Acoustic absobtion construction method

magner

Standard Member
Hi,
I am just about to build my 1st home cinema in the loft of my house. I am trying to do some pre planning before I start buying all the materials.
Regarding sound absorbers. As I will be making 1 false wall, plaster board backed on the outside, rockwoll as insulation. I was then thinking of leaving the plaster board off the inside and covering the walls with muslin type material.
my question is... if I leave to much rockwool exposed will this dampin the sound of the room too much. If so is there a rule of thumb for a rectangular room on how much sound treatment should be installed.

Also has anybody tried leaving rockwall insulated walls exposed and adding reflective material (i.e plasterboard) to walls to control acoustic performance, or is the old method of adding absobtion to a reflective room the best way to go.

Thanks for your time.
 

gareth_alien

Active Member
I'm assuming this is the wall the screen is going on.

Not that it makes too much difference - your plan sounds good as long as it is only on that one wall. I've heard that rooms completely cocooned in sound absorbing materials (rockwool is good stuff for its price) sound very flat and unnatural. Covering just one wall should improve the acoustics.

As for the covering you've not mentioned if a projector is being used. If so then dark material is good. Many people have raved about devore black fabric (Whaleys) as it is acoustcally transparent and very black. This would be good for the screen wall and at least a short way out from the screen wall on the walls, floor and ceiling to reduce light reflections.

For the rest of the room you could go for home-made acoustic panels like these: http://www.cinematreadwell.com/homemadesoundpanels.pdf.
 

magner

Standard Member
I'm assuming this is the wall the screen is going on.

I will have 3 false walls and 1 brick wall. Was thinking to make the right side of screen wall the brick wall

Not that it makes too much difference - your plan sounds good as long as it is only on that one wall. I've heard that rooms completely cocooned in sound absorbing materials (rockwool is good stuff for its price) sound very flat and unnatural. Covering just one wall should improve the acoustics.
The room will be symmetrical. Would best results not come from symmetrical acoustic treatment as long as seat are placed in the middle?

I was also thinking about using the mirror reflection technique. Instead of making panels I could leave the plasterboard off the section of wall where the sound reflects off. And cover that section with a light material that will let sound through? Good or bad idea?

As for the covering you've not mentioned if a projector is being used. If so then dark material is good. Many people have raved about devore black fabric (Whaleys (Whaleys)) as it is acoustcally transparent and very black. This would be good for the screen wall and at least a short way out from the screen wall on the walls, floor and ceiling to reduce light reflections.
I will be using a projector, would you suggest leaving the plasterboard off the projector wall and use the devore you suggest?

Thank you for your information much appreciated 
 

Peter Parker

Distinguished Member
If you make walls without the internal layers of plasterboard attached you'll be losing most of the soundproofing potential of the wall.

You should build a new wall an inch in front of the existing wall, and ideally it should be around 4 ins thick and contain rockwool insulation. Then add two layers of plasterbaord so that the second layer overlaps the joints of the first. Then seal all edges with silicon rubber to ensure no sound can leak. After that you can think about adding treatments. False walls can be used as bass absorbers but that's usually done from the rooms design stage, and you need to know in detail the acoustic properties of the material and dimensions so not something many of us will be able to do accurately.

If you don't need to soundproof then you only need to concentrate on treatments.

As has been said, a completely absorptive wall/room will give a very dead sounding result, and one method is to have the screen wall dead, and all other walls dead below ear height, but above should be reflective (and even that depends on surround speaker height). So you can leave the plasterboard above ear height and use treatments below. It's not very scientific but will prevent the dead room scenario and should help reduce effects like slap echo.

I took the not-very-scientific-approach and used foam backed carpet on my lower walls and screen wall, and that killed all the slap echo in my room. I used a Behringer feedback destroyer to level out the bass response, but despite all that, I've no idea if that approach also had any negative effects - in other words I could have removed the slap echo at the expense of other elements. Not being an audiophile, I was quite happy with the results though (compared to what I started with).

Ted White (amongst others) is an expert on the forum, so do some searching/reading to see what he and others suggest. Without actually measuring the results to see what changes you've implemented, it's difficult to know if what you've done is good or bad, but you can sometimes get comparative results by setting up your speakers to see what it sounds like now and then see what effects your treatments may have. The drawback is that its' not easy to do as a temporary thing (i.e you stick the carpet to the walls and don;t like the result, so you have to remove it again).

Quite often you may find that the addition of furniture and people is enough to help deaden the room to make it sound OK, but you won;t know until it's installed.

There is some free software over on avsforum that may help in deciding what treatments you should put and where (absorption at the first reflection points), so you could try that:

*free* software to help determine your first reflection points - AVS Forum

Gary
 
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