Plastic wrap on panels test

[Gunner]

Hi all-
Hope you're going good. I am in the process of filling the spaces between my exposed joists in my new basement studio with OC703. Since the EPA recently added fiberglass to it's list of possible carcinogens I was worried about spending 8 hours a day in a room that rains cancerous dust. My plan was to cover each panel with plastic wrap but I couldn't find any definitive info about the effects on the acoustics. I finally realized I could just run a semi scientific test. I ran a bunch of test tones through my speakers (sin sweep from 20-20k, stepped warble tones, white noise) and recorded them with an omni mic in the center of the room. Then without changing any settings I covered the mic in plastic wrap and rerecorded the entire set again.
When comparing the 2 recordings it turns out the plastic wrap drops the entire spectrum down 2-4 db. there were a couple of places in the lows that dropped nearly 10 db but I attribute that to making a slightly resonant plastic wrap bubble around a mic capsule.
My conclusion is that the plastic wrap diminishes the effectiveness of the panels slightly across all frequencies but not enough to risk cancer.

Hope that was useful. Anyone like to add anything?

[Soundman2020]

Since the EPA recently added fiberglass to it's list of possible carcinogens

They did? Got a link to that? Previous research showed little evidence for that connection, so that's rather surprising.

When comparing the 2 recordings it turns out the plastic wrap drops the entire spectrum down 2-4 db. there were a couple of places in the lows that dropped nearly 10 db

That's even MORE surprising. It should not have affected the lows at all, as they should have passed through the plastic directly into the 703, with no attenuation at all, and you should have seen a much larger effect on the highs, as such a large flat surface over th3 703 will reflect highs rather well, since it is comparable to the dimensions of high frequency sound waves. So something is wrong here: How did you ensure that you got the mic back to the same place for the second measurement, after you covered the panels with plastic? What type of plastic did you use? And how did you install it? Did you wrap the 703 panels completely, on both sides, or did you just stretch it across the front?

but I attribute that to making a slightly resonant plastic wrap bubble around a mic capsule.

Hang on a sec: You put plastic bubble wrap around the MIC? I thought you were going to test the panels!!!! What possible relationship could there be between wrapping the panels and wrapping the mic?

Your test is totally invalid. It shows nothing at all about the effect of wrapping 703 panels in plastic. All it shows is the effect of wrapping a mic in bubble plastic. Why would you want to test the effect of bubble wrap on a microphone? I can't think of any good reason at all for doing that. Maybe you can explain why you think wrapping a mic in bubble wrap can tell you something about wrapping panels of 703 in plastic?


- Stuart -

[Gunner]

They did? Got a link to that? Previous research showed little evidence for that connection, so that's rather surprising.

http://www.nytimes.com/1994/07/03/us/fi ... nogen.html I guess it got added to an action list of strong possibilities.

That's even MORE surprising. It should not have affected the lows at all, as they should have passed through the plastic directly into the 703, with no attenuation at all, and you should have seen a much larger effect on the highs, as such a large flat surface over th3 703 will reflect highs rather well, since it is comparable to the dimensions of high frequency sound waves. So something is wrong here: How did you ensure that you got the mic back to the same place for the second measurement, after you covered the panels with plastic? What type of plastic did you use? And how did you install it? Did you wrap the 703 panels completely, on both sides, or did you just stretch it across the front?

I actually wrapped the mic, not the panels.

Hang on a sec: You put plastic bubble wrap around the MIC? I thought you were going to test the panels!!!! What possible relationship could there be between wrapping the panels and wrapping the mic?

I did not use bubble wrap, you misread me. The plastic wrap around the mic created a small sphere of plastic wrap around the mic.

Your test is totally invalid. It shows nothing at all about the effect of wrapping 703 panels in plastic. All it shows is the effect of wrapping a mic in bubble plastic. Why would you want to test the effect of bubble wrap on a microphone? I can't think of any good reason at all for doing that. Maybe you can explain why you think wrapping a mic in bubble wrap can tell you something about wrapping panels of 703 in plastic?

I don't, but I'm glad you spent as many sentences as you did pointing out that bubble wrap is not good to wrap around a mic. I DO however theorize that the reason plastic wrap might affect acoustic panels would be that it would not allow certain frequencies to pass through and be absorbed, they would be reflected instead. this test allowed me to make rough assessments of what frequencies would pass through plastic wrap.

[Soundman2020]

I DO however theorize that the reason plastic wrap might affect acoustic panels would be that it would not allow certain frequencies to pass through and be absorbed, they would be reflected instead.

Well, wrapping the panels is a common method used in room studio construction anyway. It doesn't need theorizing: it's already well know that thin things reflect highs and allow lows through, which is why this is normally done on bass traps, to keep the highs in the room and only absorb the lows. Your test shows the exact opposite of what actually happens, and therefore your tests are invalid.

this test allowed me to make rough assessments of what frequencies would pass through plastic wrap.

No it didn't, because you didn't put it in the right place! If you want to find out what effect plastic has on panels, then you have to put it on the panels, not on the mic! Putting it on the mic tells you nothing useful at all, except what happens when you put plastic on a mic... Acoustically, that is not the same at all. Sorry. It's like wanting to know what the temperature is in New York by measuring it at the south pole....

If you wanted to measure the effect of any type of acoustic treatment (such as acoustic panels wrapped in plastic), then you'd have to measure it the same way that acoustic labs do: in a reverberant room with known characteristics, place several sample panels without plastic on them, measure the acoustic response of the ROOM (not the mic), then put the plastic on the samples and measure again. Repeat the test several times, and average out the results. The difference between the two sets of tests is the effect that the plastic has on modifying the acoustic behavior of the panels, and the way that the affect the room. The mic has nothing to do with it.

You cannot measure the effect that acoustic treatment will have on a room by placing a tiny little piece of that treatment on a mic. That would be like trying to find out if chicken tastes better with or without salt, by just eating the salt!


- Stuart -

[Eric Best]

That report is almost 18 years old, a lot of information since then showing fiberglass isn't a carcinogen.

[Soundman2020]

Yup, I noticed that: Not very recent at all.

Somewhere on the forum Rod did a long post about this subject, just a couple of years back (I think it was Rod: might have been someone else). He came to the same conclusion: studies show that it isn't carcinogenic. But he did also point out that it's still a good idea to use plastic under your insulation, at least on the ceiling, to stop that fine dust slowly filtering down into your equipment. Glass dusts on fader tracks doesn't do nice things to the faders.... It probably doesn't enhance the aesthetics or the sound of mics, speakers or instruments, either....

So you don't need plastic to stop you getting cancer, but it's still useful acoustically if the room is over-treated, and also a good idea for keeping that glass dust (or mineral dust) out of your gear.

I don't have time to look for that thread right now, but it should come up on a search, if anyone is interested.

- Stuart -

[BriHar]

Perhaps a better analogy to wrapping the mic in plastic to simulate wrapping the panels:
If your method held true, then we could save a lot of money by stuffing our ears with a bit of cotton wadding when mixing instead of applying acoustic absorbers and trapping in the studio.

Last I heard, fiberglass had been downgraded to a class 3 or something meaning that all research indicates it does not cause cancer in humans, but remains on the list as a butt-cover. "just in case".

Remember, sun and sand are also known carcinogens! Glass is just another form of sand (silicon dioxide).

[Gunner]

Perhaps a better analogy to wrapping the mic in plastic to simulate wrapping the panels:
If your method held true, then we could save a lot of money by stuffing our ears with a bit of cotton wadding when mixing instead of applying acoustic absorbers and trapping in the studio.

Actually I wasn't testing how plastic would affect the sound of a room. I was testing what frequencies pass through it. Stuffing cotton balls in your ears would be a good way to test how wrapping compressed cotton around your OC would affect the frequencies that actually made it to the fiberglass. Though you couldn't accurately analyze the results, so maybe you should wrap the compressed cotton around a mic and record it.

While that may not be the right NY times article, it was in the paper in the past 6 months. I doubt the NY times is publishing 18 year old studies and calling them new.

[Soundman2020]

Actually I wasn't testing how plastic would affect the sound of a room. I was testing what frequencies pass through it.

No you were not: you were testing how plastic bubble interacts with a microphone. You might THINK you were testing what frequencies pass through, but in reality you were not. Acoustics is far more complex than you seem to think. It might seem intuitive that if you wrap a mic in plastic then you are a testing what frequencies get through plastic, but actually here's not much intuitive about the way sound behaves at all. There's things like edge diffraction, specularity, surface tension, damping, rigidity, resonance, refraction and a whole bunch of other things going on with your plastic bubble that would be totally different for plastic over insulation, not to mention scalability and several other factors. It just isn't even close to being the same thing.

Stuffing cotton balls in your ears would be a good way to test how wrapping compressed cotton around your OC would affect the frequencies that actually made it to the fiberglass.

Ummmm.... no it would not, for the same reasons as above.

Though you couldn't accurately analyze the results, so maybe you should wrap the compressed cotton around a mic and record it.

And that would be different again, not comparable to either cotton-in-the-ears nor to cotton-wrapped-around-insulation. If that were the case, then simply putting a foam windshield on a mic should make any room sound good!

Sorry, but there's just so much complexity to acoustics that simple tests like yours are meaningless. They don't show anything useful at all, and certainly not what you think they show.

On fiberglass as a carcinogen:

http://monographs.iarc.fr/ENG/Monograph ... lume81.pdf


- Stuart -