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Post by redrum on Mar 14, 2017 0:13:58 GMT
Hello! This is my first post here, I need to say that this is a great forum and awesome learning resource. I've been studying acoustics for the past year or so, and Ethan's posts and test have been hugely helpful! Anyhow, I'm struggling to understand how (as is often mentioned) "all fabric is acoustically transparent" for low frequencies. - Sound is vibration / movement of air molecules - Some fabrics are so dense, that there is no way that air could literally "pass through" them. The same goes for FSK (and similar) backings on some fiberglass panels. This leads me to conclude that the only way sound could "pass through" such barriers is if the incoming sound makes the barrier itself vibrate, transferring the vibration to the other side. Is this what's meant by "transparent"? In that case, materials that pass the "breathe-through" test are in fact "really" acoustically transparent, but materials such as a plastic bag (for example) are transparent because they let the sound pass through by means of sympathetic vibration. In this latter case, the important factor is the non-rigidity (flexibility) of the material. Am I correct? In conclusion, a plastic bag could theoretically be made not to pass low frequencies, if it could be made stiff enough - by stretching / tightening it in all directions. In reality, of course, this is not possible because the bag would break before it could be made tight enough  Is this all correct? |
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Post by Hexspa on Mar 14, 2017 4:47:07 GMT
There's probably a cutoff for every material.
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Post by rock on Mar 14, 2017 14:00:44 GMT
This leads me to conclude that the only way sound could "pass through" such barriers is if the incoming sound makes the barrier itself vibrate, transferring the vibration to the other side. Is this what's meant by "transparent"? No, transparent means all freqs pass through essentially un-affected. In the case of paper,plastic and FRK, some mid and high freqs are reflected so those materials are not completely transparent although LFs do pass in the way you describe. There is no sharp cutoff, instead a gradual roll-off. I never really thought about what properties of paper or plastic allow low frequencies to pass while reflecting some mids and highs but I'll guess it has to do with porosity. The larger the "holes" in the fabric, the more high frequency vibrating air molecules will pass through directly. Materials with the smaller or no "holes" (i.e. paper/plastic/foil), the more the transmission relies on the air molecules to vibrate the membrane and then in turn, the membrane to vibrate the air molecules on the other side. I imagine this action in itself will dissipate some acoustic energy into heat (which is apparently why we use it in the first place!) and reflect some higher freqs back into the room. In the case of the absorbers we make and use with paper/plastic/foil, it is spray glued to the porous fiberglass or rockwool and I don't believe the tightness is a significant or a factor on the operation. Cheers, Rock |
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Post by redrum on Mar 14, 2017 21:31:01 GMT
No, transparent means all freqs pass through essentially un-affected. In the case of paper,plastic and FRK, some mid and high freqs are reflected so those materials are not completely transparent although LFs do pass in the way you describe. There is no sharp cutoff, instead a gradual roll-off. Thanks for the answer. Yes, that's what I meant. I should have written "transparent for some frequencies". But I get what you mean, the term "transparent" should be reserved for materials that pass through all frequencies. I never really thought about what properties of paper or plastic allow low frequencies to pass while reflecting some mids and highs but I'll guess it has to do with porosity. The larger the "holes" in the fabric, the more high frequency vibrating air molecules will pass through directly. Materials with the smaller or no "holes" (i.e. paper/plastic/foil), the more the transmission relies on the air molecules to vibrate the membrane and then in turn, the membrane to vibrate the air molecules on the other side. I imagine this action in itself will dissipate some acoustic energy into heat (which is apparently why we use it in the first place!) and reflect some higher freqs back into the room. In the case of the absorbers we make and use with paper/plastic/foil, it is spray glued to the porous fiberglass or rockwool and I don't believe the tightness is a significant or a factor on the operation. Cheers, Rock I understand what you mean in this practical example of gluing plastic to fiberglass etc..... But I would argue that stiffness indeed IS a major factor in the operation of an absorber. Imagine you had a membrane which was 100% stiff and non-porous. That would be a 100% reflective membrane. Of course, in the example of a bag in front of a fiberglass slab - it is impossible to make it 100% stiff. How I see the operation of (for example) an FSK glued to fiberglass -> The membrane (FSK) on it's own is too light, and would simply vibrate with the incoming sound, absorbing very little energy. When it is glued to the surface of fiberglass, the whole thing becomes one system, heavier, damped and useful for absorption! |
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Post by Nigel Spiers on Mar 16, 2017 6:08:36 GMT
Hi All,
This is a fascinating question. In fact the same or similar question is being asked on two current posts.
I would love to hear Ethan's opinion on this matter i.e. how exactly does a a thin membrane glued to an absorption panel increase it's absorption of low frequencies so significantly?
Thanks & Best Regards
Nigel
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Post by Ethan Winer on Mar 16, 2017 16:27:34 GMT
I honestly don't know for certain why a membrane improves LF absorption. I've theorized that it's halfway between rigid fiberglass and a tuned wood panel bass trap. But I don't really know for sure. But clearly it's true based on empirical measurements.
About sound waves passing through dense material: Sound waves are not air flow. So yes, the material vibrates on one side when struck, and when the other side vibrates I believe it can be considered as a new sound source.
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Post by eurasian on Mar 16, 2017 23:34:11 GMT
Think of impedance and it's components - resistance and reactance.
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Post by redrum on Mar 26, 2017 20:47:07 GMT
Just saw the "newer" answers, thanks for the help! I still haven't decided which material to use, but I guess I can't make to big of a mistake (if low-frequency "transparency" is the main factor). And regarding "what turns a membrane into a limp-mass absorber" question.. I guess the only solution is testing |
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