MGBF (maximum
gain before
feedback) can be calculated. The biggest factors are how loud your source is, how far the mic is from the source, how far the
speaker is from the mic and how far it is from the listener. Generally we want to have a "
safety margin" of 6dB below MGBF to avoid resonance. This is called called MAG (maximum acoustic
gain). Basically to get good
gain before
feedback, get the mics as close as possible to the source, and as far from the speakers as possible. Directional transducers (both speakers and mics) will net you about 6dB more MGBF in higher, more directional frequencies. This means you'll get low
feedback first since those frequencies are
omnidirectional. Here's the formula to calculate MGBF, just a bunch of ISL (
inverse square law) calculations:
20log (D0) - 20log (Ds) + 20log (D1) - 20log (D2) = MGBF (dB)
MGBF - 6dB = MAG
where D0 = distance from performer to listener, Ds = distance from performer to mic, D1 = distance from mic to
speaker, D2 = distance from
speaker to listener
Units don't matter; just make sure they're consistent.
Feedback happens when the loop
gain reaches unity, e.g.
level from the
speaker at the
microphone reaches the same
level as the performer at the
microphone. The funny thing about ISL is that you'll
gain the same amount of MGBF from halving the distance from the performer to the mic (which is usually small anyways) that you will from doubling the distance from the
speaker to the mic which is a much larger distance.
Don't put any of the choir mics through the
stage monitors; it will kill your MGBF. Only run the accompaniment through them so they can hear what they're doing. Not only do vocals in the monitors introduce
feedback problems, it has a psychological
effect on the singers. When they hear themselves in the monitors louder than they hear themselves normally, they will have a tendency to back off a
bit which doesn't help them or you.