Troublesome wall echo

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In my years of doing home entertainment installations and tune ups I've never run into a problem quite like this one, so a little advice would be appreciated.

The stereo system is oriented along a wall with openings on each side. One side is a hallway the other goes to the dining room. The kitchen is right behind the speakers.

On this wall one side of the wall is hollow, with the kitchen wiring running through it. The other side is solid. It is constructed from sheetrock on steel joists.

When I bump my fist on the hollow section of wall it rings at about 100hz, quite noticeably. When you sit opposite you can hear an echo returned from this section of wall... it's very reactive and it's affecting the sound from the stereo setup in some pretty horrible ways.

My first thought was to drill a couple of small holes and fill it with construction foam... A second thought was to cover it with cork and paint it. The client suggested ceiling tiles ... But I am unsure any of these measures will work plus it has to look okay in a living room.

Any and all suggestions would be appreciated...
A wall partition always has a resonance, higher or lower in frequency. You can see a studwall with sheets on both sides of the studs as a membrane absorber. The drywall moves / vibrates alot at the resonance. If the partition is between 2 rooms, the (100 hz) resonance tone will ring out very loud in the other room. Especially if there is just air between the 2 drywall sheets with no insulation between to dampen the resonance. With a solid say concrete wall + stud and air + drywall sheet the (100 Hz) tone will be amplified back into the room facing the drywall sheet. If you drill holes in the drywall, the mebrane absorber will turn into a helmholtz absorber with a resonance depending on hole diameter, distance behind holes to the backing wall and ratio of total open area of the holes <=> sheet area. Remedy: Remove the drywall and fill up the hollow with insulation to dampen the resonance and put the drywall back. Replacing the air the behind the sheet with expanding polyuretan foam is a shot in the dark. Something will happen but what?

Dry wall on studs, steel or wooden, and c-c 600 mm / 2' get a resonance around 70 Hz or so with 95 mm / 4" studs. The resonance depends on the weight / area of the applied sheeting and the air depth behind. Higher weight / area lowers the resonance, larger airgap also lowers resonance in frequency. Insulation is still needed to dampen the resonance due to the air gap between the 2 leafs.

If the problem is a metallic "zinging" sound at listening position it probably is a flutter echo showing up because of 2 flat parallell surfaces with sound bouncing between the surfaces. Remedy:Angle 1 of the surfaces 10-12 degrees, use a bookshelf to split up the parallellnes or use an absorber. The absorber doesn't need to be very thick, 25 mm / an inch or so can be enough. A folded quite heavy curtain placed 100 mm / 4" out from the wall should also be enough. A large painting with glass front / heavy canvas which hangs out "a bit" from the wall at the top can also be beneficial if it is a specific spot .
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Thank you for a comprehensive reply.

However this isn't drywall it's that pre-cast plaster and stone stuff they use in commercial renovations and the studding is steel. it is a layer of expanded steel mesh with a layer of rough plaster and a thin layer of finish plaster. In one sense it's better than standard drywall because it's a lot tougher ... but I think I've found one of it's drawbacks.

I went back this morning and using an electric razor, made a map of the wall. The really active part is an L shaped area about 2 1/2 feet high starting at the centre of the wall then jumping up to nearly full height about 2 feet from the right hand side. A little clever measuring tells me the "solid" side is 1 1/2" thick but the hollow sounding part is nearly 6" thick. So they've done something to create a cavity there. We know it is where the wiring for the kitchen stove and outlets comes in, looking closer, it is likely also where the television cable and phone wires come into the unit as well. Since this is an interior wall, I would not expect to find any kind of insulation or space filler in this cavity.

To my friend's chagrin, this hollow cavity is right behind one of his shiny new speakers and I'm sure it doesn't help that they are rear ported.

We discussed re-arranging the room but I am forced to admit that with his cable internet connection, power outlets and phone connections on that wall it would be best if we didn't. Plus given his furniture I'm not sure we could arrange it any other way and still have a comfortable room.

Taking down the wall and rebuilding it is an option, but given the expense and mess of it, I would like to avoid that if I can. Plus I'm pretty sure his landlord would have an opinion about that as well...

I agree that pumping in foam is pretty hit and miss. Since we can't see inside that cavity there is no way to know where to inject it or how much to use.

So, I guess that leaves us with some kind of wall covering to deaden the vibration. I doubt wallpaper would do it, so once again I'm open to suggestions. We've already discussed cork and ceiling tiles but I am unsure of how effective that would be.
The other type of "sheet" you describe; "plaster + stone stuff", I doubt it makes much of a difference on the theory of MAM, Mass Air Mass-systems of wall partions and resulting wall resonance. As I understand it, you are describing a 2 leafed enclosed air volume; 1 leaf = the 1 side of solid "what ever" + air volume + 1 leaf = the 1 side with the plaster + stone stuff.

If the plaster side towards the room isn't extremly massive and stiff (and a typical interior wall isn't), it will vibrate because of the airborne sound energy in the room. The air in between will act as a spring, just as it does in sealed speaker. Your speaker will be tuned to a certain frequency depending on the volume of enclosed air. Same here: Your 1 leaf + 1 air volume + 1 leaf will result in a resonance for that "system". One "slight difference" :mad: is the difference in size of vibrating surfaces, a 12" driver compared to a whole wall area vibrating ... -The wall doesn't have to move much back and forth to move the same amount of air volume as the 12 driver. The movement builds up very quickly around resonance freuqency.

(If you google Mass Air Mass resonance, you find some mathematic formulas for calcualtion of wall resonance. The usual problem is about sound isolation / transmission between rooms.) You seem to have problem with the resonance of the wall inside the listening room though, producing a long ringing sound at the wall's resonance frequency.
Thank you Adhoc for yet another informative response.

I will check out the stuff you are indicating, but right now my prority is to get that wall quietened down. With respect, I'm less concerned with how this happens than I am with how to stop it, at the moment. I have a client waiting for answers.

I assume that changing the mass of the wall will change it's resonance (~100hz) and that changing it downward is preferable. I've held my hand on the wall while tapping it and even that quiets it down quite a bit. Oddly enough it almost completely stops the echo I noticed from the far side of the room. Of course where I hold my hand changes the outcome...

There isn't a large budget here and we aren't completely free to rip down the wall...

So will covering that whole wall with something work?
and... with what?

A couple of actual suggestions would help.
Foam will stiffen the wall and that alone will raise the resonant frequency. It will also by taking up space raise the frequency.

But a wall more than an inch or two thick by itself should not be able to pass much energy. So first look for a coupling hole, the tap the entire surface Replace valvesand see what sounds out. Failing that tap all the adjacent surfaces.

To test for room resonances just move large padded furniture across the room.
Thank you Turk and Simon for your responses.

I've done installations for a few years now, as a bit of extra income. But this wall is something else, you tap it with your hand and it rings like a drum head. You stand across the room and talk to someone and it repeats everything you say... If I didn't know better I would think it was haunted.

To be honest I doubt small wall panels will work. They're likely to just move with the wall.

I'll be heading over there pretty soon. I've got a couple of non-destructive ideas to try but I'm not holding on for big successes...

The first thing on the list is to move the speakers out from the wall a bit more and move his CD/BluRay storage cabinets in behind them, between the speakers and the wall. If I can't quiet it, maybe I can block it...

Next on my list is the foam idea. I think we all agree that is hit and miss, but worst case I waste a can of foam, which is pretty cheap, so why not?

The next idea is to look into panelling the wall with drywall or thin-board, held in place with something like Green Glue. In another thread we were discussing constraining layer damping... this might be a reasonable application for it.

Beyond that, I'm wide open for ideas.

I'll let you know how it goes when I get back home...
perhaps my morning scotch talking here but if that section of the wall is so lively i'd be tempted to get my hands on some panel exciters and see if i could turn it into a stealth sub.... heck i'd be even foolish enough to try a phase inversion(with a filter) between the wall and the loudspeaker just to see if i could null the effect!
I haven't understood really how the disturbing noise sounds. Is it like a buzzing noise, like the one you can get when playing "music" on a comb + a thin sheet of paper? In that case it might be either that a steel stud or the sheet is warped or steel studs besides each other are out of parallell, => result they aren't in stiff contact and you get a small a air gap which can give a rattling / buzzing noise when the sheet surface gets energized by sound energy in the room and bounces against the stud.

If you have stud detector, check out where they are placed. The one I have indicates where wood, steel and / or electricity is placed behind the surface.

If the noise goes away when the sheet is firmly pressed against the stud behind, you might only need some extra screws. If it gets quieter when you press in the middle between 2 studs, it might be trickier to solve. Perhaps a hole for a wooden dowel? Adjust the length so it gets a correct length between the leafs. Saw a kerf (right word in English?) at the end towards the room and tap in a wedge in the kerf to secure that particular wall surface from vibrating.

If you use expanding foam, drill 2 holes as a start. Inser the foam in the lower hole , when the foam starts to come through the upper hole, -stop and wait a while. Then drill a new hole above and so forth.

I don't understand why you can hear the sounds bounce back to the other side of the room. Is it rounded or oval shaped, like a whispering gallery?

(You can download a tone generator from the internet to your computer, hook it up to the stereo and slowly change the frequency to find out when the wall sound starts to be annoying.)
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I haven't understood really how the disturbing noise sounds.

It's an echo ... for an example speak into a bucket or cardboard box, right up close.

The wall is stressed, like a drum head and the one side where the wiring cavity is, is very active. Knock on it with your knuckles and it will ring for quite some time, almost half a second, at about 100hz.

Thank you for your suggestions. I was thinking about the dowel as a brace idea as well...
Mass seems to be the universal trick to fix problems

I tend to agree ...

What we did this morning was to rearrange a bit... We moved the speakers a little bit further from the wall and took a pair of large-ish CD storage racks -- approximately 40" tall, 30" wide and 8" deep-- (Edit: Like the thumbnail) that were on a side wall and moved them to that wall one on either side, right near the corners. The speakers now stand a little further into the room on either side of the equipment stand and mostly in front of the racks.

I'm off on lunch while they feed the kids and play a bit of music to make their own assessment. I already know it's cut the voice echo down substantially but have not done any measurements yet.

BTW, I discovered the product used to cover that wall is called "sheetrock". Apparently it's becoming a favourite in apartments, row houses and offices. I am so not looking forward to those problems!


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Okay, back home from my wall problem.

Moving furniture around decreased the echo quite a bit. It's still there but only a fraction of what it was. Oddly, installing the safety pins in the wall for those media stands actually helped a bit. The pins I use are basically a drywall anchor, the type with spreading wings, with a longer than normal bolt and a short piece of soft rubber tubing that you cut to length. These go through the back of the shelving unit and into the wall behind, to prevent tip-over risk around kids.

A couple of REW sweeps are attached. The first one is before the second one is after... there is an improvement, but not what I was hoping for. It would be nice to sit in that room without the voice echo...

My client/friend has signed off saying it's good enough.

But for the record I don't much like this solution. I hid the problem rather than fixing it...

Sorry for the different scales on the graphs... My bad.


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Cool to see measurements before/after, so thanks for that.

I'm curious, though - was the mic very far off axis from the speakers?
It looks like there's pretty much no treble at all, which would make me hesitate for sure.


The speakers are a done deal. It was the wall we were worried about.

It's the cheapo voice mic I used ... it rolls off pretty badly around 8khz.
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