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When an audio signal is compressed, its amplitude above the threshold is reduced according to the ratio. The gain reduction, sometimes labeled GR, tells us how much threshold interaction and ultimately how much gain reduction or signal attenuation is taking place in the compressor at any given time. In simple terms, gain reduction tells us how much the compressor is turning down our signal, measured in decibels. Gain reduction can be monitored in most compressors. When audio is played through a compressor, the amount of gain reduction at any moment is shown on the GR meter.
Watch the Gain Reduction meter on this Wave's plug-in as I play some compressed audio through it. (music playing) The lower we set the threshold, the more audio signal is compressed, resulting in more gain reduction. Also, if the ratio is set high, say 8:1, then we will also see more gain reduction than with lower ratios. Because our working compressor is actively attenuating or lowering the level of the signal, most compressors feature makeup gain, sometimes labeled just gain or output, to recover any lost volume after the compression has been applied.
I like to think of it this way. The threshold comes down to interact with the louder parts of the program triggering gain reduction and attenuating the louder signals. Makeup gain is then used to raise the overall level of the signal, bringing up the uncompressed values to fill in the signal and blend it into the mix. For basic compression tasks, start by lowering the threshold to a value that achieves between 4 to 6 dBs of gain reduction. That's a good place to start. Next, since parts of the signal have been compressed, use the makeup gain to match the signal's level to around the same as before compression.
This trick allows you to turn off the effect and evaluate your processing before and after without a net gain change. So you can really hear if you are getting somewhere, rather than the "oh, it's louder and must be better" reaction so many engineers make the mistake of. I like to use my ears to do this, but you can often get away with setting the makeup gain to whatever the gain reduction meter is showing on average. So if that hovers between 6 and 8 dBs, try using a makeup gain of 7 dBs. Some signals will require more gain reduction, while some will require less.
This is where using your ears in the context of the material becomes critical. No preset on a compressor plug-in can tell you how much gain reduction is going to work for your specific signal. So close your eyes, open your ears, and tweak accordingly. Note that some compressors do not have controllable thresholds or any visual threshold at all. These compressors are said to be fixed threshold in design. The UA 1176 is an example of a famous fixed-threshold compressor. With these types of compressors, it's even more important to read the gain reduction meter and use your ears to adjust the makeup gain properly.
So if you can understand and operate the threshold and makeup gain controls, as well as read a gain reduction meter, you can successfully use most compressors.
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