I played in an Australian Bush band for many years, in halls that ranged from nicely lined barns to tin sheds. We used a range of acoustic instruments from fiddles to mandolins, banjo, guitar, whistles, etc. I remember the first time I played in one of the tin sheds, and the noise was unbelievable. When you played, say, a guitar chord, you didn't hear it out of the monitor, but off the back wall half a second later! I remember trying to stay in beat by watching everyone else's hands. Trying to turn up the foldback lead to feedback.

Eventually we got ourselves good 31 band graphics on both front of house and foldback, and went through a set procedure.

1. Set FOH and FB master faders about two thirds up. Set all graphics flat. All other faders off.

2. Starting with vocal mics one at a time, set fader about 60% up and adjust trim so that FOH is about right. Set tone controls to suit vocalist. Adjust FB send so that vocalist is happy. Turn fader off.

3. Set instruments one at a time using similar procedure. If you know particular instrument has a particular feedback problem (e.g., boom), then make some compensation with tone controls.

4. Wind back master faders 20dB, and all individual faders back up to preset value.

5. With each vocal mic place a hand flat about 2 inches away, to duplicate sound bouncing off your face into the mic. Leave all acoustic instruments on.

6. Slowly wind up FOH master until something starts to take off, and then wind back a notch.

7. One channel at a time, bring up the FOH graphic until feedback occurs, and then wind back a set amount ( e.g., 12dB).

8. Repeat procedure for foldback.

9. By this time you should have notched out the booms and most of the other room resonances. However, you can still get high frequency reflections off flat instruments, so get every acoustic musician to hold their instrument about the right distance from their mic, and run through the same procedure for the 2k - 10k range.

The advantage of this procedure is that you have tuned the PA to the room, and the overall sound will be pretty smooth, because the overall gain is fairly flat over the whole frequency band.

Once the whole band is used to doing it, it takes about 5 minutes, and everyone ends up happy.

You also have a fair bit of head room if you need to crank up a bit.

I have only limited experience with active filters, but I formed the opinion years ago that they are not as good as properly tuning the graphics, because you start off with an unequal frequency response, and the filters act randomly. The sound is not so good.

I have played in loud clubs with concrete walls and ceilings, and was not happy with the result. In these cases the 12dB range on my graphics was not enough to notch out the resonances. In one extreme case I remember putting two graphics in series and notching up to 20dB - but that was unusual.

My band has played in large carnivals, etc., with professional sound guys, who seem almost universally to be deaf ex rock musicians. I have experienced boom problems with them, because they did not understand acoustic instruments.

I hope this is of some help.

Regards
Chris