Excellent suggestion but there are a couple of problems with diffusers, absorbers and foam tiles in this room...
It's the family's living room so decor is very important to them. It's not that easy to arrange the room around the equipment and accessories. We can't have the speakers 2 or 3 feet from the wall and if we followed the usual rules the sweet spot would have them sitting on their coffee table... The goal is to fit the system into their lives, not the other way around.
Also this isn't a reflection from a hard wall, like most places, this is a wall that actively reacts to and re-radiates sound. If you tap it gently with a rubber mallet it rings like a drum head. Booooom boooom not thud thud, and you can actually feel the after-ring on the wall. Anything hanging on that wall will likely vibrate with the wall, defeating it's purpose.
The current fix in there is not going to be the permanent, they will get tired of going behind the speakers to get their movie disks eventually, so for me this has become an ongoing "investigation". In all my years in this game, I've never seen this one before so it's a bit of a mystery to me.
I'm currently favouring a constrained wall solution... lots of green glue and some hardboard panelling, then a nice coat of paint so nobody's the wiser. But the problem is in doing it to test it, it becomes very permanent (no way to get it off the wall) and then there's the question of how many dead ends we want to follow...
I'll update once I find an answer... hopefully sooner than later.
I was planning on covering the entire wall behind my hifi with a full edge-to-edge 1st off for the decorative purpose, 2nd made in red western cedar for that wonderful smell, and 3rd acoustics.
But you have to deal with your wife, not mine :^)
dave
This is the first time you’ve ever seen Sheetrock?
I’ll help you out from experience.....spray foam (especially closed cell like what’s in the cans) is useless for sound proofing and can actually amplify an issue.
Now if it’s a rattle due to something loose then yah.
Would they be open to hanging a thick wool rug/tapestry on the wall that’s large enough to cover the area behind the speakers?
I’ll help you out from experience.....spray foam (especially closed cell like what’s in the cans) is useless for sound proofing and can actually amplify an issue.
Now if it’s a rattle due to something loose then yah.
Would they be open to hanging a thick wool rug/tapestry on the wall that’s large enough to cover the area behind the speakers?
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.
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.
Something I once read, makes me think this is both practical and effective.
Gedlee said:
Of course not (I just didn't have a name for it)... but it is the first time I've seen it behave like this.
This problem is not soundproofing (keeping noise out) or sound absorption (dealing with reflections inside the room). Those I can usually figure out on my own... a couple of panels here, some heavy drapes there... but this thing is on a whole new level.
Ok ... pick up a bucket, preferably a clean one, hold it directly in front of your face at distance of a foot or so... now say "Hello" directly into it a few times.
Hear that reverberation and echo? It sounds hollow, deep and nasty... That's what this wall does. You get back about 6 feet or so look it straight in the eye and say "Hello" or "Echo" into it and it repeats it right back with that same hollow sound.
People talking, speakers, street noise... same result.
That's sort of what we have going on now. I've temporarily moved their CD storage racks behind the speakers to block the wall. It mostly blocks the effect but that's not the same as fixing the problem. We discussed tapestries, and heavy curtains behind the system but they didn't seem very big on the idea.
We spoke to the landlord about it and his answer was totally classic... "That wall is strong, it's in good shape, nothing is loose, so there's nothing to fix". (I think I understand why the previous tenants moved out.)
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At this point a constrained layer approach seems the most likely to solve the problem... I've kept in touch with the family that's stuck living with it and they are open to anything that will shut that wall up.
Of course, their concern is that we can't try repeated experiments because something like CLD gets to be pretty permanent once installed and if it doesn't work we've just made ourselves a bigger problem.
Totally classic ... but I suspect not to their taste.
Although I will say the tapestry idea has some merit. Perhaps a nice woodland scene or an abstract...
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.
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.
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.
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)
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.
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.
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.
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.
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.
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.
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.
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