deanm
New Member
Posts: 38
|
Post by deanm on Aug 26, 2018 10:51:08 GMT
This may not even qualify as acoustics 101, but... Insomnia, you know, and, I'm really just thinking out loud trying to figure this out, and asking y'all for confirmation or correction:
Given: above a threshold of thickness, pink fluffy absorbs lows better than rigid fiberglass. And below that threshold, rigid absorbs lows better than fluffy.
I've heard it said that once the total resistance of the trap gets too high, additional sound energy starts "bouncing off" the front of the trap.
What exactly does "bouncing off" mean?
A node is a point of highest compression along a standing wave, and lowest velocity. As a wave of oscillating air particles approach the node, the increased pressure slows the particles down and rebounds them back towards the antinode. If a trap is thick and dense enough to completely stop the wave from passing through it (via friction on the particles), this creates a node somewhere in the midst of the trap, and the increased air pressure at/around that node resists/deflects/repels additional waves.
Here's what defines the threshold: If this node happens to be closer to the face of the trap than the wavelength (or more practically, 1/4 of the wavelength), then the trap doesn't absorb the wave as well as if the wave penetrated deeper, possibly reaching the forced node at the wall....
right?
<tinFoilHats> So this would mean "soundproof" is usually the enemy of "sound absorbent". And when insulation manufacturers label a product as an "acoustic treatment", or "best for acoustics", my guess is they're usually talking about soundproofing. </tinFoilHats>
-d
|
|
|
Post by Michael Lawrence on Aug 26, 2018 13:20:37 GMT
Hi deanm-
Welcome to the forum.
I think things are simpler than you're making them out to be. I recommend a quick read-through of the opening bits of the Master Handbook of Acoustics.
One mental image you've got to rid yourself of is that of "air particles" bouncing around like ping pong balls. We're dealing with oscillation here, not wind. A better visual is the waving stalks of wheat in a field as the wind goes through. The wavefront certainly propagates forward but the individual particles of air all end up where they started. High frequencies can bounce off surfaces in a specular fashion, like a laser beam bouncing off a mirror, but again that's the wavefront's motion, not that of the individual particles. Velocity absorbers such as fiberglass work by literally creating friction against the oscillating air molecules, so they lose energy to heat. As you point out, in a standing wave, the velocity is at maximum a quarter wavelength from the boundary, so that's where we can generate the most frictional losses with our absorbers.
I'm not exactly clear on what you're asking; does that help answer your question?
|
|
|
Post by rock on Aug 26, 2018 16:12:22 GMT
<tinFoilHats> So this would mean "soundproof" is usually the enemy of "sound absorbent". And when insulation manufacturers label a product as an "acoustic treatment", or "best for acoustics", my guess is they're usually talking about soundproofing. </tinFoilHats> -d Soundproofing the enemy of sound absorption? NO, they are two completely different things. "And when insulation manufacturers label a product as an "acoustic treatment", or "best for acoustics", my guess is they're usually talking about soundproofing." NO, I think they ARE talking about "Acoustic Treatment". Read below, I will try to explain what I understand. "Soundproofing" I think is better described as "Sound Transmission Attenuation". Think of sound in one room passing through a wall to an adjacent room. If you want to "soundproof" your room, you will attenuate or reduce the sound going from one room to the next. Techniques involve adding mass to walls, ceilings and floors, insulating between drywall, brick or whatever the mass components of the wall is made of and "isolating" structural members so vibrations don't conduct to the next room. Sound absorbent or absorbing are techniques used in what I'll call "Acoustic Treatment". I'll define Acoustic Treatment (that which is this forum's primary focus) as anything done to the interior of a room to affect the quality of the sound IN that room. Note that these two items (sound transmission and sound treatment) are really two different things. You can treat a room and not "soundproof" it or you can "soundproof" it and not treat it or you can do both. The one does not do (or undo) the other.
|
|
deanm
New Member
Posts: 38
|
Post by deanm on Aug 26, 2018 21:34:46 GMT
Hi Michael, well, this was mostly just insomnia-driven rambling. I guess if there's a question in there, it's, "why exactly does denser fiberglass absorb low frequencies less efficiently once it surpasses a threshold of depth?" And then I tried to answer it. Reading is an excellent idea, rather than spinning out convoluted theories when I can't sleep about why wheels are round. Yes, I should buy some textbooks to read, instead of just forums. Regarding waves vs. particles - right. I get that - about waves traveling, while the particles end up right where they started. The particle oscillations create the wave. When I was talking about particle movement, I was "zooming in" to try to understand why longer waves would be reflected from a trap dense/thick enough to force a node (with a high-pressure zone) to exist within the trap. It would be more accurate to say "as the WAVE approaches the node, the increased pressure..." I'll edit this, thanks Hi Rock, thanks for the response. My gist here is that when manufacturers say "acoustic treatment", we cannot interpret that as meaning "sound absorptive", because they probably mean "sound transmission attenuating". At least, that's what I'm finding when I go insulation shopping. You're correct that they're not enemies of each other as much as they're just unrelated. In a roundabout passive-aggressive kind of way, I guess I was ranting about the local insulation salesperson telling me adamantly that I need dense material for my planned 12" thick ceiling trap, instead of fluffy stuff. "See here, where it says this one is an 'acoustic treatment'? That means it's what you need! Trust me, I read about it on the interwebs." And in this circumstance, making the trap more soundproof by adding density would also make it less absorptive at low frequencies. -d
|
|
|
Post by Hexspa on Aug 27, 2018 0:27:12 GMT
I just looked through the Cox/D'Antonio book and it looks like no one knows. They don't even mention it, from what I can tell. What they do talk about is the myriad ways which a porous absorber can be measured and its performance predicted, none of which have exact correlation.
In other words, all we know about absorption is a cobbling together of tests and predictions but it's complicated as all get out and no one knows the whole answer.
|
|
|
Post by Ethan Winer on Sept 23, 2018 15:14:50 GMT
I wonder if the answer is actually this simple explanation: As a soft absorbing material is made more rigid and dense, eventually it becomes like a piece of wood and reflects more than it absorbs. Somewhere in between those extremes it absorbs bass well but mids and highs are reflected off the semi-hard surface. Is this what's being asked by "What exactly does 'bouncing off' mean?"
|
|
|
Post by Hexspa on Sept 24, 2018 4:56:44 GMT
I wonder if the answer is actually this simple explanation: As a soft absorbing material is made more rigid and dense, eventually it becomes like a piece of wood and reflects more than it absorbs. Somewhere in between those extremes it absorbs bass well but mids and highs are reflected off the semi-hard surface. Is this what's being asked by "What exactly does 'bouncing off' mean?" I agree that the principle can probably be reduced to something simple. While I like my 'rattling license plate' hypothesis, I've yet to see hard data describing why and how FRK works.
|
|
|
Post by Ethan Winer on Sept 24, 2018 16:29:40 GMT
I don't really know for sure how it works either. I've hypothesized that it takes on some properties of a wood panel bass trap, but being bonded to the fiberglass makes the thin membrane less resonant. Or some such.
|
|
|
Post by Hexspa on Sept 25, 2018 8:20:20 GMT
That's pretty much what I'm assuming. The bass is strong enough to vibrate the paper, paper is dampened by the insulation, energy is converted into heat.
EDIT: I thought about this some more. Thinking in terms of vibration is the wrong time scale. If you slow it way down, the waves have pressure. The FRK must function like boxing ropes or a baseball glove. The wave pushes against the material and becomes dampened. Lighter, higher-frequency, waves just bounce off. Lower, bigger waves just plow past. Or maybe I'm trippin'.
|
|
deanm
New Member
Posts: 38
|
Post by deanm on Sept 25, 2018 8:55:51 GMT
practical question: Does FRK / thin plastic behave differently with thick (12-ish inches) of pink fluffy FG, as opposed to 4" rigid FG panels? Should I follow the same guidelines -- i.e., membrane facing out, must be attached to FG (not just floating in front of it), and only used at non-first-reflection points..? to clarify my earlier ramblings about "bouncing off": in my original post, I asked myself what was actually happening when a sound wave "bounces off" a surface that's too dense to allow the wave to penetrate "fully" (meaning, appreciably nearing the 1/4 wavelength point). Then I tried to explain it to myself (out loud) by envisioning what happens at a particle level. Ethan's simple explanation makes a lot of sense too. Geez, I'm glad I put my foil hat on for the last part of my OP. I coulda looked dumb otherwise. Thanks again for all the help, -Dean
|
|
|
Post by Hexspa on Sept 25, 2018 17:27:48 GMT
practical question: Does FRK / thin plastic behave differently with thick (12-ish inches) of pink fluffy FG, as opposed to 4" rigid FG panels? Should I follow the same guidelines -- i.e., membrane facing out, must be attached to FG (not just floating in front of it), and only used at non-first-reflection points..? to clarify my earlier ramblings about "bouncing off": in my original post, I asked myself what was actually happening when a sound wave "bounces off" a surface that's too dense to allow the wave to penetrate "fully" (meaning, appreciably nearing the 1/4 wavelength point). Then I tried to explain it to myself (out loud) by envisioning what happens at a particle level. Ethan's simple explanation makes a lot of sense too. Geez, I'm glad I put my foil hat on for the last part of my OP. I coulda looked dumb otherwise. Thanks again for all the help, -Dean I think we're all on the same page. See my edit in the previous post. The bottom line, upon which we can all verifiably agree, is that the membrane acts within a certain bandwidth one way and has different properties within another. My guess is that pressure is the mechanic and you're adding a measure of resistance but not too much as in a solid concrete wall.
|
|