Sorry for late reply. Thanks to everyone for great advices! I knew most of that, but still thank you.
However, many advices are not applicable in my case, especially the ones to do a proper sound check. Because I’m talking about a festival here, where bands will be rotating quickly and there’s basically no sound check for every new band, except for the first one. They play couple of notes – “Can I hear myself? Ok.. ” – and rolling the song into PA. They don’t want to play a whole song into monitors first – nobody wants to spoil the songs before the audience, you know…
Actually, for the upcoming event I found someone who will direct me on monitors mix over radio when I’m no available on stage. But just as a backup/experiment I will probably tape a wireless lavalier to a mic stand and try to monitor that in headphones and compare that with directions from the monitoring person. We’ll see what happens...
I do see what your getting at but you won't get enough information from that mic to really do much besides hear an obvious bit of feedback, hopefully find the offending frequency and then cut it in all the wedges because you don't know which mix it was. And when down in the trenches of "combat audio" sometimes you have to do such drastic things but I prefer not to.
When I'm handling wedges either from a monitor desk or from FOH I always want to determine the source of the feedback when at all possible. This is often the most difficult task. Is it the house? Is it in the wedges? Which wedge? What input? If I'm lucky enough to be at a monitor desk with a que wedge I have a fighting chance.
If I'm out front during a show I first have to determine if it's house or wedges. During a spot between songs I'll push the whole house mix up a bit and see if I can find a hot spot that just starts to take off. I then use my ears or SMAART on my ipad or laptop to pinpoint the offenders and make cuts at at the graph. I do the same with a single channel if I suspect it's just a single channel. Acoustic guitar, mics on quiet acoustic instruments and quiet shy vocalists are the most common.
If I've determined it's in the wedges then I have to look for the most obvious culprit. Usually vocal mics and acoustic guitar are the problem so I'll use the seat of my pants techniques like knowing the gain structure of your system to determine what the hotest mic in the monitors is and once again use SMAART in spectrograph mode or my internal SMMART to make a cut. I'm not into guessing anymore what frequency it is so if I'm not pretty damn sure I defer to the software. It's $50us for the app on an ipad and iphone and it's the best $50 bucks i've spent in at least 10 years.
This entire scenario is to be avoided by properly setting up wedges ahead of time however. Before almost every show I walk up with my ipad and get the vocals and/or instrument mics up to a nice loud starting level with appropriate high pass filter engaged. I do this even if I have to run back and forth myself for 10 minutes or so. If I have another engineer with me I have them bring up the level slowly until we get the beginnings of some feedback and I find the frequency and pull it at the graph and repeat this several times. I do this till I've pulled 3 or so bands but not much more then that because I don't want to hack up the wedges till they sound bad. I also may pull a couple of bands for over all tonality. You can do this with ear plugs in and watch SMAART if you want for the feedback sniffing. When All that's done I've got a barking loud wedge and I back it down to a reasonable starting volume. This way I know I have that headroom for later on in the show. Now if someone swaps mics on me things change but That's to be avoided if you can help it all.
The other way to avoid your problem scenario is to manifest a monitor desk and split and have a second engineer handle that big. I built a 40 channel split from parts for about $200 and it's been solid. The club had an older unused Allen and Heath lying around and it's now a decent monitor desk. Rack your monitor EQ's up separately so they can be used at either end of the rig. If your thrifty like me you can get a whole simple monitor world worked out for under $1000.
Until then I'd say ring out your wedges ahead of time, keep your eyes open for the usual suspects and by SMAART or similar for the ipad or iphone. Life will be much easier.
Also depending on your monitor rig you may need to add some PEQ into the chain to flatten out the response of the actual wedges. They'll become MUCH more stable and less prone to feedback. That's if they're older passive designs like the Yamaha Club series or similar. All our wedges are passive and I use BSS Soundwebs for this to great effect. Many loud rock shows go by that I don't touch the monitor graphs at all.
