BIRTH OF FEEDBACK

Feedback, when it comes out of the stage's womb, is quite a silent kid, but within milliseconds or seconds it becomes that rebellious teenager, throwing out its howling tantrums and then finally dies out when 'father or mother engineer' slaps down some fader and then keeps the child under control. Some days and in some environments, this notorious child can get out of hand and behave like an itinerant toddler that irritates everybody in the audio engineering family and pretty much everybody in the 'listening family'. So now that I've added my metaphor, let me summarize how feedback arises:

To understand this event better it would be helpful to understand a bit of system gain and overall headroom; a definition at this point might be handy:

Potential Acoustical Gain (PAG): is defined as the calculated gain that the system can achieve at or just before the point of feedback.

Every live sound system would have its own PAG, which gives the engineer the reference of how much gain the entire system can achieve before feedback begins to occur. Now, don't think that the engineer starts to calculate each time before a show begins, but as I said earlier, it only is a reference. Therefore every time an open microphone is added on stage the PAG or potential acoustic gain of the system decreases by 3dB leaving you with lesser decibels and a higher probability of inducing feedback in the system. Suppose a system had a PAG of 20dB with one microphone on stage, by adding one more microphone the PAG of the system now becomes 17dB. This is a tricky part because there's no way you can run a show with just one microphone, so the various ways in dealing with this problem would be:

Bear in mind that there are other elements on stage, perhaps a drum kit in close proximity or a bass guitar amplifier, these are normally operating at loud levels and these can be picked up by the vocalist's microphone; the intensity of the feedback can be severe if the venue is small or if the band is loud on stage. So you can now imagine that if there were more microphones on stage or open microphones (mics that are picking up levels continually), if all these microphones pick up the stage levels and then are re-amplified through the stage monitors and house speakers, you know you're 'fader pushing-up action' was the primary cause of it. In these scenarios the only action which saves eardrums and possibly your reputation is bringing down the monitor send level or if you're mixing from the FOH end just bring down the master stereo buss.