# Feedback Destroyer



## avkid (Apr 29, 2005)

I am currently looking for a feedback destroyer of some type that is fairly inexpensive, used(in good condition) or new. If you have any suggestions please relate them to me in a timely fashion.


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## koncept (Apr 29, 2005)

http://www.google.com/search?client...l=en&q=feedback+eliminator&btnG=Google+Search

it looks like one or two of these might be of interest to you


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## BNBSound (Apr 29, 2005)

My advice: don't get a box to do your thinking for you. Feedback Destroyers aren't a good substitute for a well set up EQ. You can train your ear in a matter of weeks to recognize the frequency of feedback and just notch it out manually.

Destroyers are notorious for "deleting" things like flutes, guitar feedback and even female vocals that they interpret to be feedback. And they often don't react quickly enough to correct for catastrophic feedback and even if they do they tend to destroy your frequency response as well as the feedback.

So I would seriously consider getting a good graphic EQ and using this free trainer to get your ears in shape. http://sft.sourceforge.net/


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## freshmantech (Apr 29, 2005)

I agree 150% with Jon (bnbsound). A well structured EQ would be your best bet as well as training technicians. I would also suggest watching where your feedback is coming from - ie: singer standing 3 feet from one of the main speakers is going to cause problems even for a good tech. You have to figure out the dynamics of your space and give an approiate microphone as well. We've taken all of our feedback eleminators out of our racks and use them as paper weights now - they were (as was said above) eather too much or would react too slowly to do anything that our audio tech's couldn't do themselves if trained.

Good Luck!


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## VipermanGTX (Apr 30, 2005)

I Agrre with the others.Training your ears is the great feedback destroyer. But if you realy want a machine to do the work for you i'd say Berhinger makes some cheap yet effective.


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## JP12687 (Apr 30, 2005)

is there anything out there where i can select the frequency and thats what you hear? cuz this gives it to you and expects you to know it..but if there was a way to play everything 1st and listen then this would be more useful.


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## koncept (Apr 30, 2005)

i just gave it a try and would definately agree. i wouldnt mind learning what they sound like but with out knowing then im just guess n check ing


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## stantonsound (Apr 30, 2005)

Although I agree with the last few posts that a well set up system with a trained person at the EQ is best, I will argue that these destroyers do have a place. As a pro sound technician I often am forced to do a quick PA set up (1-2 hrs) in a location that I have never been in before and jack-a**es using the mic's that do not care enough to learn how to use them. (Corporate meetings are the worst). Although it is common sense, they do put the mic in front of the main stacks and countless other stupid things. You can try to teach them, but they don't listen.

I feel that the destroyer gives me a safety net. It allows me to cue the next CD or to put the bodypack bact together that someone dropped. If feedback occurs, the audience rarely ever notices that a part of the signal went out for a minute when the destroyer acts and I can fix the problem without the squeel. Feedback squeel is always seen as an error of the "sound guy" .

I don't use anything fancy, the Behringer unit is only a couple of hundred bucks and it has worked fine for me. It is easy to use and fairly versatile.


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## mbenonis (Apr 30, 2005)

However, given a few weeks to set up for a show, an EQ is the only proper instrument to take care of feedback. In my eyes, a feedback destroyer is a tool to be used when there is insufficient time to properly notch out feedback frequencies - a last ditch resort, if you will.


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## BNBSound (May 1, 2005)

stantonsound said:


> As a pro sound technician I often am forced to do a quick PA set up (1-2 hrs) in a location that I have never been in before and jack-a**es using the mic's that do not care enough to learn how to use them.



That's all I do, and I've gone months at a time with no squeal at all. EQ's not easy to learn, but once you do you can just about smell a room and know where it's going to ring. 

And to the guy playing with the feedback trainer, if you click above each notch you can sample the sounds.


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## avkid (May 7, 2005)

Problem solved, came in last night and found a Sabine FBX automatic feedback controller: either The FBX1200 or FBX2400, with SMARTFilter™ Technology. (I have not had a chance to use it yet because I had to help replaced a dismissed lighting crew, in the last week of a 4 week show no less!)


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## JasonH (May 7, 2005)

stantonsound said:


> As a pro sound technician I often am forced to do a quick PA set up (1-2 hrs)


Whats a slow PA setup?

What is the saying? “How long is a piece of string?”

I guess the answer would be relative to the size of the set up and how much equipment is “in house” and how much set up time is wasted by others.


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## blademaster (May 9, 2005)

I don't know but about the quickest ive done a setup for my church was maybe 1hr, tops. Slowest took about 2 or 3hrs


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## Mayhem (May 9, 2005)

JasonH said:


> stantonsound said:
> 
> 
> > As a pro sound technician I often am forced to do a quick PA set up (1-2 hrs)
> ...



What is the saying? “How long is a piece of string?”

I guess the answer would be relative to the size of the set up and how much equipment is “in house” and how much set up time is wasted by others.


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## blademaster (May 10, 2005)

LOL only too true


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## Eboy87 (May 25, 2005)

I may be a little late on this, but, I hook a Behringer feedback destroyer (*dodges townsfolk with pitchforks*) into the inserts for the mics next to the monitors. It really helps ease the headaches when running the show and playing guitar at the same time.


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## avkid (May 25, 2005)

We no longer have the need for one, we had a consultant come in and exhaustively eq our uni-points (main source of feedback)


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## BNBSound (May 28, 2005)

A well EQ-ed system will almost never be in need of a Feedback Destroyer. I have one on my Behringer digital EQ and it never trips becuase I always take a minute or two to ring out the room.


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## bahaha (May 28, 2005)

On the driverack 260 the "advanced feedback suppression" is split into two parts, fixed and live. Am i correct in thinking that the live section is like the feedback destroyers mentioned earlier, and that the fixed section works like an eq by cutting at certain frequencies?


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## Andy_Leviss (May 31, 2005)

Not quite; feedback eliminators generally have two types of filters, fixed and dynamics. Both are very narrow notch filters, the difference being that fixed ones are set when you set up the unit in a new venue by bringing up the system until the first feedback frequency occurs, letting the unit find it, then turning up again, and repeating until all filters are set. These filters get locked to these frequencies, on the assumption that they are created by room characteristics, similar to when you EQ a room manually, but with narrow filters than you'd have on a 1/3 band graphic (you could get ones that narrow on a parametric).

Dynamic filters are ones that get set during a performance automatically to catch feedback in the split second before it becomes audible (in theory). They can change frequency. Once released when the feedback stops, they can reset to a new frequency. Thus they'll cover stuff caused by mics moving within the speakers' patterns, that may be constantly changing.

I hope this makes sense--if not, try reading the manual for Sabine's products, which explain it really well.

--A


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## bahaha (May 31, 2005)

Made perfect sense. Thank you.


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## jkowtko (Jul 27, 2007)

This is an old thread, but I'll continue the discussion as I need some quick feedback (no pun intended) on a couple of units --

My problem is that the center cluster is several feet behind the front lip of the stage (quarter-round flex theater with stage on floor) and we have not been able to move the speakers forward due to lack of gridwork above where the speakers really need to go (and lack of time to deal with these obstacles)

So, for the time being, I just pull down the volume of the center cluster on the wireless lav input channels (I'm using one of the aux outs for this) by about 10db or so. This allows the actors to go out to the front edge of the stage without feedback, and I can still keep the front wing speakers at full volume.

However I'd like to get that center cluster back up to full volume. I can throw a 1/3 octave EQ on the center cluster, but I'd rather use a 1/60 or 1/80 octave notch filtering feedback suppressor if it will work properly.

Does anyone have experience/comments with the Behringer DSP1100P or DSP1124P, or the DBX AFS224 or Driverack PA? Specific questions:

1) When running live filtering, do they apply the right amount of filtering, or too much?

2) Aside from the filtering, do the units introduce any noticable audio quality degradation or noise?

3) Does the balanced IO work well (since I will be plugging this in-line between the board and the self-powered speaker, I do not have an insert available on the board)?

Thanks. John


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## museav (Jul 28, 2007)

If the problem is that the mic is in the speaker coverage then a feedback suppressor or notch filter probably isn't going to help that much. You are not dealing with narrow band room modes or resonances, you are likely dealing with a broader frequency range from the speaker getting directly into the mic(s). If so, you will likely end up needing to address a broader frequency range rather than narrow notches in order to really get much gain.

If you run the feedback suppressors with all filters 'live' or dynamic, then you will likely end up with them constantly cycling through, which can sound bad. If you do use a feedback suppressor you might want to try to 'fix' several of the filters and not go any narrower than 1/20 or 1/40 octave. However, a smaller, broader cut may actually work better in this situation.

If it is just one or two mics involved then you may want to think about addressing only those channels rather than the entire mix. You could simply use the relevant channel EQ on the mixing console to apply EQ for the mics when needed, just work out ahead of time what EQ needs to be applied when they walk downstage and apply it live. Or you could insert a feedback suppressor or EQ on just those inputs, Sabine makes some units specifically for this application.

The best bet if at all practical is closer micing. Halving the distance to the mic gives you 6dB more gain, which is why headset mics that are very close to the mouth can allow for so much more system gain.


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## jkowtko (Jul 28, 2007)

Brad, 

The actors go to just behind the edge of the "published" sound field of these speakers. So they're not picking up full volume sound, maybe -10db or so coming from the side.

Right now I can more-or-less hear the feedback frequency ... it's been the same for the run of this show. So I figure it I can notch it out, and maybe a few others, the remaining frequency spectrum will still be below the feedback threshold and I'll be able to bring the volume back up a bit on the center speakers -- if I'm lucky, all the way to where I have the wings.

All of my lavs are Countryman B3s, very good smooth response and pretty feedback resistant, and I haven't detected any one unit is worse than the others. So for now I would just plan to filter them as a group (in fact I can't separate the lavs from the other input channels, board's too small, so unfortunately for now I'll have to filter the speaker itself.

Yes, I would plan to ring the house a bit to set a few fixed filters, then monitor during the first two shows to see what else pops up. If the suppressor applies more than a few filters during a show I would be inclined to shut it off for the remainder of the show and just lower the speaker volume back down.

If you've used feedback suppressors, other than the filtering, were you able to notice any degradation in sound quality?

Thanks. John


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## jkowtko (Jul 28, 2007)

Oh, yes, I also have the lavs taped to with 2-3" of their mouths. unfortunately they're kids, so not as loud as the adult actors we have during the season. But yes, I've noticed that it makes a huge difference in volume and gbf when you get those mics close to the sound source.


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## museav (Jul 29, 2007)

jkowtko said:


> The actors go to just behind the edge of the "published" sound field of these speakers. So they're not picking up full volume sound, maybe -10db or so coming from the side.


The published nominal "coverage" simply defines the -6dB point for the overall response, which while it has some use for assessing whether a speaker can cover an area, does not provide a lot of information. The sound from a speaker does not stop at the defined nominal coverage angle, in some cases it may actually increase further off axis for some frequencies. The coverage of a speaker also typically varies dramatically over frequency, for example below crossover you may have very limited pattern control so that while the overall level off axis is down considerably, some frequencies may be down little if any in level. You really need to look at the actual polars or response balloons for a speaker to get a better idea of what really happens, especially at different frequencies.

Notching offending frequencies will help with GBF, but it is not uncommon is to notch out the worst offending frequency and then find others just a few dB below that, much like the old peeling an onion analogy. 10dB is a significant amount of additional gain, especially when the problem may caused by the speaker levels on the stage and there are multiple mics involved, which is why I am concerned about using a feedback suppressor. In order to get 10dB of additional gain you will likely find yourself needing to address multiple frequencies and/or a wider frequency range, while at the same time some frequencies may require much less than 20 to 40 dB in reduction a feedback suppressor would typically apply. For example, you may be able to put a small dip in the response over frequency range rather than having a bunch of very deep notches all assigned in that same range and also end up with a more stable system.

Especially as you feel the problem is the same every time, a multi-band parametic EQ may be a better choice than a feedback suppressor as it allows you a lot more options in the settings possible. However, it does require a little more work or experience since you are making manual settings and have several parameters to adjust. Some of the devices you mentioned include multiple bands of parametic equalization and I would probably try to get the gain desired using this fixed parametric equalization capability and then use active feedback suppression only to address minor changes that may occur as the actors move around, if at all.

The sound quality, as always, depends on the device. It is another box in the signal chain and thus enters into the resulting gain structure. Very steep, very deep notches may cause some ringing that is audible in some applications.


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## avkid (Jul 29, 2007)

Here's a follow up.
We got two Sabine FBX 901's, and I really don't like them. 
They are very loud when inserted into the signal chain, and because of that I hardly ever use them.
Learning how to better utilize the parametric equalizers on your channel strips is a much better way to curb feedback than using an outboard device.


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## SHARYNF (Jul 30, 2007)

This is like the joke of the boater who complains this his bilge pump will not keep up with the leak in his hull. The solution is not to get a bigger bilge pump it is to fix the leak. You need to either move the cluster, or make that section of the stage off limits. You are going to find that you keep reducing frequencies, that by the time you have finished you have effectively reduced the volume of the center cluster. 
Sharyn


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## jkowtko (Jul 30, 2007)

Okay, I got a cheap Behringer DSP1124P and tried it out on three shows this weekend. And ... it worked ... reasonably well, actually 

I connected the unit in-line from the board to the (self-powered) center cluster. Phil, I also noticed the noise when I first connected the unit, but then I changed the "nominal operating level" from +4 to -10db and the noise dropped out with no change in audio level . The Behringer units are probably nowhere near as quiet as DBX, but this adjustment worked for me. So if you haven't tried this already, it's possible this could solve your noise problem.

I was able to ring out the feedback on individual actors pretty easily, to the point where they could stand at the front lip of the stage and I had their mikes up +20db louder than I ever had before, with their voices just screaming through the house.

Dealing with lots of mics on stage and a band blaring in the background was much more difficult for this unit, so I found myself having to pull down the center output gain -3db or so just as a precaution. Since I was also getting a slight amount of reverb during normal dialogue, I left the center cluster gain down for all except for the songs.

So, for this unit I found myself having to babysit it a lot. It cut a lot of the feedback, but without adjustment could not deal with it all.

Now the one thing I do like about the Behringer is that it has great adjustment capability. Using the numeric display I can review and adjust the frequency, Q and gain values for any of the filters it sets, and I can selective set and freeze any of the filters of my choosing. So the unit also effectively gives you 12 fully parameter EQs per channel. (what I don't know is how good the EQ is, though). So on my second pass at using it I did a ring-out, checked the values of the 4-5 filters it set, and then widened them up a bit. Fyi the filters it did set generally had a 1/60 bandwidth, but a couple of them had 2/60, so there is some extra logic in there to widen the notch if needed. The gain it chose was anywhere from -6 to -12db. Also with the Behringer you can specify the sensitivity of the feedback detector, so it's a fairly tunable unit.

In comparison, the DBX does not have a numeric readout and therefore no manual filter adjust capability, so although the white paper describes what appears to be a very good algorithm for detecting feedback and applying filters intelligently, using the DBX would have to be a "set and forget", or "leap of faith" operation. My next goal is to try out the DBX and see if it does a better job.

I agree with everyone about the 1/3 EQ being a reasonable alternative here -- 1/3 octave is obviously better than dropping the speaker volume (what's that, an 8 octave EQ?) I do have a DBX 31-band EQ and can run it in-line with the feedback suppressor, so over time I should be able to identify the problem frequency areas and make adjustments to allow me to run full volume without issues.

Sharyne, yes the ultimate fix here is to remove the problem (i..e move the speakers downstage), but what I'm also realizing is that with a flex theater (which ours is) the stage ends where the audience begins so I also need to make sure the PA covers the front rows of seats. So unfortunately it's a balance between sound coverage and feedback avoidance, and I think there will always be a grey area in there where I have to walk on eggshells. As an engineer I always welcome a good challenge, and this one is no exception. So between standard EQ and some notch filtering I'm hoping to have this problem licked by the time our next show starts in September.


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## Hughesie (Jul 31, 2007)

Feedback destroyers, seem like the simple way out
the quick fix, quick fix they ain't

i agree with everyone else good eq is the way to go, always

that's my two cents


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## SHARYNF (Jul 31, 2007)

The problem also seem to be if I understand you correctly the center cluster is not lined up with the side speakers. to properly set this up you also need to have time aligned the cluster and sides so that the signal all appears to have come from the same plane.

When you use a eq to reduce feedback in a monitor situation it is one thing, and in most cases the monitors are positioned behind the mics. In front of house feedback reduction should be first resolved with placement, and then the eq is being used only to reduce frequencies that are being emphasized by the speaker system. In your case I would be concerned that you now have higher GBF but with a simple sound check it is difficult to determine the quality of the audio that this cluster now provides.

Using the center cluster to cover the front rows in what I think is your setup also is not a good idea, because you are likely to get complaints about higher sound levels than the audience wants, the sound from the distance cluster and the actors on stage will cause sound problems . Typically in your sort of situation small speakers are arranged across the front of the stage, and the level controlled such that the audience in the front rows gets the correct level but is not blasted out 

I guess it is like the idea of pointing the speakers away from the audience to reflect on lanai panels, it "works" but you quickly learn that the quality of the experience is not what you are looking for.

I've used Sabine 901's they are a quick fix, are effective to a certain extent in a situation where you have little time or control, but I would ONLY use them on monitors.

Again everyone has their own theory and practice on designing systems , so... 

Sharyn


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## Andy_Leviss (Aug 8, 2007)

jkowtko said:


> Sharyne, yes the ultimate fix here is to remove the problem (i..e move the speakers downstage), but what I'm also realizing is that with a flex theater (which ours is) the stage ends where the audience begins so I also need to make sure the PA covers the front rows of seats. So unfortunately it's a balance between sound coverage and feedback avoidance, and I think there will always be a grey area in there where I have to walk on eggshells.



Again, though, the problem has a more "correct" solution, namely properly selected and implemented frontfills or downfills. It's a matter of tight pattern control in the right spot. 

I'm not quite sure what you mean by flex theatre, but if it's what I'm thinking, at least in part (the first row of the audience is seated on the same level as the stage?) you want to try towards downfills, which are harder to do tightly, but can be done. Remember, in that case, that you don't need to hit the audience's lap on-axis, just their ears!

Pattern control is very often overlooked in sound design, and it really can't be if your design is to be effective


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## jkowtko (Sep 23, 2007)

As I'm getting my head above water on the million things that need to be done at this theater in sound, I'm starting to get a handle on the sound dispersion requirements.

The Mackie SR450s that I have hanging have only a 45 degree vertical dispersion. From their position about 12 feet up, the LF/RF speakers are just in front of the front row, which means a single 45 degree sweep won't cover the first and the last row. They are already tilted down a bit, but I'd have to tilt them a lot more and I still don't think the 45 degrees will cover front to back audience. Or, I can turn the speakers sideways to get the 90 degree side dispersion of these speakers.

Or ...

I spoke to one of the design guys at Meyer Sound (our lighting guys' son is the software engineer on Galileo product there) and they said they will come out to do a design review on our site  Their UPM-1P loudspeaker has a 100x100 degree dispersion range, is much smaller than the Mackie and can easily be hung horizontally under the lighting grid, making it excellent for our theater application. One of these speakers should be able to reach front to back including downfill needs, and three of them could handle the entire quarter-round sweep of the audience seating area. Unfortunately Meyer products are extremely pricey compared to Mackie, but if we can get the money somewhere to fund this it would likely be worth it. Anyway, Meyer is about a half hour down the road so it's not a huge deal for them to come pay us a visit.

In the meantime, yes, I'll have to struggle with the dividing line between sound dispersion limits and rolloff, to try to cover that first row of seats but not the actors standing downstage ...

Back on the feedback suppressor front ....

I found a cheap used Behringer DSP1124p for $40, and I have it running passively to see what sort of feedback it picks up. So far a mix ... with single mic ringing exercises it came up with a few 1/60octave notch filters in the 600Hz-1kHz range. However when running it with 10 mics turned up and music blaring, I think it loses it's touch a bit (too deep, too wide, but still only 3/60 or 4/60). Still, this unit is great in that it will adjust width and depth of the notch filter as it applies them, you can set the sensitivity of the feedback control, and you can view the specs on every filter it applies -- frequency, width and depth -- and freeze them or set them manually and store in a separate program. So for me this is a great little tool to understand where the problems are occurring. I can then either set up some of these filters in parametric mode on the Behringer and leave them on, or I can use the DBX 231 to pull down those frequency areas a bit.


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