/h2]There are a few ways compressors tell you how hard they are working apart from just using your ears. The most common are a gain reduction meter, or a graph.
[h3]Gain Reduction[/h3]The amount of compression can be visually expressed by a gain reduction meter showing the amount of compression in dB.

[h3]Graph[/h3]Another way to visually express the compression is by a graph. Graphs are often used on computers. Typically the input gain is on the x axis spanning across. The output gain is on the y axis going upwards. As input signal increases the audio moves right following the line on the graph. The threshold is the point where the line strays from a slope of 1. The ratio controls the slope after the threshold.

Many times it is possible to simply click points on the graph and drag them where you want them to be.
[h2]Multiband compressors[/h2]Multiband compressors are special compressors that allow one to apply a different compression characteristics to different frequency bands of audio. A multiband compressor with three frequency bands of audio would process lows, mids, and highs independently of each other. Most offer selectable crossover points. Multiband compression is most often used in broadcast or mastering situations.
[h2]Limiters[/h2]Limiters are special compressors that allow no signal to be above the threshold. They usually drop the ratio, attack, release, and knee controls leaving the input, threshold, and output controls. The ratio is fixed at ∞:1. Metering on limiters can be as basic as a simple light that turns on when the limiter is adjusting the audio. Sometimes limiting is referred to as brickwall compression.

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