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Wrapping 703 / 705 / mineral wool in plastic sheeting?
Old 16th May 2008
  #1
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666666's Avatar
Wrapping 703 / 705 / mineral wool in plastic sheeting?

I am very concerned about particles / dust / fibers from assorted bass trapping materials getting into the air. I am generally quite allergic to airborne particles.

Most folks wrap bass trapping materials / insulation with tight fabric which likely holds in most of the fibers / particles. As well, unless you're beating your bass traps with a stick every day, you shouldn't really have a problem anyway.

However, I am still neurotic about keeping the air quality in my room 100% fiber-free. Occasionally I'll have fans or an air conditioner blowing hard while rehearsing, I'm sure having a fan blow against a fabric wrapped trap could potentially push out some fibers into the air, etc.

When making a custom bass trap out of 703 / 705 / mineral wool etc, one thought is to first cover the insulation with a sheet of plastic (like 3 mil, like a plastic trash bag), and then cover that with decorative fabric. The plastic should positively hold in all fibers from reaching the air... even if you beat it with a stick. I like that level of "protection" from airborne particles.

Realistically, is a 3 mil sheet of plastic over your mineral wool yet under your decorative final layer of fabric going to harm acoustics in any way?

Maybe the plastic is just not necessary, but it would give me good peace of mind knowing that zero fibers can reach the air inside my room.

One thought I had for one of my very small rehearsal / tracking rooms...

Against one entire wall, floor to ceiling, install a row of 2"X3" studs on 24" centers, 6 inches out from the main wall... then add some 1"X4"s or furring strips to the back side of the new studs, horizontally... then add 703, 705 or mineral wool between the studs against the furring strips so it's all 6" out from the main wall. Next, staple plastic sheeting over this entire new wall of insulation. Final treatment, add a big hanging curtain made of decorative fabric in front of this new wall of insulation. Then the entire wall, floor to ceiling would be one huge bass trap.

The advantage of using a hanging "curtain" in front is that the fabric could then be removed easily for periodic washing. If the fabric was stapled to the frame as with most bass traps, this makes cleaning more difficult.

Behind the curtain would be the plastic sheeting which would seal out 100% of any fibers that otherwise might make it into the air (even if just a few).

I realize that this would be an extreme level of bass trapping, but since this room is quite small with a low ceiling, having one wall treated like this might be a good thing. In this scenario, would the plastic sheeting have any sonic drawbacks?

Thanks
Old 16th May 2008
  #2
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Weasel9992's Avatar
 

I hear you about keeping the air clear, but wrapping the 703 in plastic would have a definite negative impact on the its effectiveness in the room. You said it though: unless you're beating them with a stick, the amount of fibrous material in the air wouldn't even be measurable...so long as they're made well. You'd have far more risk from other allergens...dust mites, pollen, animal hair...the usual suspects.

Frank
Old 16th May 2008
  #3
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Glenn Kuras's Avatar
Well putting the plastic to the front may work as membrane, but as Frank pointed out your really do not want to wrap the full panels in plastic.

Glenn
Old 16th May 2008
  #4
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jinksdingo's Avatar
3mm is too thick, you want plastic film to contain each absorber. The film will have a negligible reflection effect. If you need to compress the fibreglass use luggage mesh or even chicken wire. The plastic film can be neatly wrapped and hot glue sealed. Once up and completed listen for any bass movement noise on the film and dampen with additional fabric if needed.

Hanging the fabric in front is what I too have done as I'm recycling used Movie Theatre curtains which are loose and pleated over the traps.
It is true about the mites and dust from the fabric, carpets etc and regular cleaning helps prevent dust build up in your studio and gear as well.

Make no mistake about the the fibres becoming airborne without sufficient containment.

Health is a safety issue for any of us.
Old 16th May 2008
  #5
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666666's Avatar
Thanks for the input guys!

I'm still trying to understand though WHY a sheet of plastic, whether 3 mil or even 1 mil thick (which is darned near like Saran Wrap) would have a negative impact, or ANY impact, on sound waves.

I could understand that if your final exposed layer was a 3 mil sheet of plastic that it may reflect a certain extent of upper frequencies as opposed to absorbing, but if the plastic is covered with soft fabric, or even better, a pleated curtain of thick fabric hanging a few inches out in front of the plastic, I'd think that whatever upper freqs that would normally be reflected by the plastic would be absorbed by the fabric / curtain BEFORE they even hit the plastic... and whatever freqs would actually reach the plastic at that point would go right through the plastic like it wasn't even there.

Where is the error in my reasoning? I just want to fully understand this.

As for dust, dust mites and such, thanks for the advise, I am well aware of those things, I keep everything so incredibly clean at all times it's ridiculous. I have a custom air ventilation system in the works that will provide fresh HEPA filtered air into the room, putting positive air pressure on the room... also an air conditioner / dehumidifier, remote vacuum system, etc... the air in the room will be kept at a near "perfect" state at all times. With this level of insanity in mind, you can see why I'm concerned about making sure not to have ANY potential for fiberglass fibers floating around in the room.

Again, I'm just trying to understand why, for argument's sake, a mere 1 mil sheet of plastic, "sandwiched" BETWEEN a thick fabric curtain (exposed side) and say 4 inches of mineral wool would / could have any effect on sound. And if it definitely does, exactly what effect would that be? How will that 1 mil of sandwiched plastic sheeting harm the sonic response of the trap?

As always, much thanks
Old 16th May 2008
  #6
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If you did a very thin plastic and then covered with a deliberately absorbant material like velvet, it would be fine.

Obviously, this is going to be a no-go for any panels that deliberately have a facing on them to deliberately reduce HF absorbtion or act primarily as membrane absorbers.

Bryan
Old 19th September 2019
  #7
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Christof's Avatar
Resurrecting this thread, as I got a very similar problem: Building acoustics for a recording room in a public school, I'm concerned about all the things mentioned in the beginning of this thread. Additionally everything will be "B1"-certified (hardly inflammable).

In the past I just used B1-certified Molton-fabric to wrap the mineral wool of most broadband-absorbers, but also a thick B1-certified foil (250my/0,25mm thickness) for others. Every absorber will get a frame with B1-certified Molton-fabric in front of it (300g/m²).

Wrapping the mineral wool into the foil and sealing those packages with a special duct tape (Siga sicrall) would effectively keep all fibers from being exposed to air, but of course the foil will change the acoustic behaviour, thus reflecting some of the high frequencies. But the Molton in front of each absorber will absorb some of those frequencies, so maybe this would compensate the effect of the foil?

The absorbant effect of one layer of 300g/m²:Wrapping 703 / 705 / mineral wool in plastic sheeting?-molton-absorption.jpg

I would prefer to do it this way and in some cases (big wall-absorber) it probably won't make a big difference (and could even help to avoid too much dampening of high-frequencies), but in others I would not like to compromise on high frequency-absorption (in case of mobile gobos).

Maybe some of the experts could share their thoughts?
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Wrapping 703 / 705 / mineral wool in plastic sheeting?-molton-absorption.jpg  
Old 21st September 2019
  #8
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Christof's Avatar
No replies... means "no problem"?`

Do you think it's sufficient to cover the absorbers with the molton-fabric?
Old 4th October 2019
  #9
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Quote:
Originally Posted by Christof View Post
No replies... means "no problem"?`

Do you think it's sufficient to cover the absorbers with the molton-fabric?
I'm not sure about particles emission, but if your Molton is (like Berger Textiles writes) 300 g/sq.m - most probably it's something like thick denim. So, if you will not kick all the rockwool out of absorbers, most probably everything will stay in place.

If you have real concerns and need some kind of legal permission, you can make small example of absorber and bring it into some lab with subwoofer. After prolonged exposure to LF sound waves people in lab can measure if the air still clean or not.
I have no other idea how to find this out in reality.
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