Feedback Issue - Need Urgent Help!

[rostikodak]

I have a question regarding feedback control in a church sanctuary.
I will describe the system a bit and the main problem. I would like to get some responses as how to go about fixing the current situation.

We have an allen heath gl3300 with 9 choir mics, 8 vocal (sennheiser 300 G3), 2 temporary shure beta 58’s as x/y stereo in almost closed piano – we are planning on replacing with a stereo set of condensers to close the grand lid all the way, and an earthworks fm360 podium mic.
This whole system is send into a time-delayed array of 8 speakers with 2 of them being subwoofers.
The FOH 1st set of speakers is about 5 feet in front of the stage. And it is aimed downward as the whole array is flown. The next set of speakers is around 16 feet back and so on. We currently have a limited monitor situation and have two jbl floor monitors and 2 older speakers facing the choir – they are set fairly wide and are nearly at the same plane as the 1st set of FOH speakers.
THE PROBLEM: we cannot get an adequate signal in the sound system because of feedback. We have isolated the feedback and it seems to come from the FOH system. The floor monitors do not have any issues with feedback and the top monitors only have an issue when we start sending the choir mic signal back at the choir.
Our podium can never be turned up to a comfortable listening level as the microphone starts feeding back because of the FOH sound.
Also, the choir mics cannot be brought up to be loud enough without the rear mics feeding back the subwoofer signal! The subwoofers are at the sides of the stage with their backs to walls that close off a hallway behind the stage. We do have higher levels of subwoofer frequencies in that hallway and I suspect that the subwoofers create a bigger sounding box with that hallway and maybe leak that signal into the mics? Going out on limb there…
We are currently running the amps at full on all the speakers and the sabine navigator processor seems to be not helping either with constant preset changes.
Our goal is to get rid of the feedback that is stopping us from getting even adequate listening levels for the speakers and worship groups that sing to soundtracks and the grand piano to the right of the stage. We would like to go with a live band – but in this situation it’s impossible.
How would I go about fixing this issue? Is the placement of speakers the issue? It was tuned by a “professional who has decades of experience with live concerts” and I can say that if you play a song off a cd through the system it does sound good – response has been tailored well. But everything else does not because of the issues.
I can draw up a diagram of the hall if that will be necessary. Also gear list and routing as well, if needed.

Thanks for any help/tips/ and suggestions!

 

[gcpsoundlight]

From what you are describing, it sounds as though the gain on the choir mics/podium mic is too high, or the microphones are too sensitive. If you dont already, familiarise yourself with the operation methods of Dynamic and Condensor microphones. I think what you will find is that the feedback from the podium is just due to a very sensitive mic being too close to the foh speakers. The reason you would be getting sub feedback is that bass is very omni directional, and if you are in a hall with wooden surfaces this will increase the problem.

Hope this helps!


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[bishopthomas]

I agree that it could be your gain structure. You'll need quite a bit less gain for condensers such as that podium mic than you would for dynamics like the handheld vocal mics. There's no reason for any of those microphones to be have signal going to the subwoofers. High pass them and it should at least help. An aux fed sub will completely eliminate it but that's an explanation for another post/thread (do a search). As you've found out, choir mics into the choir monitor will feed back, just avoid it. Choirs should be able to sing and blend acoustically. If you have to put piano or tracks in that monitor that would be acceptable.

What is the Sabine processor doing? Why would you change the presets? A drawing might help, or at least the answers to these questions: What are you using for FOH speakers? What would you say the angle is from the podium to the face of the speaker? Can you see the front of the speaker from the podium? Getting the installer back for some more in depth training might not be a bad idea. There are a TON of variables that we will never know here in Internet Land. Good luck!

 

[museav]

You might start with some of the obvious factors such as what the choir microphones are and whether you really need 9 of them. Also, the actual speaker models and their height and aiming. Do you have the 100Hz high pass/low cut filter engaged on the mixer for the the podium and choir microphone channels? What are the frequencies and slopes for the subs to mains crossovers?

Any chance that you have a person at the podium with them also having one of the wireless mics? What is the podium surface? Do people take care to aim the podium mic at their mouth? The FM360 is a cardioid mic with a nice rear notch at 2kHz, but it also has a fairly broad pattern and much less rejection at lower and higher frequencies. The FOH speakers also probably lose pattern control at lower frequencies, and if compact speakers well into the speech range, and may even exhibit 'pattern flip' where the vertical coverage becomes greater than the horizontal coverage for some frequencies making for a potentially poor combination.

I agree with bishopthomas that this sounds like the kind of situation that someone might walk in the room and figure out in a few minutes but that could be challenging to try to address via an online forum.

 

[mbenonis]

Lots of good advice above, and as all have said there are a ton of factors here. Instead of repeating what's been said, I'll try and describe how I would tackle this if I walked into the space.

Start by isolating the lectern* microphone. Mute all other microphones and make sure they're not going to any monitors either. Get someone to stand at the lectern and speak into the mic the way folks usually do. Take the mic out of all speakers except the front array (killing amps may be the easiest way to do this), drop the console gain control all the way, put the fader at unity, and ask the person to start speaking. Bring the gain control up slowly (and put the mic in PFL so you can watch your levels). Make sure all other master faders are at unity. Watch your meters and see how things behave. Can you get enough volume without feedback?

By the way, this is a very good time to examine the gain structure. If the volume is way too loud/feeding back with barely any gain on the console, or levels way below the unity mark on the meters on the console, your processing and amplifier chain is set way too hot. In a properly configured system, at what you might consider "average" listening levels, you should probably see ALL of your meters between maybe -10 dB and 0 dB, which gives headroom for the louder parts. Adjust all of the gain controls until you see something like this. There's no one single correct answer but there are definitely wrong answers.

If you still can't make things work, it's time to start futzing with the system. Get the mic closer to folks' mouths, adjust the angle to try and null out the speakers, physically move things around, etc.

If you can get that happy, then work mic by mic, individually, until you've gone through the whole setup. Then start adding/layering things one by one.

Not sure how you are actually mixing the show, but you may need to be very proactive in ducking mics not in use rather than leaving everything up at once. This takes a skilled individual, but it is something you can learn.

Please let us know how things are going and we'll help as much as we can!

*A podium is something you stand ON, a lectern is something you stand AT.

 

[museav]

*A podium is something you stand ON, a lectern is something you stand AT.

I agree, however even the folks at Merriam-Webster have apparently given up and now include lectern in the definition of podium. Apparently if a sufficient number of people misapply a word often enough they change the definition to fit the common use.

Also, as has been pointed out to me, a microphone may be attached to a lectern but the lectern is often on a podium and thus the microphone could be a lectern microphone (specific location) or a podium mic (general location), similar to the concept of a microphone on stand located on stage being a stage mic even though it is physically on a stand rather than actually on the stage.

 

[65535]

If using handhelds with feedback, lower the mic gain and tell the talent to speak/perform INTO the mic not straight past it.

 

[FMEng]

There is a lot going on here but a few things come immediately to mind for you to consider.

1. The potential system gain before feedback drops 3 dB each time the number of open mics doubles. It isn't uncommon to see more mics on a stage than is necessary and they never turned off.

2. Any mic that isn't there to pick up a source with low frequency sound should have its high pass filter engaged on the console channel. For example, choir mics don't need to reproduce anything below 100 Hz, so if they are high pass filtered, they won't be feeding back from the sub woofer.

3. Nine choir mics seems like a lot. If they are spaced with the proper 3 feet apart to every 1 foot from the source, that would be a very big choir. Too many mics actually sounds worse when the 3:1 rule is violated.

4.. Turn off the subs and see what happens. It may help identify what is happening. You may that they are too hot or not needed at all.

 

[David Ashton]

your phrase, "all the amps are at full" is very concerning, it usually means your gain structure is wrong, unless pads have been put in the system.Gain structure, as others have stated is the single most common problem.Too many mics is the second most common problem, especially if they are all left on at the same time, every mic added reduces the overall gain before feedback.Cut the bass in your choir mics, there is no choir content in the sub frequencies.Choir mics are omni-directional, they pick up the choir and also pick up the sound from everything else including the speakers, you cannot send an omni signal back to the choir without feedback.Get your gain structure sorted, try using less choir mics and forget trying to foldback a high level back to the choir.

 

[museav]

your phrase, "all the amps are at full" is very concerning, it usually means your gain structure is wrong, unless pads have been put in the system.Gain structure, as others have stated is the single most common problem.

Very true, I'm also not clear about the "the sabine navigator processor seems to be not helping either with constant preset changes" comment or what that means.

Choir mics are omni-directional, they pick up the choir and also pick up the sound from everything else including the speakers, you cannot send an omni signal back to the choir without feedback.

I don't think we know what the choir mics are, they could be anything and cardioid choir mics are quite common. In fact I know of situations where aiming the choir mics to maximize rejection of the speakers rather than maximizing pickup of the choir improved gain before feedback. If by adjusting the choir mix aiming you lose 2 or 3dB for the choir but reduce the speaker contribution by 10dB then that is 7-8dB more gain before feedback.

Similar for the lectern microphone, you might have to play with whether it is better to maximize pickup of the wanted sound or minimize pickup of the unwanted sound. And don't forget the potential contribution of reflections off the lectern surface for both the person speaking and the speaker sound, the relationship of the direct sound and the reflected sound can often lead to phasing issues and if you happen to have a summation at a critical frequency that could be up to 6dB less gain before feedback than if you had no reflected contribution.

 

[dboomer]

Proper "Gain Structure" can certainly help with some things ... but it does NOT cause feedback. Don't confuse it with "Gain" which does cause feedback when it rises above unity level (in other words when the mic hears the sound coming back from the speaker at the same level as it hears the sound going into the mic. Pretty difficult to diagnose across the internet.

If you have a Sabine Navigator and you have some unused I/O you could break out a channel or two just for FBX. The best way to use it would be to insert it one the most problematic mic or mics. If it is a large number of mics then you should buss them all to a subgroup and insert the FBX on the subgroup.

 

[TimmyP1955]

How about some photos, especially of the choir.