Re: Advice for dealing with singers who wander out in front of the PA
To attempt to help Phil with his lack of education on compressors (just in case the two posts before this don't help):
Compressors are essentially an automatic gain control. The reason this won't help with your feedback problem is that compressors reduce your dynamic range. Assuming there is a 15 dB difference between where the feedback starts and where you absolutely need to be to push the vocals over the rest of the mix/band, when you add a compressor lessens this value by however much you compress it (you'd probably eat up 7 dB in your case). This means that you will only have about 8 dB of usable dynamic range left. This certainly doesn't help you in any way, and in fact makes the problem worse.
Now if you want an explanation of how a compressor does this, read on:
What a compressor does is relatively simple. It takes a signal, and after it exceeds a certain level (a parameter called the threshold), it applies a set amount of negative gain at a certain ratio to the original signal (a parameter called the ratio-go figure) at a certain rate (a parameter called the attack) and allows the gain to stabilize to the normal level after the threshold is no longer exceeded at another rate (called the release). However, because negative gain is applied, the signal going out of the compressor will always be less than when it went in. To accommodate for this, there is a final amount of additional gain to return it to pre-compressor levels (this parameter is called makeup-gain).
However, all sounds below the threshold are not affected by the negative gain, while the makeup-gain still applies. This means that both the noise and the feedback floor are increased more than before. Thus, it will cause more feedback and also most likely make the singer sound worse- especially if they are inexperienced already.
For more information on the use of compressors and many people's opinions of them, see this recent thread concerning them here.
Best of luck, and break a leg!
