What Is an Audio Compressor and How Does It Work?

An audio compressor is a tool that reduces the volume of loud sounds by narrowing or "compressing" an audio signal's dynamic range. Simply put, it keeps loud sounds from getting too loud. An audio compressor is a great tool for studios and live sound production. Whether you are mixing a new CD, a choir at your local church or a small band in a coffee shop, having one or two handy is always a great idea.

Audio compressors have many uses. Some of the most common are:

    Vocals - Controlling an overbearing vocalist that has a very wide dynamic range
    Drums - Giving a loud drummer a little more finesse by limiting his dynamic range
    Guitars - Because guitarists have a volume knob on their guitar, they are subject to turning it up when it doesn't really need to be turned up.

Almost any instrument or vocal can benefit from a little compression, but you never want to compress everything that you are mixing. If you can't control the volume and dynamic range of a specific instrument/vocal with the gain and fader, then maybe you need to use a compressor.

Basic Controls

A compressor has many settings, some may be unique to specific models, but we will look at them all here:

Threshold - This is the level of sound at which the compressor will "turn on" and begin to compress the audio signal. It is generally represented in dBu's.

(dBu's are the same units that most meters are set on. Use your console's meter bridge for the selected channel to determine the dBu's to set the Threshold at.)

Ratio - This control sets the amount of compression for the audio signal. It is shown as an Input/Output ratio: A setting of 3:1 means that if the inptt audio signal exceeds the Threshold by 3 dB's, the output signal will be 1 dB over the Threshold, effectively compressing the signal by 2 dB's.

Attack and Release - Some compressors provide controls for the speed of their action. These controls will allow you to set how fast or slow the compressor compresses the signal when it exceeds the Threshold and how fast or slow it releases the compression when the signal falls below the same Threshold.

Soft and Hard Knees - This control is specific to certain compressor models and determines whether the bend in the compression curve is a sharp angle or has a rounded curve.

Everyone has different settings for a compressor, depending on their use and preferences, but a good starting point for almost any instrument or voice is:

    Threshold - -20 db
    Ratio - 3:1
    Attack - 30 ms
    Release -300 ms

These settings will give you a starting point for your compression. Next time you have the chance, play with the compressor while rehearsing so you can hear what it does and you can better understand it.

My name is Gerad Clarkson and I have been working in and around Pro Audio for over a decade. I started my own Pro Audio company, Audio Avenue, in 2011. I work with church tech teams, mobile DJ's and local clubs, to help them find the best gear that fits their needs and their budget.

Article Source: Gerad K Clarkson

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