Construct absorption panel with rug and Owens Corning 700 boards

[thefsb]

I need to add absorbers on a couple of walls in room where I use an LDC mic and, not simultaneously, a pair of 5" loudspeakers (Genelec 1029A).

For example I could try to hide four 2' x 4' panels of 4" Ownens Corning 705 behind a decorative 4' x 8' rug like the one shown.

Do y'all have experience or advice to share on the materials, construction and mounting? For example, 2" 703 and 705 is available with facing which might simplify my work. And should I construct air gaps into the panels?

 

[abdo123]

Persian / Middle-Eastern rugs are way too dense to cover a fake wall or floor.

try to use fur (sans the skin) or very thin fabric or paper.

 

[thefsb]

Persian / Middle-Eastern rugs are way too dense to cover a fake wall or floor.

try to use fur (sans the skin) or very thin fabric or paper.

What problems does that density cause? Is the concern mechanically mounting a heavy thing to walls?

 

[abdo123]

What problems does that density cause? Is the concern mechanically mounting a heavy thing to walls?

the concern is sound (well air) not penetrating the rug and as a result it doesn't get absorbed.

 

[thefsb]

the concern is sound (well air) not penetrating the rug and as a result it doesn't get absorbed.

In other words a rug is a good acoustic reflector?

 

[abdo123]

In other words a rug is a good acoustic reflector?

I wouldn't say that either.

 

[puppet]

It's safe to say that the denser a material becomes the reflective potential increases.

 

[Vict0r]

Ideally you'd want an acoustically transparent material to cover your actual absorber. The most basic broadband absorber consists of rockwool (or a similar material) covered in thin cloth to keep the fibers in, within a wooden frame/enclosure that is covered by an acoustically transparent material, like speaker cloth for example.

 

[Inner Space]

Ideally you'd want an acoustically transparent material to cover your actual absorber.

Not really - all it needs to be is non-reflective. Some absorption by the fabric will help. It makes no sense to say, "I'm making an absorber, but I want the front face to absorb nothing." And a thick Persian carpet is ideal - an open-weave backing populated by wool fibers end-on to the arriving sound pressure. Perfect!

 

[audio2design]

I tend to agree with @Inner Space , the rug sounds ideal, though watch for frequency notching, i.e. heavy absorbing at a particular frequency. I cannot see that construction, being very reflective at any frequencies.

 

[Vict0r]

Not really - all it needs to be is non-reflective. Some absorption by the fabric will help. It makes no sense to say, "I'm making an absorber, but I want the front face to absorb nothing." And a thick Persian carpet is ideal - an open-weave backing populated by wool fibers end-on to the arriving sound pressure. Perfect!

Thanks! Most of the advice I've gotten in the past was to have the material covering the actual absorber interfere as little as possible with the absorber. Also, if you take a shot every time someone writes the word "absorber" here, you'll be off your feet in no time.

 

[Inner Space]

For example, 2" 703 and 705 is available with facing which might simplify my work. And should I construct air gaps into the panels?

The facing (usually foil bonded to kraft paper) is good for maintaining some HF life in the room, while absorbing lower frequencies. But 2" won't give much LF absorption, so to keep the absorption balanced, don't use facing.

Air gaps are misunderstood. Leaving an air gap is not an improvement over no air gap - it's merely better value for money. There's a diminishing return in trying to absorb very near the wall, so economically you're wasting a percentage of the unit price, but some small amount of absorption will still be achieved, so if you have the money, fill the gap.

 

[Hipper]

Three issues occur to me with using rugs to cover an absorber:

• reflections, if any, as already discussed.
• weight. Securing the absorber to walls etc..
• how well it contains the absorbent material so its dust doesn't come out into your room.

 

[thefsb]

Three issues occur to me with using rugs to cover an absorber:

• reflections, if any, as already discussed.
• weight. Securing the absorber to walls etc..
• how well it contains the absorbent material so its dust doesn't come out into your room.

I'm considering building a 4"-deep box on the wall that will accept 2' x 4' slabs of unfaced Owens Corning 705 side by side. The existing painted drywall would be the rear of the box. Use 3/4" x 4" lumber for the sides. Caulk the joint with the wall. Install the 705 and tightly staple cloth over the front. Then decorate however I like, e.g. with a rug, or hang guitars from it, or both. A filled box 8' wide 4' high would probably cost around $300 for materials. A single 2' x 4'9" x 4" trap from Real Traps costs $320.

Alternatively, two Corner MondoTrap 2' x 4'9" x 4" would cost $740 and might be more efficient in taming my A2 (110 Hz) honk. They could go above my desk attached to ceiling and wall. Nothing else happens in that space so it would have less impact on the room.

Or both.

 

[Inner Space]

I'm considering building a 4"-deep box on the wall that will accept 2' x 4' slabs of unfaced Owens Corning 705 side by side. The existing painted drywall would be the rear of the box. Use 3/4" x 4" lumber for the sides. Caulk the joint with the wall. Install the 705 and tightly staple cloth over the front. Then decorate however I like, e.g. with a rug, or hang guitars from it, or both.

Good plan. A cut-up cotton flannel bedsheet would be ideal for the stapled fabric. Suggest if you use a rug, fix it at the top edge only - most such rugs aren't exactly regular, and will distort (in shape) if you fix all four edges. Plus, by hanging free, it will have a mild "limp mass" effect that might help a bit at lower frequencies.

 

[NTK]

You may want to take a look at the BBC's wideband sound absorber design.

A modular wideband sound absorber

A modular wideband sound absorber.

www.bbc.co.uk

They have measurement data on the absorption coefficients of various depths of rockwool.

And this is their final design.

[Edit] They also have a "matching" very low frequency absorber design.

The design of a modular sound absorber for very low frequencies

The design of a modular sound absorber for very low frequencies.

www.bbc.co.uk

 

[DonH56]

Disclaimer: I have not read the whole thread.

I would use 1/2" or even 1/8"~3/8" plywood for the frame to save weight. Plywood is a PITA to edge-finish, however, so when I've gone that route I wrap the fabric around so it covers the edges and/or use trim on the front (or just wrap all the way and staple in the back.

I would not caulk to the wall. More work, not needed sonically, may peel the paint as they move, and the caulk will open and shift as the wall and panels expand and contract as well as dry out and flake off over time.

You could make small blocks to stand the panels off the wall about 2" for better absorption if desired. Look at the CNRC, Kinetics Noise Control, or many other sites to see how a small air gap can help absorption especially for lower frequencies. Absorbers like these are velocity-based so depend upon turning the velocity of sound waves into heat. At an ideal (perfectly stiff) wall, the velocity is zero, so a small gap gets the absorber out where it does more good. Real walls are not perfectly reflective but an air gap still helps significantly.

The poor man's corner trap is to just cut triangles out of the material (e.g. 2' per side so you get four per 2'x4' sheet of OC-70x) and stack them floor to ceiling (more or less) then cover with cloth (or use a drape hung in the corner).

 

[Timcognito]

https://nzwool.co.nz/wp-content/uploads/2017/04/ACOUSTIC-PERFORMANCE.pdf FYI

 

[thefsb]

I would not caulk to the wall. More work, not needed sonically

That thought came from concern about glass fibers falling out over time.

You could make small blocks to stand the panels off the wall about 2" for better absorption if desired.

If the front face of the absorber will be 6" in front of the current wall then I might as well just build the box 6" deep and fill it up. It would cost only $90 more to fill that extra 4' x 8' x 2" with 705 rather than air. @Inner Space explained in #7 above that air gaps are really about cost saving.

The poor man's corner trap is to just cut triangles out of the material (e.g. 2' per side so you get four per 2'x4' sheet of OC-70x) and stack them floor to ceiling (more or less) then cover with cloth (or use a drape hung in the corner).

Yes, and it makes sense but the room has no corners I am willing to sacrifice for that. Only the horizontal wall-ceiling corners are available.

Thank you for the advice. Plywood noted!

 

[thefsb]

https://nzwool.co.nz/wp-content/uploads/2017/04/ACOUSTIC-PERFORMANCE.pdf FYI

The rug on the floor is a polyester Ikea Persian-style job. It looks ok and the dogs seem unconcerned about the material. The off-gassing when it was new was intense and demanded ventilation. I wonder if it continues to do that, just at a lower level. I would prefer wool.