It seems to be common for small churches to suffer from instability in the audio system design. I think it has to do with older buildings not having been designed for modern sound reinforcement technologies. So problems can occur when those technologies are introduced, such as sensitivity to feedback. A good 30-band equalizer can help stabilize such venues, as well as provide other benefits.
One of the problems that occurs is that sanctuaries can have one or two dominant resonances — frequencies at which they feed back very easily. This makes it impossible to turn up some, or all, microphones as much as the sound tech would like, because the system starts to squeal. Or perhaps the system is not squealing, but another microphone is simply turned on without changing volume. Each active mic on the stage increases sensitivity to feedback by a small amount, so turning on another mic can kick the system into unstable operation, and it squeals.
Although sophisticated devices exist for eliminating such feedback, it’s often true that the picky frequencies are relatively stable because they depend on the overall dimensions of the sanctuary. That doesn’t change. So if those frequencies can be tamed, the whole system behaves better. The key is to find out which handful of frequencies (the main resonance and perhaps some harmonics, or multiples, of that frequency) are the issue, and notch those out with an equalizer (EQ).
A good 30-band EQ provides fine enough notches that one band can be brought down quite far without anyone noticing. The human ear just isn’t sensitive to that sort of change for most people.
There are several ways to detect problem frequencies. Some people have trained hearing, so they can determine the problem frequency just by listening. I once worked with an audio consultant who could do that, but I can’t! Instead, I have an app for my smartphone that shows the signals it is picking up. With normal sounds, signals are present over broad ranges of frequencies. But when feedback occurs, you can see clear lines on the display at the problem frequencies.
At that point, all you have to do is notch the EQ at that specific frequency. (Sometimes it’s helpful to take the bands on either side down a little, too, but not more than 3-6 dB.) You may need to do that at several harmonics, but once that’s done, the feedback should be much harder to generate and the whole system should be more stable.
Note that this needs to be done for both the monitors as well as the mains. I’ve only dealt with single-monitor systems, so two channels are enough to handle the whole venue.
An EQ can provide other benefits, as well. For example, by softening high frequencies, the overall sound is more mellow and acceptable to a broad range of people. Although many people like a “bright” sound that results from crisp, accentuated highs, the reality is that such sounds are fatiguing for most people. And some people find them irritating. When the intent is to bring a diverse group of people into worship, you want to avoid things that irritate people. This is just my opinion, of course, but I’ve seen this born out at least once.
Sometimes an EQ can be handy for filtering out discrete noise sources, too. This is less common, and I don’t have as much experience with it, but might be handy for some people. You can also make arguments for utility in controlling overall “bass-y” sounding low end. But again, I don’t have much experience with that, so can’t provide much guidance.
A good, 2-channel, 30-band EQ isn’t that pricey, and can make public experiences much better for everyone. In my experiences, it’s been a good investment.