Feedback Management

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Re: Feedback Management

Post by blinddrew »

Nice hall but I can see why feedback is a problem. And, frankly, is probably always going to be a problem.
Is there any scope to get the speakers moved? Perhaps a flying installation above the stage so that they can be at least in line with the microphones?
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Re: Feedback Management

Post by Bob Bickerton »

The problem is clearly the speaker positioning and as has been said feedback eliminators have very limited value if you’re breaking the laws of physics.

You say your main issue is with recital lectures. Are you talking about a lecturer speaking whilst musicians play acoustically? Average lecturn mics can be feedback prone and maybe an alternative solution could be found there.

If the problem is from amplified musicians then you really need to deal with speaker placement.

Having said that, I found the QSC TouchMix desks pretty handy at eliminating feedback. Check out https://training.qsc.com/mod/book/view.php?id=595 if I was in a hurry I’d use their eliminator to identify and notch out rogue frequencies, but never used the roving function.

You could/should train your students in basic feedback management before letting them anywhere near a sound system anyway ;)

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Re: Feedback Management

Post by Exalted Wombat »

I hope we can assume the microphones we see in the picture are a recording rig, not for pa?
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Re: Feedback Management

Post by twotoedsloth »

Hello All,

Thanks for your helpful posts.

First, Exalted Wombat, yes the ORTF rig in the center of the stage is for recording only.

Next, blinddrew, we're not likely to have budget to move the speakers, but I will continue to ask for this.

Bob Bickerton, the reason lecture recitals are problematic is that you have to alternate between settings suitable for speech, and settings suitable for classical chamber music, or small jazz ensembles. I am not likely to buy a new mixer, but if I do, I'll look closely at the QSC Touchmix. Is the touchmix that much better than the dbx AFS2 when it comes to notching out offending frequencies?
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Re: Feedback Management

Post by Bob Bickerton »

twotoedsloth wrote: Sat Jul 16, 2022 4:11 pm Bob Bickerton, the reason lecture recitals are problematic is that you have to alternate between settings suitable for speech, and settings suitable for classical chamber music, or small jazz ensembles. I am not likely to buy a new mixer, but if I do, I'll look closely at the QSC Touchmix. Is the touchmix that much better than the dbx AFS2 when it comes to notching out offending frequencies?

I can't really comment as I don't know the DBX.

I'm a bit confused about the input source of your feedback (cognisant that the speakers are in a poor position).

Are you saying that feedback is a problem for the three scenarios you mentioned? Is the 'speech' scenario from just one mic, multiple? Lavalier? Headset? Lecturn?

I don't understand why you'd get feedback for classical chamber music - is it from a presenter or lecturer mic?

With Jazz Ensembles, is the feedback from miked instruments, soloists?

Not being critical here, just trying to understand the problem more fully - and not sure I can help.

And another thought - are the speakers shown in shot the only FOH speakers or are others flown? They would be working hard to fill that size of auditorium and pretty much impossible to do without feedback. Can you move them?

Bob
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Re: Feedback Management

Post by twotoedsloth »

Hello again, and many thanks for all of your advice.

The problem with the classical lecture recitals is that to avoid feedback I have to notch 2.5kHz and 4kHz for speech to prevent feedback, but for recording I do not want those notches present, so I have to return to a flat frequency setting when the presenter is not speaking. Usually the presenter is speaking through a wired mic at the lectern (most of the time a Sennheiser e935, but we can do an SM58 if the presenter wants it), on request we can also do a lav mic.

With the Jazz recitals we get feedback on the double bass mic and the piano mics, the solo horns and guitar are usually fine without tweaks.

So, I guess you are all saying that a feedback eliminator is a waste of time and money... do you think it could have even a small incremental improvement in feedback rejection?
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Re: Feedback Management

Post by Wonks »

To operate the system in the way you've described, someone is going to need to turn the feedback eliminator on and off or bypass it in the way you are currently doing with the graphic.

It sounds like you probably aren't setting up the FOH graphic EQ and the recording path correctly if the graphic is affecting the recordings from the desk. Possibly worth starting another thread if you want to describe the desk and how the FOH graphic and recording outputs are configured.

The different scenarios you've given mean they each will have its own problem range of feedback frequencies, and what a feedback eliminator learns for one set-up will be wrong for another.

There is normally a mode where a feedback eliminator will reset a notch filter if there hasn't been any feedback on that frequency for a given period of time. That's the sort of setting you'll need if you want to leave it to do its own thing and not have to manually switch the unit in and out. But it takes time, so after setting up for feedback suppression for speech, if the presenter then plays back some example music, that music will still be affected by the feedback filter notches until the filters have fully reset themselves.

But go on a pro PA forum and no-one likes leaving a feedback suppression filter in auto mode as the notches do tend to get deeper and wider and affect the sound more and more. With your double bass example, you could end up with almost all the bass end notched out. Or if you limit the filter notch width, then a filter may not be able to eliminate all the feedback.

Some eliminators allow you to select the number of fixed filters vs the number of filters that are automatic. The fixed filters are set for the system before any performance by turning up the system volume until feedback occurs and a filter kicks in, and then you turn the volume up again until the next filter kicks in, and repeat until all the fixed filters have been activated. You can then leave some filters as automatic to catch any feedback that occurs during the performance. But these would normally be limited in notch depth and width (if the system allowed) to minimise the damage these can do the sound.

Whilst the feedback detection algorithms have got better over the years, there is still the chance that they can mistake a musical (or non-musical) note for feedback (say a guitarist with an amp on stage creating feedback deliberately), then the filter would try and notch that out from the FOH sound. Which is why FOH sound people prefer to do things manually.

According to the dbx literature, the dbx AFS2 and the dbx Venu 360 have the same anti-feedback algorithms, the Venu just seems to have more inputs and outputs than the AFS2. Which, as you only appear to need two channels, the AFS2 should do the job.

The dbx Drive Rack series started out as a combined 1) crossover for passive component speaker systems with power amps, 2) a FOH graphic EQ auto setting device using pink noise and a real-time analyser, 3) a time delay function for delaying the signal to secondary speakers forward of the stage speakers and 4) some feedback elimination thrown in to use up the remaining available DSP.

They've got more sophisticated as time has gone by, but the crossover function is required less and less as people install active speaker systems. Most of the other functions offered are standard featured in digital mixing desks, so there's often now no need for one at all.

I'd imagine that it should be possible to hire one for a week or so, and it would be informative for you if you could install one and play with it if you have a week of mixed performance styles that covered all the normal scenarios. Hopefully these would be non-critical internal events where if feedback did occur, it would be known beforehand that tests were being carried out to avoid complaints.

It may do what you want, it may not. But you'll never really know until you try one out.

We've tried to highlight the issues with this type of device and that they have definite limitations if used in auto mode for a wide variety of amplification uses. Note that it only takes someone to say substitute an omni pattern mic for a cardioid one because they prefer the sound, before you get to a situation where a feedback suppressor can't cope and you need a knowledgeable FOH engineer to step in and fix things.

But it does sound like you may need some help in the recording vs FOH feed configuration on your desk.
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Re: Feedback Management

Post by Murray B »

What wonks said, put an eq in the path of the lectern mic, You might need to purchase a separate one to use and a insert in the channel depending on the routing/ patch-bay options on your current mixer and/or if you are using a inbuilt graphic or still need to have the existing graphic in the path of the overall mix.
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Re: Feedback Management

Post by Bob Bickerton »

Wonks has covered off on this well and I think you now understand feedback eliminators can be more trouble than they're worth - especially in the hands of students who may not understand the consequences of the negative impact.

Here's a few comments on the latest information you have given us:

twotoedsloth wrote: Sun Jul 17, 2022 5:54 pm The problem with the classical lecture recitals is that to avoid feedback I have to notch 2.5kHz and 4kHz for speech to prevent feedback, but for recording I do not want those notches present, so I have to return to a flat frequency setting when the presenter is not speaking. Usually the presenter is speaking through a wired mic at the lectern (most of the time a Sennheiser e935, but we can do an SM58 if the presenter wants it), on request we can also do a lav mic.

It sounds like you're taking the recording straight off the desk - what model is it? Best practice would be to take your recording feed pre Graphic EQ - or even better set up an alternative mix for the recording altogether. That way your notches can stay in place and not effect the recording.

twotoedsloth wrote: Sun Jul 17, 2022 5:54 pm With the Jazz recitals we get feedback on the double bass mic and the piano mics, the solo horns and guitar are usually fine without tweaks.

Again with so little information it's hard to comment, but it wouldn't be unusual to run separate mics/channels for FOH versus recording. The fact that you're having to deal with feedback means your sound is compromised anyway (largely by the speaker positioning). For the piano you might consider popping a contact mic on the piano for FOH or a condenser with a (home made) rubber ring into one of the frame holes on the piano but send a nice pair of condensers to the recorder (this can work well for a situation where you're having to send piano/bass to monitors around the stage). For the bass could you send a pick-up, contact mic or clip on to FOH and a nice mic to the recorder?

Just ideas as we're working in the dark here.

twotoedsloth wrote: Sun Jul 17, 2022 5:54 pm So, I guess you are all saying that a feedback eliminator is a waste of time and money... do you think it could have even a small incremental improvement in feedback rejection?

Maybe - but only in the hands of someone who knows what they're doing and you appear to be looking for a catch all solution that students can use.

Bob
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Re: Feedback Management

Post by shufflebeat »

It's a shame that people go to such lengths and expense to design and build such lovely spaces then equip them in such an ill-informed way. I've seen it often in lecture theatres where speakers seems to be placed for PowerPoint media playback rather than mic'd presentation.

Whatever you do to fix this scenario will be a compromise, solving one problem and causing others. The only proper solution is to move or replace those speakers.

I've used a couple of feedback eliminators and have found them useful but not when used as designed. The "seek and destroy" function is a handy shortcut but I always set one or two filters to catch the worst offenders, reset the depth of cut from -20 to -5/-8 and widen the bandwidth. It was really just "ringing out" with the listening part partially automated and not the fully automated feedback system which promises a simple solution to a complex problem but can't possibly deliver.

Is it possible to suggest just putting the speakers on tripods when appropriate? For reinforcement purposes I'd like to see these in front of the mics and off to the sides but even up in the gallery (is that a gallery above the projector screen?) might be an improvement so they're not pointing directly at the mics.
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Re: Feedback Management

Post by twotoedsloth »

Hello,

I truly appreciate the help you are offering.

The signal path is as follows: mics (e935, SM58, e614 or ewG4 wireless lav),
cables (star quad), mixer (A&H SQ5) and speakers (currently Tannoy System 15, but will be swapping in Meyersound Ultra X40s, and Yorkville NX610 monitor speakers).

I use the graphic EQ on the SQ5, and I manually adjust this to prevent feedback, and switch it back to flat for music recording.

For big band (Jazz) we usually put put Tannoy speakers on tripods at the front of the stage, but for small Jazz ensembles we just use the hanging speakers.

Does that clear things up?

Sincere apologies for not being more forthcoming.
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Re: Feedback Management

Post by Wonks »

Still not quite sure where you are taking the recordings from so the graphic EQ affects them.
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