Hi, I want to improve the acoustic of my homestudio. I don't want to go with fiberglass or rockwool. My plan is make a 4' x 2' x 4'' foam panel made by 2 units of 4' x 2' x 2'' of 35kg/m3 density open cell acoustic foam glued one over each other to place them to the left and the right of the listening mixing position for flutter echo and early reflections and plus two panels with 4' x 2' x 6 '' , assembled the same way, at the corners for bass traping. I did read that a space of 4'' between the foam and the wall with just foam spacers (like truespacers from auralex) make a air gap that improve the perfomance of the panel. If I make a wood or metalic stand to mount the foam panels on them and place making the same 4'' air gap, it works too ?

Yes. The issue is that foam (and mineral wool) absorbers work by taking kinetic energy out of the sound wave and converting it to heat instead of sound. They can only do that if the air particles are moving, of course (static air has no kinetic energy). However, adjacent to the wall surface the air can't go anywhere, so there is maximum pressure but minimum velocity. Consequently, it works much better if the fopam (or mineral wool) is spaced away from the wall to put it into a region of greater air particle velocity.

The optimum place is at the quarter-wavelength of the frequency of interest, and in pragmatic terms, that means that the absorber should be spaced from the wall by roughly the same distance as the thickness of the absorber.

It doesn't matter how you space it -- foam blocks, wood blocks, free-standing stands, etc.

And finally, calling a 6-inch depth of foam a 'bass trap' is entertainly optimistic .. but every little helps!

Hugh

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Technical Editor, Sound On Sound

And finally, calling a 6-inch depth of foam a 'bass trap' is entertainly optimistic .. but every little helps!

Ok, so what is the thick of the acoustic foam panel to work like a bass trap ? I thinking in 8''. Is possible to make a good bass traps with acoustic foam panels ? are there other alternatives to make a decent bass trap without fiberglass or rockwool ?

Why do you want to avoid fibre panels?

The stuff you use to build them is mega-compressed compared to the flyaway puffy rolls that most folks use in the loft. Yes, you get fibres fly when you cut it but the trick is to do this bit outside then put the fabric front and back. No probs then.

Thing is, it's the density that does it. Foam just doesn't have the mass to take the lower end oomph out unless you have oodles of the stuff vs the Rockwool. Or that's my understanding anyway but the 'Rockwool vs foam' above is 'learned wisdom' from here vs personal experience - I made rockwool panels that have been fantastic so I've not had the urge to build foam stuff to a/b . Foam fans can of course put me straight

Before i post, i did a search on the net and there are many and many citatiions and NRC curves showing the efficiency, mainly at lower frequencies, of the fiberglass or rockwool panels. I dont want to go with them, because the volatile organics compounds, like styrene. Look well, i dont talk about " fibers in the air" but gas. Someone have a article or citatiion that is secure for the enviromment ? If yes post to link and ok.

Quote Iggor:

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Before i post, i did a search on the net and there are many and many citatiions and NRC curves showing the efficiency, mainly at lower frequencies, of the fiberglass or rockwool panels. I dont want to go with them, because the volatile organics compounds, like styrene. Look well, i dont talk about " fibers in the air" but gas. Someone have a article or citatiion that is secure for the enviromment ? If yes post to link and ok.

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Several of the fibre/wool panel producers are environmentally aware. Here is an example from just one...

"All Blue Frog Acoustics products use Ecose Technology Mineral or Glasswool fillings. Wood from sustainable forest sources is used where practical. The fabric used for the traps is BS-476 and EN 1021-1 compliant for fire and cigarette safety regulations, and can be wiped clean and vacuumed."

Anyway, isn't a lot of foam full of chemical nasties?

Quote Hugh Robjohns:

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... Consequently, it works much better if the fopam (or mineral wool) is spaced away from the wall to put it into a region of greater air particle velocity....

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This interests me. You are much more knowledgeable on these matters than I, so don't take this the wrong way, but...

... if we're concerned here particularly with velocity, surely spacing a panel away from the wall surface is of little benefit as, like for example a snooker ball speeding towards a cushion, the 'particle' is still at maximum velocity right up to the point that it hits. Then, when it rebounds, its fastest point will again be closest to the wall and only diminish the further away it gets.

Therefore, surely the closer to the wall the panel is the more energy it will absorb, or do I have my logic wrong?

Sound waves involve an alternating pressure longitudinal wave, with compressions and rarefactions. The particle movement is not all in one direction, it ebbs and flows so that particles effectively move back and forth from their starting point, passing the energy in one direction. The particle displacement is in quadrature with the pressure, so where the pressure is max or min, the particle velocity is zero, and where the pressure is passing through the normal ambient pressure, the particle velocity is maximum.

Hugh

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Technical Editor, Sound On Sound

I'll chip in here and agree that many absorbents can be completely safe if you look around. No mineral wool or fiberglass has been proven to do much harm other than simple irritation while being handled, though we use ECOSE Greensafe material at GIK just to be safe. Our fabrics are also acoustically transparent, and the Guilford of Maine 701 fabric we can provide is fire treated as well.

While foam is relatively safe as well, it isn't a very good absorbent for anything over 250 Hz. You can check out the test we did here: http://gikacoustics.com/comparing-foam-to-gik-244-bass-traps/

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Alexander Reynolds
GIK Acoustics USA (770) 986 2789 | GIK Acoustics Europe +44 (0) 20 7558 8976 (UK)

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While foam is relatively safe as well, it isn't a very good absorbent for anything over 250 Hz.

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I think that's a typo! Foam doesn't work particularly well for frequencies below 250Hz

hugh

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Technical Editor, Sound On Sound

Haha, thanks for the catch Hugh. Of course, that is what I meant.

Odd, there is a cut off time for editing posts here?

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Alexander Reynolds
GIK Acoustics USA (770) 986 2789 | GIK Acoustics Europe +44 (0) 20 7558 8976 (UK)

Hi. In my country there's no formadehyde and VOC's free fiberglass. there's no ECOSE or Cotton panels. My room is OK about high and Mid Highs frequencies. i had place 2'' open cell acoustic foam in the main positions. I am thinking in make a panel bass trap with 4 '' of High density Rockwool or fiberglass and wrapped then with impermeable ( no acoustic transparent) polyester or plastic fabric. Does it work ?

For those who read this post, there is a article on sounsonsound that is intersting.
http://www.soundonsound.com/sos/oct10/articles/green-studio.htm

The fabric will need to be able to let air pass through, otherwise it wont work. Provided you can breath through the fabric, then that is all you require.

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http://soundcloud.com/richie-royale
http://www.mixcrate.com/richieroyale

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Odd, there is a cut off time for editing posts here?

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Yes... 45 minutes. It's to prevent posts being changed after someone else has responded...

hugh

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Technical Editor, Sound On Sound

You'v probably heard it all before but.... don't bother with foam panels.

I had foam panels all around my listening position and the difference was so minimal that I shouldn't have bothered. I replaced it with Rockwool panels and the difference was amazing. Iotally tightened up my stereo image and the bass traps made my bass so much smoother.

Rockwool panels also works out far cheaper and your concerns about environmental friendless are totally misinformed. You make panels wrapped in material.... fibers DO NOT escape! I don't know what eco extremist society told you any differently.

The OP is concerned about volatile organic compounds being released from the mineral wool (eg. formaldehyde) rather than particles.

There have been lots of studies on this, and most don't seem to think there is any significant problem with mineral wool. (eg. http://pubs.acs.org/doi/abs/10.1021/cr800399g ).

There are inherent risks in pretty much everything we do, make and use and what we have to do is assess those risks and take the appropriate precautions to minimise them. The studio flooring and other contruction materials are probably just as likely to be as hazardous as any mineral wool used to deal with the acoustic problems... and foam is definitely as bad in its own way...

The obvious solution, if you are concerned about air pollution in the room, is to ensure an adequate ventilation system to change the air...

Hugh

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Technical Editor, Sound On Sound