Hanging DIY Panels: Best Gap Space?

BradJacob · **Posted:** Thu Jan 17, 2008 7:01 am

I'm building the typical 4'x2' acoustic panels.

They'll be stuffed with 2" 703 rigid insulation.

The wood is 3/4" x 3 1/2".

The insualtion will be flush on one end and 1-1/2" away from the other edge of the wood.

How far OFF the wall is ideal for these panels? I was under the impression that it was 2-3 inches??

Thanks!

jwl · **Posted:** Thu Jan 17, 2008 7:45 am

There has been some debate on this issue. When you pull an absorber out from the wall, you are basically lowering the frequency to which the absorber will work by giving the absorber more of an opportunity to soak up bass frequencies. In general, the further from the wall the better, but you rapidly reach a point of diminishing returns, not to mention eat into room space.

A good rule of thumb is that the panel should be spaced from the wall the same thickness of the panel, ie, your 2" thick panels should be 2" from the wall. I'm assuming these will be used for RFZ creation and general room deadening, as opposed to bass trapping, since they are 2" thick.

If it were me, I'd probably just put the 1x4s flush up against the wall, and live with the 1.5" spacing. That'll work well for RFZ absorbers, and it will probably look the best.

I'd also consider cutting some holes in your 1x4 frames, to allow air/sound to breathe through the sides of the absorbers. A 2-2.5" holesaw would work great for that. You don't need a lot of structural strength, just enough to hold the frame together, and the effectiveness of your absorbers will be increased.

AVare · **Posted:** Thu Jan 17, 2008 8:49 pm

I wrote this several months ago. It is quite relevant to your project.

Quote:

703 is the commonly used term in studio design for acoustic absorbent material. It is formally the registered trademark of Owens-Corning for OC’s 3 pound per cubic foot fiberglass. There is nothing unique to 703. You can use any company’s product. At some places mineral wool is cheaper or available. With mineral wool, the 4 pound per cubic foot material is acoustically equivalent for our work.

In my part of world, 4” 703 is $3.80 per square foot . Roxul Safe n Sound (2.5 pounds per square foot and 3.5” thick) is 27 cents per square foot. A little off on the density thickness but ONE FIFTEENTH the price!
It is difficult to keep track of all the materials avaiable . The BEST source of information that I know of is Bob's excellent site.

An interesting effect of the pricing is that gapping becomes a non issue. People at times consider having an air gap between the material to increase low frequency absorption. If a gap is used, there has to be some way to keep the material from falling into the gap. Typically this done with chicken fencing and strapping to which it is attached. The chicken fencing costs 50 cents per square foot here. 1"v1" stapping about 12 cents per foot. Putting a 3.5” gap behind the Safe N Sound would be almost twice as expensive as filling the entire space!

Economically,
Andre

Ethan Winer · **Posted:** Fri Jan 18, 2008 4:00 am

BradJacob wrote:

How far OFF the wall is ideal for these panels?

The ideal air gap is equal to the panel thickness as explained here:

http://www.ethanwiner.com/acoustics.html#air%20gap

However, this is theoretical, and in practice using an air gap up to twice as thick as the material is fine. So the answer is between 2 and 4 inches.

--Ethan

BradJacob · **Posted:** Fri Jan 18, 2008 9:18 am

Ethan Winer wrote:

BradJacob wrote:

How far OFF the wall is ideal for these panels?

The ideal air gap is equal to the panel thickness as explained here:

http://www.ethanwiner.com/acoustics.html#air%20gap

However, this is theoretical, and in practice using an air gap up to twice as thick as the material is fine. So the answer is between 2 and 4 inches.

--Ethan

Thanks Ethan... BTW, those videos on your site are great. I learned a lot.

awilki01 · **Posted:** Mon Feb 25, 2008 2:51 pm

BradJacob wrote:

I'm building the typical 4'x2' acoustic panels.

They'll be stuffed with 2" 703 rigid insulation.

The wood is 3/4" x 3 1/2".

The insualtion will be flush on one end and 1-1/2" away from the other edge of the wood.

How far OFF the wall is ideal for these panels? I was under the impression that it was 2-3 inches??

Thanks!

If your box that housed the panel was 4" deep, you could probably place the panel flush to the wall leaving a 2" air gap between the foam and surface of the wall. I'm not expert - just a guess.

jwl wrote:

I'd also consider cutting some holes in your 1x4 frames, to allow air/sound to breathe through the sides of the absorbers. A 2-2.5" holesaw would work great for that. You don't need a lot of structural strength, just enough to hold the frame together, and the effectiveness of your absorbers will be increased.

What kind of spacing would you recommend for these holes? One per foot? I think perhaps an evenly spaced 3/4" hole cut with a forstner bit would look good to. I only say that because I just bought me that bit for something else :lol:

jwl · **Posted:** Tue Feb 26, 2008 2:24 am

Ideally, the entire panel should be exposed to air, ie, there should be no frame at all. Of course this is impossible unless you go with a frameless design.

So the answer is, get rid of as much of the wood as you can, without weakening the too much.

awilki01 · **Posted:** Thu Feb 28, 2008 1:30 pm

What if you take a panel and connect it to the wall at the bottom and at the top, you lean it away from the wall at around a 10 to 15 degree angle? That would give you a varying space gap from top to bottom.

Thoughts?

Ethan Winer · **Posted:** Fri Feb 29, 2008 3:28 am

Just determine the ideal gap and use that everywhere. Angles gain you nothing in this case. If angles were useful there I'd have said so in that part of my FAQ linked above.

--Ethan

awilki01 · **Posted:** Fri Feb 29, 2008 12:53 pm

Ethan Winer wrote:

If angles were useful there I'd have said so in that part of my FAQ linked above.

--Ethan

Ah!!! You are that Ethan! Great job on that FAQ. I've read it about three times over now.

However, you did suggest what I was talking about. I quote your FAQ here:

Quote:

You could also use a few different gap sizes, or make some panels much thicker than others, or install the fiberglass at an angle so the top of a 2x4 panel mounted vertically is touching the wall and the bottom is spaced away half a foot.

awilki01 · **Posted:** Fri Feb 29, 2008 2:32 pm

AVare wrote:

The BEST source of information that I know of is Bob's excellent site.

If I am reading this site correctly, it is suggesting that OC 703 has a higher rating for bass frequencies than 705?!?!?! :shock:

Everywhere else indicates the opposite....

AVare · **Posted:** Sat Mar 01, 2008 1:28 am

awilki01 wrote:

If I am reading this site correctly, it is suggesting that OC 703 has a higher rating for bass frequencies than 705?!?!?!

Everywhere else indicates the opposite....

As I have written already, acoustics is not intuitive. Welcome to counter intuitive item 17.

Porous absorbers work by the vibrations of the fibers in the material. If there is not enough fibers, then the sound does not get absorbed. If there too many fibers, the sound is reflected by the material. The location is also important, in that if there is no velocity, the the porous absorbers do nothing. The acoustics term for the significant factor is acoustic impedance at the surface of the absorber. Denser materials have greater impedance. So for low frequency absorption, thick material (or located a space away) is used, and to keep the impedance where it is needed to air's impedance, less dense material is used.

You didn't comment on it as it is not important right now, but notice that the denser materials also reflect highs more.

To gap or not to gap? That is the question. The answer is to look at the total cost.

Economically,
Andre

Ethan Winer · **Posted:** Sat Mar 01, 2008 1:51 am

awilki01 wrote:

However, you did suggest what I was talking about. I quote your FAQ here:

LOL, I'll change that right now.

--Ethan

awilki01 · **Posted:** Fri Mar 07, 2008 3:41 pm

Ethan Winer wrote:

awilki01 wrote:

However, you did suggest what I was talking about. I quote your FAQ here:

LOL, I'll change that right now.

--Ethan

But, it makes so much sense. Why the change of heart? :shock:

Ethan Winer · **Posted:** Sat Mar 08, 2008 5:09 am

awilki01 wrote:

But, it makes so much sense. Why the change of heart? :shock:

It doesn't really make sense because some areas will have less than an ideal gap. Better IMO to have the best gap for the entire panel.

--Ethan

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