absorption behavior of a panel trap

[Brian Dayton]

this is a question related to the thread where Bert is clearly exposed as an idiot

clearly

the absorption behavior of a panel trap is as this, correct:


1.  it's a resonance phenomenon, and absorption takes place via sympathetic vibrations (which are peaked at the resonance).  Basically, sound impinging on the trap winds up "negated" because the motion of the trap cancels the pressure of the impining wave.

boy am i not sure i made adequate sense of that.  but i am pretty sure at least i know what i mean.


2.  the concept of these type of traps re-radiating sound into the listening environment is as this:  they "absorb" in a totally different manner than a porous absorber (this cancellation effect) and as such we may wind up with a situation where

1.  the sound stops but
2.  the panel keeps vibrating so
3.  the panel now becomes a sound source in the room

you fellas have posted about this from time to time, and if you've a moment to offer what you know, please do

[Brian Dayton]

http://www.customaudiodesigns.co.uk/art ... ranes.html


there, and various places herein people recommend not smashing the absorbing bunk up against the panel, lest we damp the thing and reduce vibrations

eric commented something along the lines of "but by the time i got the damping high enough to eliminate this, it didn't work as an absorber anymore"

i found this explanation:  http://www.kemt.fei.tuke.sk/Predmety/KEMT320_EA/_web/Online_Course_on_Acoustics/absoprtion.html


which i found un-satisfying.


so until someone hits me with something hard (or, alternately, with something very sharp) i'm going to presume that my intuitive guess is correct, and the "absorption" that takes place with this type of trap is better described as "cancellation"?  

i asked once previously if damping the panel of an absorber would lower the peak level of absorption, and Eric confirmed.  FWIW, the www.kemt...  link above shows rubber/limp membrane having higher peak absorption than plywood.  It absolutely stands to reason that if resonance is the phenomenon that causes absorption, then killing resonance would kill absorption.  a critically damped panel trap would be a moot point.

FWIW, i don't particularily enjoy the "the resonating panel pushes sound into the cavity, where the fiberglass can absorb it" explanations one finds here and there either

[knightfly]

Brian - "FWIW, i don't particularily enjoy the "the resonating panel pushes sound into the cavity, where the fiberglass can absorb it" explanations one finds here and there either" -

You may want to buy a copy of Dr. D'Antonio & Dr. Cox's new book, Acoustic Absorbers and Diffusors - Theory, Design and Application; on page 159, section 6.2.1, (Bass trap membrane absorber) where they state (in part)

"The membrane converts pressure fluctuations into air motion. As the membrane sympathetically vibrates over a selective low freuency range, determined by its mass and the air spring stiffness, it pushes air through an internal porous layer producing low frequency absorption."

There's a lot more, but that's the gist; in all honesty, I've got probably 325 of the 400 pages left to read for the first time, just went to the index for this quote because I wanted to make sure I'd not made some of those "explanations you didn't like'  based on un-educated readings instead of "the high-priced kind"  :?

Said book ain't cheap, but TANSTAAFL... Steve

[Brian Dayton]

hey steve, awesome

so that implies that i'm wrong, eh?  well, if i am i am

so if we built a panel trap with an absolute bare-bones minimum of damping (steel panel, steel frame, all welded construction, no damping material in the cavity) it would then (per this theory where the absorption takes place in the porous/fibrous/fiberglass material) exhibit no absorption?


an interesting related phenomenon:  putting damping material on the panels in the trunk of a car RAISES the SPL level of subs in the car by reducing sympathetic vibration in the panels (said vibration wokring against the woofers efforst to pressurize and de-pressurize the cabin of the car)

this i know to be so, as i've done it in cars i had in days gone by (loud little buggers)


i guess i'll have to buy some books

[knightfly]

"so if we built a panel trap with an absolute bare-bones minimum of damping (steel panel, steel frame, all welded construction, no damping material in the cavity) it would then (per this theory where the absorption takes place in the porous/fibrous/fiberglass material) exhibit no absorption?"

Not entirely; the section I quoted from also mentions absorption due to panel flexing itself, calls it "minimal", and comments about the difficulty of predicting accurately what a panel trap will do partly because of the nature of the joint between the side panels and the "panel" panel, etc - then, there's always the interminable pages of formulae, etc, which I tend to avoid until some "future" time when I'm not trying to juggle 19 other projects (yeah, right  :bang  )

[Paul Woodlock]

hey steve, awesome

so that implies that i'm wrong, eh?  well, if i am i am

so if we built a panel trap with an absolute bare-bones minimum of damping (steel panel, steel frame, all welded construction, no damping material in the cavity) it would then (per this theory where the absorption takes place in the porous/fibrous/fiberglass material) exhibit no absorption?

Well I would think it would absorb. Surely the only panel that wouldn't absorb is a panel that sits in free air, is infinitely stiff, and has zero mass.






Paul

[Paul Woodlock]

My question is what's the better absorber?

The panel or the wool behind it ( touching or not )?

#1 Is a Panel ( no damping wool ) a great absorber at the resonant frequency, but nowhere else. ( high Q )

#2 Is a  Panel+Damping Wool a not so great absorber than #1, but better at other freqeuncies ( low Q )

#3 Is Pure Absorption ( Chunks etc ) a much better absorber at all freqs than #2, but not as good as #1 at #1's resonant freqeuncy?


Paul

[Brian Dayton]

My question is what's the better absorber?

The panel or the wool behind it ( touching or not )?

#1 Is a Panel ( no damping wool ) a great absorber at the resonant frequency, but nowhere else. ( high Q )

#2 Is a  Panel+Damping Wool a not so great absorber than #1, but better at other freqeuncies ( low Q )

#3 Is Pure Absorption ( Chunks etc ) a much better absorber at all freqs than #2, but not as good as #1 at #1's resonant freqeuncy?


Paul

that's kind of my thought as well, save perhaps #3 being better at all freq's if the panel trap was tuned very low.


bu ti'm asking in this thread, not telling.  Pauls logic contradicts that quoted by Steve & the link that i presented above.  


I enter two pics presenting what i think (the first pic) paul is saying adn what i think (second pic) the theory steve quoted and i linked above is saying

I am not 100%, but i think eric confirmed #1 at a previous date?

[Brian Dayton]

ok, fellas, we have options 1, 2, and 3, from the top above


each show the well-known widening of bandwidth with damping, all show different behaviors WRT levels and so forth


option #1 reflects mechanical resonances as you'd find in a bell or what-have-you

[Bob]

Brian

There's two types of damping available here - damping the spring (fiberglass in the cavety), and damping the membrane (limp mass, cymbal, stiffness).

It absolutely stands to reason that if resonance is the phenomenon that causes absorption, then killing resonance would kill absorption.

Isn't it the difference between structural resonance vs mass(membrane)-spring(air) resonance. I'd assume the mass-spring would be dominant, particularly for larger panels and flimsier membranes. To get the entire panel, rather than just the middle, to vibrate, the panel can be mounted in something flexable like neoprine.

Resonance usually means it's bouncing a bunch of times. But in the case of the membrane absorber I thought it was more about where the mass-spring became effective and started sucking the energy out of the pressure by allowing movement. Any movement of the membrane is one that converts sound to mechanical motion to heat, with a bit more due to friction where the membrane is fastened. There's also losses of energy in the porous material in the cavety. Other frequencies the panel moves less.

"The effect of bending stiffness is to increase the resonant frequency." pg 171 of Cox.

Also on page 171 "More complex modelling of panel absorbers does exist [8, 9, 10] but the prediction models presented are not that useful in designing practical surfaces. It is possible to model the plate vibration, and then use a mode matching apraoch to derive the power absorbed. This is complex, and many parameters concerning real surfaces, such as the mounting conditions of the panel, will not conform to simple conditions that the precition models use."

[Bob]

Paul

#1 Is a Panel ( no damping wool ) a great absorber at the resonant frequency, but nowhere else. ( high Q )
#2 Is a  Panel+Damping Wool a not so great absorber than #1, but better at other freqeuncies ( low Q )
#3 Is Pure Absorption ( Chunks etc ) a much better absorber at all freqs than #2, but not as good as #1 at #1's resonant freqeuncy?

Cox said somewhere that (I'm paraphrasing, so may be WRONG)
a) it's hard to build a membrante trap that actually works at the target frequency, the larger the better
b) if the sound hits the panel at an oblique angle, you may need a different tuned frequency
c) helmholtz, including slotted, are much easier to get right.

[Brian Dayton]

Brian

snip

Resonance usually means it's bouncing a bunch of times. But in the case of the membrane absorber I thought it was more about where the mass-spring became effective and started sucking the energy out of the pressure by allowing movement. Any movement of the membrane is one that converts sound to mechanical motion to heat, with a bit more due to friction where the membrane is fastened. There's also losses of energy in the porous material in the cavety. Other frequencies the panel moves less.

i guess that's what i was sort of trying to say .  you said "sucking the energy out of the pressure by allowing movement", i said cancelling.  I proposed a non-conversion-to-heat mechanism of yielding the effects of absorption (cancelling).  

"The effect of bending stiffness is to increase the resonant frequency." pg 171 of Cox.

that's awesome to read.  i know this half-crazed midwestern dude who says that all the time.

Also on page 171 "More complex modelling of panel absorbers does exist [8, 9, 10] but the prediction models presented are not that useful in designing practical surfaces. It is possible to model the plate vibration, and then use a mode matching apraoch to derive the power absorbed. This is complex, and many parameters concerning real surfaces, such as the mounting conditions of the panel, will not conform to simple conditions that the precition models use."

that's how it always goes with models.  but at least complex models assuming some of reality are better than Fmam models.

this is cox that Steve quoted?  if i order that i'll get this stuff?

[Bob]

Cox = "Acoustic Absorbers and Diffusers: Theory, Design, and Application" by Trevor J. Cox, Peter D'Antonio, ISBN: 0415296498

[bert stoltenborg]

Why don't you kick an awful lot of passive membranes (speakers without the magnets & voicecoils) in a large box, eventually vented, tuned according to TS-parameters (the bit that is applicable to speakers without magnets) and you have a hell of an absorber.  I even wonder if angle of incident is very important with such a contraption. Use a membrane with a weight adjustable system and you can tune the shit out of it.

Dammit, I should ask for a patent.

(I'm still looking for a forum to discuss South Park quotes, so I'm still hanging around here).

[Scott R. Foster]

Too late I just slapped a cab with an unloaded basket on the bookshelf and called it macaroni... prior art.

=p

[bert stoltenborg]

Scott:
Too late I just slapped a cab with an unloaded basket on the bookshelf and called it macaroni... prior art.

=p


Wow, that's deep, man....

[knightfly]

Bob, your "b" comment above,"b) if the sound hits the panel at an oblique angle, you may need a different tuned frequency" - I'm not sure I'd agree with that one without more discussion; seems to me that, in all but pretty large rooms, any frequencies you would need a panel trap for would be low enough to fall into the "pressure mode" basket instead of the "wave mode" basket; that being the case, I would think direction wouldn't matter.

Brian - Yes, the Cox book is where that came from; pony up yer $160 and enjoy  :?

Also, why GUESS about curves - on page 208 of Everest's MHOA, 4th ed, there is a graph of 3 ACTUAL panel traps with specs - the one WITHOUT internal absorption has a lower absorption, FLATTER curve than the two (with different panel thicknesses) which both had 1" mineral wool with 1/4" gap between the wool and the panel - without wool, peaked absorption at .32 at resonance; WITH wool, one peaked @  .6, the other @ .65 with a MUCH MUCH peakier curve.

Not that this is the greatest acoustics book in the universe, but it's the ONLY one I've found that's available in electronic format - I have both, and even though the PDF version costs a few $ more than the print version, it's been really handy as a quick reference. For this example, I opened pdf reader, double clicked on the pic of the cover, did [ctrl]F, typed in "panel", hit the down cursor 4-5 times, and voila; graph of ACTUAL panel absorbers referred to above.

No commish from Amazon, but I STILL think this is a good deal for nearly ANYONE reading these pages, if you're this far along you'll not be fooled by the typos in this version,

I know it woulda been faster to show the graph, but even WITH my "free advertising" I'm not sure it would be appreciated by the author or his heirs... Steve

[Brian Dayton]

hmmmmmmmmmmmmmmmmmmmmmmmmmmm

following steves lead to pg 208 of everest, i note that his description was spot-on

we have higher absorption and a NARROWER bandwidth, which is 180 degrees contrary to what every single comment i've ever seen about adding insulation to panel traps says

EW sent an email with some thoughts about his experience with panel traps, and offered that the absorption seems to take place in the panel, and that adding insulation broadened bandwidth (lowered the Q), which is 1000% in-line with conversation about these buggers, but in contrast with everests two diagrams (See below as well as steves reference above)



also in everest is page 211 where they show the absorption of poly diffustors with and without insulation.  noted is a boost/narrowing of bandwidth again

FWIW, not to sound like a preposterously skeptical or cynical chap here, but often in books covering broad slices of TL theory they have measurements which aren't all that realistic, or applicable.  at times we see things like two un-studded panels, and are shown that these reasonably follow the general theory as presented in said text...

but real walls have panels, etc.  and i know that example is different from this one.



there is no IR-761 for absorbers, and as such there is not absolute reference that at least allows the comparative effects of things to be "known".  FWIW, the cost of running an IR-761ish study on absorbers would be only about as high as testing one single Green Glue/Gypsum concrete floor assembly in a lab.  that's all, folks, it could easily be done with only modest donation from any commercial company.

[Brian Dayton]

FWIW, it's commonly recommended to not smash the insulation up against the panel, lest we reduce absorption, is it not?

this would seem to imply that the panels motion is at the core here

[Paul Woodlock]

....

this would seem to imply that the panels motion is at the core here

I dunno how related this is.....


Yesterday when I had my almost finished computer silencign cabinet, I did some Isolationtest with the fan on full speed.

The scenario....

The box was sealed on all sides except the top.

then I laid ( not fastened ) a sheet of 18mm MDF on top of the box, and noted how loud I could hear the fan noise. ( pretty quiet )

then... as a further test, and actually in order to press the sheet of MDF tighter over the box to IMPROVE TL, I put a heavy'ish weight ( A full 300' reel of power cable ) on the MDF sheet in the centre.

To my suprise, I noted the TL was LESSENED. Soon as I lifted the weight off the top panel the TL Improved.


MY first though was the resonant frequeny of the panel was changed ( Lowered ) to more match the freqency of the fan noise, and perhaps let more sound through. But I'm not entirely sure this is the case.

Any thoughts, as to what this phenomen might be due to, and whether it's related to this discussion?




Paul