Live sound is always a battle against feedback. It is in the nature of microphones to pick up sound, and in the nature of loudspeakers to produce sound. When sound finds its way around the system in a circle, you have feedback, that howling or screeching tone at the frequency where the gain of the system is highest.
Any live sound engineer makes it his or her priority to stay in absolute control of feedback. This involves checking the level at which every mic will feedback. A careful engineer will make a mark on the fader where this no-go zone starts.
But during the course of a show, things might change. The singer might go closer to the monitors than expected. He or she may cup a hand around the mic. This creates a resonant chamber that increases the gain at a certain frequency.
So the engineer needs to ready to pull down the fader at an instant's notice. And to quick enough to realize which channel is causing the problem.
But despite all this care and professionalism, it isn't at all unusual to hear feedback, even at major concerts.
Is this due to amateurish engineering?
In truth, feedback often occurs because it is seen as a natural consequence of live sound, and it doesn't matter. I don't find this acceptable, but many engineers seem to.
But also, the occurrence of feedback tells the audience that the system is working 'on the edge'. And they like that.
Feedback tells the audience that the system is working to the peak of its capability. It tells them that the sound is loud and exciting.
So if a live sound engineer is entirely professional and in control of feedback, it wouldn't hurt to create a little every now and then to add to the excitement.
Oh, there is one more beneficial aspect of feedback - it shows that the singer is performing live rather than miming to a recording!
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