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The compressor is a very useful studio tool. It has the ability to control dynamic range, but these days that is better handled by automated mixing. But it also has the remarkable ability to add fullness and sparkle to a sound.
For instance, a vocal recorded without compression will sound flat and lifeless, even with EQ. But apply suitable compression and it will leap out from the speakers and give you a great big sloppy kiss.
Compression is also extremely useful on drums and bass, for added punch and fullness, and on acoustic guitar for a lovely shimmery effect on high frequencies. It isn't so useful for electric guitar since most of the compression you could ever want is provided by distortion in the amplifier and speaker. But go ahead and try it if you like. You never know what might happen, which I guess is one of the joys of sound recording.
Compression is therefore an invaluable studio tool. But what about taking the compressor on stage, or at least to the front-of-house processor rack, and using it in live sound?
All of the benefits of the compressor in the studio work just as effectively in live sound. So yes, go ahead and do it. But there is one problem...
Compression reduces the margin before feedback.
So suppose that without compression, you have 6 dB of gain in hand before you know, through experiment prior to the sound check, that the system will feed back. Now put the compressor in the signal path, and adjust the levels so that the peak level is the same as it was before. But now you will find that your margin before feedback has diminished. Perhaps now just an additional couple of dB will set the system ringing.
The reason for this is that compression works by lowering peak levels. So when the signal goes above a level set on the threshold control, the level of the signal is brought down. Signals that are lower in level than the threshold are not affected.
But this would make the signal quieter overall because the peaks have been lowered. So 'make up' gain is provided, usually by a control in the compressor, to bring the peaks back to where they were.
But the low level signals have now been brought up too, and this is precisely where potential feedback lurks. If you use 6 dB worth of compression, and 6 dB make up gain, then you have reduced your margin before feedback by, yes, 6 dB. And for a system that is already quite close to the edge, that would be scary.
So a live sound engineer who uses compression has to be very much on top of his or her profession. It can make the sound better, but it makes feedback harder to control.
Worse still, since feedback is always more of a problem in small venues, it is the sound engineer who is still working up the ranks that has the most difficulty. Still, no-one said live sound would be easy. Perhaps being on the edge is what makes it fun!