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Troublesome wall echo
Troublesome wall echo
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Old 22nd February 2020, 03:51 PM   #31
Douglas Blake is offline Douglas Blake  Canada
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Quote:
Originally Posted by russc View Post
Going back to the foam idea -
Rather than filling the cavity, just injecting foam in a few localised spots may have a significant effect. It would break up a large vibration prone panel between studs into smaller less vibration prone areas. Rather like bracing a speaker cabinet. At least it would be easy to try before going down the redecorating route.
Good suggestion.

I've done that one before with drywall... drill a small hole, feed in a plastic bag then fill it with air to create an open cavity then fill it up with foam... The result is a puck of foam behind the hole that puts a couple of psi of pressure on both sides of the wall and forces both sides of the wall to move together, which takes considerably more energy. It also changes the resonant nature of the wall itself, so the two sides are no longer alike and less prone to react to the same stimulus.

It does work ... mostly... on drywall. I've never tried it with sheetrock which is an entirely different animal.
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Old 22nd February 2020, 03:54 PM   #32
mountainman bob is offline mountainman bob  United States
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Do the foam bags and a tapestry to cover the holes......problem solved!
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Old 22nd February 2020, 04:07 PM   #33
Douglas Blake is offline Douglas Blake  Canada
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Originally Posted by Adhoc1 View Post
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?
Is seems like the wall is actually vibrating with the sound and re-radiating it. It is somewhat directional, but mostly whatever is going on in that room is rebroadcast with the appropriate time delay for the distance.


Quote:
Originally Posted by Adhoc1 View Post
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.
I apologise for not catching this part of your commentary sooner... Yes that is what it is like.

I just called them and asked them to try an experiment for me... Their entertainment stand is about 6 feet long, 2 feet tall ... and flat as a pancake on the back, thanks to a nailed on piece of hardboard (pressed paper and glue panelling about 3/16" thick).

I'm waiting for a call back, but your comments about 2 parallel surfaces makes me wonder if they don't have what I would call a "cavity resonance", something like an acoustic reverb unit, with the stimulus originating between the back of their furniture and the wall itself... I've suggested they roll the stand out to the middle of the room and see what the echo does... I'll update this message when they call me back.
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Old 22nd February 2020, 04:11 PM   #34
Douglas Blake is offline Douglas Blake  Canada
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Originally Posted by mountainman bob View Post
Do the foam bags and a tapestry to cover the holes......problem solved!
The bags with foam is easy to hide... just cut everything off flush with the wall and sand. A little spackle, a dab of paint and nobody's the wiser.

It's not my wall, thus not my decision... but you might be right.

As I mentioned in my previous comment... I called them a few minutes ago and they are trying a simple experiment for me... I'll talk to them about your idea when they call me back. (hopefully later this afternoon)
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Old 22nd February 2020, 06:01 PM   #35
Douglas Blake is offline Douglas Blake  Canada
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Well, that was quick... I just heard back about moving the stand.

As I asked they did this in steps and made notes as they went...

1) Take the TV off the stand ... no real difference.
2) Pull the stand out a few inches ... minor reduction.
3) lay a blanket over the stand near the wall... big reduction
4) put the stand well out of the way ... only trace echos remain.

Apparently moving the stand away from the wall reduces the echo to what you might expect in a normal room, they described it as "a warm room echo"... This might be okay. Nobody wants to live in an anechoic chamber.

So, it seems that @adhoc1 was onto something right from the beginning.

I'm theorising but it seems like the small cavity (2") between the back of the stand and the wall was stimulating the wall vibrations. With a wall this active, it probably wouldn't take much.

So, I suggested we remove the stimulus... acoustically treat the back of the stand itself.

They used some leftover carpet they had from moving in for step 3. It's a medium pile carpet with a foam backing, so it's easy to glue to the back of the stand and should have excellent sound absorbing qualities. I figure worst case, they end up with carpet on the back of their equipment stand where nobody can see it anyway.

That, along with @mountainman's suggestion for a tapestry on the wall, just might get the job done.

So, I'm off to help them with the stand tomorrow morning.
I will update this when it's done.
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Old 23rd February 2020, 11:46 PM   #36
Douglas Blake is offline Douglas Blake  Canada
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Okay, back home from the client's apartment... here's the latest.

A friend of mine who is a contractor/carpenter that I've worked with on a couple of home theatre installations agreed to join me for my outing.

As we were putting the carpet on the back of the stereo stand, he went around with a small rubber mallet and tapped all the walls in the apartment. Apparently this one wall I was worried about isn't the only one that rings when you tap it. Fortunately he seemed to know what to do about it... all he kept telling us was that my friends needed to "add mass" to the wall.

He explained that dissimilar materials vibrate differently, so something softer needed to be put on top of the rock hard wall. He suggested drywall compound... spackle.

As we finished the stand he returned with a bag of spackle and after we tested and found some improvement, we rolled the stand and speakers back out of the way and he took a wooden block to the wall and hit it in several spots with a hammer, apparently to break up stress in the wall, then put on a very thick coat of fast drying drywall finish.

So we stuck around for a nice spaghetti dinner as everything dried and ... guess what.

No More Ringing Echo!

There is a little bounce back from the TV screen (expected) but the wall is mostly dead now. If still sounds hollow, like drywall, but does not ring in a sustained way.

So the combination of @adhoc's advice about parallel surfaces, and my friend's plastering skills fixed the problem. My client/friends are even considering @mountainman's suggestion of a nice tapestry (but probably not Dogs At Poker).

So apparently the solution in this kind of case is to use dissimilar materials and add more mass to the wall surface...

Thank you everyone for your help with this, some very good suggestions from all ... Even Scott.
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Old 24th March 2020, 01:18 AM   #37
BottomEnd is offline BottomEnd  Canada
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Troublesome wall echo
Well, you have to make the wall "not hollow". Cork will do nothing to reduce the resonance because the wall is still hollow. However, cork will significantly reduce reflections above 250Hz and looks quite nice. Since you want to leave the wall, you could look into commercial foam injection which would be permanent. And finally blowing in Roxul or as a last resort blowing in Pink but you would have to seal the wall from particulates. Cloud ceilings make a huge improvement acoustically and Tectum is better in an institutional solution.
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Old 24th March 2020, 06:41 PM   #38
Just Dave is offline Just Dave  United States
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Quote:
Originally Posted by Douglas Blake View Post
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...
Fist bump at ~100Hz, echo from that surface. What do your measurements indicate?

I just now read your initial post, so, sorry - if this has already been resolved.
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Old 25th March 2020, 02:46 PM   #39
Douglas Blake is offline Douglas Blake  Canada
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Originally Posted by BottomEnd View Post
Well, you have to make the wall "not hollow".
Good idea but not possible in this case, since the kitchen electric and plumbing runs through the hollow cavity. We later discovered there's a wall like this in every room of the apartment.

As I detailed, my contractor/helper put on about a 1/4 inch layer of spackle. The first layer was put on with a notched trowel, like they use for the glue under tile. This built up some depth. The second layer was put on about 2 hours later and trowelled smooth. A little paint and it's good to go. Looking at it you can't even tell anything's been done.

The solution was to add mass to the wall, using a material of a different resonant nature. Seemed to work just fine.

We did take measurements later and that 100hz bulge is gone as are several other audible artefacts in the room. The thing was acting like a drum head. If you've ever heard a snare drum rattling on as a bass guitar plays, you'll get the idea of what this was like.

Now that they have their art work hung above the TV, it's really a very nice setup.

But, as I said... I'm pretty sure I know why the previous tenant moved out after only 6 months. That echo would have driven me absolutely insane.

Last edited by Douglas Blake; 25th March 2020 at 02:48 PM.
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Old Yesterday, 10:40 PM   #40
SirDystic is online now SirDystic  Australia
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I had a similar problem, so I made these, there is a similar but different large artwork on the opposing wall, Heavy canvas on timber or aluminum frames, I was going to put accoustic panel behind the canvas, but they solved the problem as they are..
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