Acoustic Foam Bass Trap Question

[WmAx]

What approximate dimensions of a mass of standard medium density acoustic foam would it take to absorb, say, 50Hz and above?

I would not normally use such an in-efficient product as foam for low frequencies, but I may have the opportunity to aquire some stacks of smooth faced acoustic-type foam for free. I could get enough, for example, to have 2 columns of foam, 3'Wx3'Dx4'H, if I so desired. The foam is in standard 12"x12"x1" thickness units.

-Chris

 

[Ethan Winer]

Chris,

> What approximate dimensions of a mass of standard medium density acoustic foam would it take to absorb, say, 50Hz and above? <

The more the better. It's really as simple as that. Even a wash cloth will absorb something at 50 Hz, just not very much.

For real acoustic foam of a high quality, you'll want at least a 2x2 foot column from floor to ceiling in each of the four wall-wall corners. If you have room for 3x3 feet all the better. Then next step up is to do the same in the wall-ceiling corners. But this assumes acoustic foam. A lot of foam sold on eBay etc is not acoustic foam, even when it claims to be. And non-acoustic foam won't do much no matter how thick it is.

--Ethan

 

[WmAx]

Chris,

For real acoustic foam of a high quality, you'll want at least a 2x2 foot column from floor to ceiling in each of the four wall-wall corners. If you have room for 3x3 feet all the better. Then next step up is to do the same in the wall-ceiling corners. But this assumes acoustic foam. A lot of foam sold on eBay etc is not acoustic foam, even when it claims to be. And non-acoustic foam won't do much no matter how thick it is.

--Ethan

Yes, I am aware of the fraudulent foam. However, the foam I have found is a medium to high density open cell foam, that appears to the same material as real acoustic foam. The on-the-spot test I used was to place about 4" of the material over my ears to see if it reduced the amplitude of midrange frequencies noticeably[something that I notice that regular foam is not effective at doing], and it did. The foam has no markings of manufacture. Worst case scenario: I get the foam for free, set it up, it ends up having insufficient effect upon measured LF response. I transport foam to dumpster. Cost = gasoline for transportation + time.

I was looking for some sort of average co-efficient per area/volume of a standard good grade acoustic foam vs. frequency, so that I could get an idea of how much of this material I should aquire. The 2' x 2' x room height columns in the corners is reasonable for at least 3 corners[one rear corner has the door located in the corner in such a position as to make it non-feasible to have foam in that corner].

-Chris

 

[bpape]

IF it's good acoustical foam, I'd just extrapolate from something like the Auralex LENRD. I tried to get numbers for the corner cubes (closer to what you'll be doing) but couldn't find anything. From 100Hz up, it will likely perform the same. Down lower, you can't get reliable test results anyway but the thicker material should definitely do more. 2ft wide x 2 ft thick should do a decent job down fairly far if the foam is decent.

 

[Ethan Winer]

Chris,

> The on-the-spot test I used was <

One easy test is to grab a sheet at least 1 foot square and "talk into" it. Hold it right up to your face and go "Testing testing" etc. If it sounds like you're talking into a total void, it's probably absorbing. (Though it helps to have done this before with a known-good absorber.) BTW, this same test works for diffusors too.

--Ethan

 

[WmAx]

Chris,

> The on-the-spot test I used was <

One easy test is to grab a sheet at least 1 foot square and "talk into" it. Hold it right up to your face and go "Testing testing" etc. If it sounds like you're talking into a total void, it's probably absorbing. (Though it helps to have done this before with a known-good absorber.) BTW, this same test works for diffusors too.

--Ethan

I do a varient of this test initially: I put 4 inches of material against a wall and bark at several different tones to see if it deadens the sound. However, I find it difficult to tell how well it's working at lower midrange[<500Hz] with this method. Therefor, I listen for a white-noise like broadband source[air vent usually] and put 4" over my ears to see if it cuts out a good portion of the lower midrange band. The foam in question passed both on-the-spot tests superbly.

-Chris

 

[Savant]

WmAx,

You can get fairly decent results from foam at 50 Hz, but you will need the space to do it. If the foam is of a decent density, you will have to incorporate some sort of airspace behind the absorbers. Stacking the panels such that they are at least 3" to 4" deep and placing them at 45° over a corner is one approach. For 50 Hz, this "panel" will have to be at least about 24" wide.

You can also use stacks, as discussed above. I would generally agree with the minimum stack dimensions discussed, but would add that experimenting with different placements will be key to effectively addressing problems in the 50 Hz range. If you know the exact nature of the mode or modes that are causing these problems, I can help you with good starting points for placement. Since the wavelength in the 50 Hz range is on the order of 20' or so, you will want to incorporate average spacings of roughly 24" to 30" from, say, a corner. Placing foam "columns" or "gobos" into the room roughly the same distance from the walls (or ceiling) will help considerably if you (a) have the space to do it and (b) the mode or modes you're addressing are axial in nature.

Finally, I would advise against pitching the foam until you've exhausted all your options.

 

[sploo]

I was looking for some sort of average co-efficient per area/volume of a standard good grade acoustic foam vs. frequency, so that I could get an idea of how much of this material I should aquire. The 2' x 2' x room height columns in the corners is reasonable for at least 3 corners[one rear corner has the door located in the corner in such a position as to make it non-feasible to have foam in that corner].
-Chris

Hi Chris,

I was reading a copy of F.Alton Everest's "Master Handbook of Acoustics" a while back, and I seem to remember there was a long list of materials with absorption coefficients in the back.

Whilst you'd need more than one trip, you could get some boards, and then use an SPL with software like ETF to measure the effectiveness, and determine how much you need.

Sploo.