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Audio Compressor Plug Ins

Audio compressors scare a lot of people off, but folks don't need to be scared By Graeme Hague

Audio compressors scare a lot of people off. They’re confusing to novices and sometimes don’t seem to do a thing- or they seriously crunch up your sounds, if you try presets. Like, what’s going on? Yet experienced sound engineers swear by them as an essential tool. Compressors of the plug-in variety are no different. The signal processing has translated well from the hardware devices into the virtual, digital environment. They are a great effect, if you know what they do and how it all works.

Compressors perform one of two tasks, or both at the same time. Confused already? Take a deep breath and stay with me. The point is that a compressor set up to increase the volume of a low signal is often called an Expander, while one that is putting the brakes on any loud material is called a Compressor. But they’re actually the same device. Calling them an “Expander” or “Compressor” describes the job it’s doing, that’s all.

You might say the basic purpose of a compressor is to average out an audio signal by boosting the quiet parts and softening loud sections. This can be true, if that’s how you use the compressor, but no one wants their music to sound “average” right? The dynamics in a recording can be very important.

Kjaerhus Classic Compressor. Kjaerhus Audio has a compressor plug-in that faithfully recreates the classic hardware rack-mount devices of old. Straightforward and uncomplicated- and very effective.

To explain what compressors can do, here are three situations where you might need one. First, the very quiet moments of a classical guitar track are too soft and get lost among other instruments. By boosting the input and lowering the output, plus telling the compressor to gently handle sudden transients, you can lift the volume of these sections without everything getting very loud in normal parts. In this case, the compressor is being used as an Expander. Next, a vocalist has a good, strong voice and occasionally really screams or yells, peaking your recordings into the red. A traditional compressor setting will “catch” these peaks, before they start clipping the input.


Last, a combination of both- again using the vocalist example; A singer barely whispers some lyrics and shouts others. You want to build up the quieter moments and still put a safety valve on that yelling! By describing the purpose of each of the compressor’s controls you’ll understand more how they work. Note that some compressors also have a “Gate.” We’ll do this last.

Input: This is like a gain control on a mixer and is used to set a good, strong signal on your input. It isn’t processing anything as such, just adjusting a level.

Threshold: This determines when the compressor actually kicks in and starts to do its job. You want to compress that loud yelling, but leave the other parts untouched. It’s the volume, if you like, when the compressor begins to work. Below the Threshold, even though you might see flashing signal indicators, the compressor is doing nothing.

Ratio: This is the hardest thing for many users to understand. The Ratio is how much the signal is compressed once the Threshold is crossed. Some high school math might help here (serious mathematicians please don’t cringe). For example a Ratio of 4:1 can be thought of as one quarter or twenty five percent. Every bit of audio that is louder than the Threshold setting is reduced down to one quarter of its volume (one part in four). We can look at some values that are too precise to be real, but you’ll get the idea.

Let’s say the singer is too loud at exactly 100 decibels (dB). You have a Ratio of 4:1 set at a Threshold which will cut in when the vocalist’s volume is at 80 dB. That means the extra 20dB of sound is above the Threshold. The 4:1 Ratio will squash it down to 5dB giving a total of 85dB. That’s the 80dB below the Threshold and the extra five the compressor allows through. If the Ratio was 2:1 that 20dB would be cut in half to 10Db and the total is 90Db. If it was 10:1 only 2dB of that 20dB would get through and the total is 82dB. Because a certain amount of signal always gets through, then on a graphical GUI like in Adobe’s Audition 2.0’s compressor plug in it looks like this.

Audition 2.0 compressor. This is a graphical read-out of a simple compressor setting of a 3:1 Ratio and a -30dB Threshold. The plug-in is from  Adobe’s Audition 2.0.

Attack and Release: You can set the compressor to react fast or slowly to an audio signal crossing the Threshold and also how long the compressor keeps working. A loud acoustic guitar being plucked is a good example. With a fast Attack time the musician’s playing will be compressed almost instantly. What if you want to hear that definite click of the plectrum plucking the strings? You set a slower Attack time which allows that plucking noise through before the compressor cuts in to reduce the overall volume. It’s like shutting the digital barn door after the horse has bolted. Okay, now you want the compressor to turn itself off again in time to hear the next plectrum pluck. So you want a fast Release time to get that compressor out of the way. Otherwise with a slow Release time the compressor will still be on and effecting the following picking.

Different situations call for other approaches. A fast Attack time will help remove plosives, such as microphone popping. If you get your Attack and Release settings wrong it can create a pumping effect as the compressor struggles against the signal.
Output Level/Gain: Any compression has the effect of lowering the overall volume of a signal. The Output lets you compensate for this by bringing things back up to normal. 

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Related Keywords:Audio compressors , digital audio, audio post production,

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