Building Membrane Absorbers (Forumbeiträge)

building membrane absobers - Page 4 - Gearspace

I know : in your opinion, 1 big trap vs 2 smaller trap but with the same area gives the same results?

correct me if i’m wrong but won’t a larger surface area (holding L:W:D constant) result in a larger amplitude of the motion of the panel? the moment on membrance/support joint exerted by the pressure near the center of the panel should grow with increased surface area, as the distance from the center of the panel and the joint would increase. with a larger moment, the panel should deform more, basically meaning the maximum displacement (amplitude) will grow.
with an increased amplitude, wouldn’t it follow that more air (or whatever the outside damping material is) will be displaced, which would create a larger damping effect?
just my two cents

edit:
I’ll post FEA simulations demonstrating this when I can get back into the labs come Tuesday if anyone wants me to. Let me know if you’re interested

Last edited by deadbolt24; 3rd September 2011 at 06:05 AM.. Reason: clarity

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This thread is a little old, but I want to add that one of the ways of measuring at what frequency is the panel oscillating, is to use a contact microphone. Connect the mic to your computer, put it on the panel and blast the panel with pink noise. Using a program like a RTA, check what frequency is peaking, that should be the resonating frequency of the panel.

I’ve done that. It’s a good idea to measure two or three locations. I can’t remember the name of the guy that makes the limp rubber rings to set the mics on. It’s fun stuff and even if there is some measurement error, it’s data.

Quote:

Originally Posted by deadbolt24 ➡️

super +1
my thinking was regarding rather small panel, where I think a rubber membrane should be more usefull than a stiffer one, for the reasons I wrote before….
but I can be wrong of course

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Quote:

Originally Posted by deadbolt24 ➡️

with a larger moment, the panel should deform more, basically meaning the maximum displacement (amplitude) will grow.
with an increased amplitude, wouldn’t it follow that more air (or whatever the outside damping material is) will be displaced, which would create a larger damping effect?

Yes, I’d guess it depends (a) what model the equations for a specific device were derived from and (b) whether or not the facing acts in that way.

With (a) the last time I saw a panel absorber derivation, iirc the assumption was made that the panel acted pistonically (rather like a passive radiator… hmmm… oh and I bet it was in that SLAM paper), not by bending into the shape of a rigid sheet with distributed loading. That relates directly to (b) as I’d expect the MLV to be much more accurate to the model than a “hard” facing like plywood or even steel (though admittedly G.E. was extremely successful with steel and had a tuning scheme to boot) since the MLV doesn’t really have bending rigidity. It would, however, have an excursion limit because it does have some rigidity in tension. From that standpoint, mechanically, it’s kind of like a two-dimensional rope.

I’d guess that bigger, then, is better (as it were) for a rigid-panel style unit. It stands to reason (to me right now anyway) that the MLV facing would behave more like the model (if memory serves that said model is pistonic), but also that it would absorb more than one faced with a rigid panel since average excursion across the panel face would be larger. Additionally, it tells me that in making the MLV version it’s important to leave it as limp as possible while still sealing the unit. The other kicker is that the closer the unit acts to pistonic action, the more they add linearly with increasing units. For example, if the base unit is 1 m^2 face, two 1 m^2 units will absorb twice as much with a good pistonic action. But with rigid faced units it would probably be better to build a single 2 m^2 unit than use two 1 m^2 units.

I suppose the most effective facing would be very stretchy (no tension rigidity), like balloon film, but obviously of the required distributed mass also. Not that such a material is practically possible, but a guy can dream.

I bet a steel facing with a foam “surround” would work great. Maybe made from surround repair kits for a square high-excursion driver? It would also be highly tunable using G.E.’s method. But I also think that once you take the concept to this level it is probably profitable to go down the VPR road.

little update….
I finished two limp membrane absorbers and the result is 0 (zero) a complete failure…..the membrane do not vibrate at any frequency…..
it can not be a sealing problem because there is silicon on any juncture and in any case it will vibrate even if is not sealed….I say this because I have big honeycomb cartons in front of the big superchunk on the two front angles, the carton is not sealed but it does vibrate on a certain frequency…..
I believe there is something wrong on the rubber membrane but I can’t investigate more than this because I’m getting depressed, that room is not going to be for ever so I’m not going to invest more time in this.
I will replace the Rubber membrane with 3mm MDF panel which I have already and which I used to make the other two which are tuned at 90Hz and they work….
If even this is not going to work I will give up and go back to simple BroadBand porous absorbers, my knowledge and skills on this matter are evidently not enought and probably never will.
I will keep the room as it sound and get use to it….in the near future I’m going to move again and it will be for ever, so I start again when it will be time for that….

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I am rather newbee for this but I would not be depressed for this failure.

Someone in France (on another french forum) posted some encouraging results showing very good results using mass loaded membranes. The main difference is that the membrane is not tuned using an air cavity but is hanging freely.

The result is that the modes are killed.

Traitement “Control Room” et choix d’un diffuseur DIY - Acoustique

Have a look to waterfall links.

I still wonder anyway how it works… not tuned..?

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Quote:

Originally Posted by Nexus.6 ➡️

…. The main difference is that the membrane is not tuned using an air cavity but is hanging freely.

That’s what I am talking about.

It is a damping mechanism and is not tuned. smokin smiley

Cheers,
John

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Quote:

Originally Posted by PaulRain ➡️

Hey Paul,
Thanks for the update and really sorry to hear they are not working. Keep us undated though if something does work in the future. howdy smiley

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Quote:

Originally Posted by PaulRain ➡️

little update….
I finished two limp membrane absorbers and the result is 0 (zero) a complete failure…..the membrane do not vibrate at any frequency…..

Paul,

did you test room response? or did you test the membrane for vibration? It’s almost like testing for vibrations on surface of broadband absorbers except the density of the MLV will cause the unit to have an acoustic crossover at around 400 to 700Hz. I would do a measurement of the room response with/without the limp bag traps. Also note that the bag must not touch any of the absorbent material.

Cheers,
John

I didn’t test the room, I was very disappointed……but there is also another reason I removed the Rubber, it smells like crazy, the room is small and I would have probably die within few hours with those thing in the room Smile smiley
so I didn’t even bother testing….
The membrane was not touching the absorbent material.
maybe I’m wrong but a membrane on the front of a sealed box should vibrate to make it work otherwise how can the air move inside?

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Quote:

Originally Posted by PaulRain ➡️

John is spot on, but if it smells like %$#$ then……………..

just for make a little bit of light in the dark I’m roaming, is it suppose to vibrate or not? Smile smiley

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Paul,

Did you use a bitumen product? Yeah, that’s horrible stuff. haha! - If it’s a fresh vinyl product, you can let if off-gas for about a week in an open area & then install it.

Yes, it should have some noticeable movement, but it will be very damped. We’re building some panels next week; I will take photos and post our in-house test results.

Cheers,
John

It’s real rubber sheets but it has some kind of component inside, I suppose it’s made for garages or places where there is agressive fluids like gasoline or whatever…..
….long story short, even after 1 week in the open air still smells like the first day Smile smiley
I know there are companies which produce PVC or rubber flooring which for sure doesn’t smell like this one but, unfortunately, I don’t have much time to search around, maybe I should wait fot your tests before gluing the MDF panels and then decide to search better for suitable products.
Anyway, the trap with the rubber didn’t have any sort of vibration, not at all, at any frequency I tried….
after I removed the rubber I also took off the rookwool and paint the wood with some kind of filler paint, just to be even safer on the sealed side Smile smiley

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Paul, sad to hear it did not seem to work out and that the rubber smelled too bad. My rubber membranes did vibrate, maybe move would be a better word as it was a bit hard to notice. The sheet was not stretched at all and answered with a low “thud” when struck. In my work we receive samples from various rubber manufcturers. One thing I have noticed is that low priced rubber often smell more.

John, it will be interesting to see how your panels will turn out.

small update.
I replaced the rubber membrane with 4mm MDF (on 3mm ?) anyway it’s 2,6kg/m3…
the tune should have been around 75Hz
the traps are 2, 60x70cm inner depth 18cm, 16cm of rockwool 40kg/m3
Still didn’t make any measurement because I’m sitting on the sofa with high temperature and an headhake that is driving me crazy.
the final tune is different, it’s something like 85/90Hz, I know that just feeling the vibration is not enought but it must vibrate to work Smile smiley and there is where it vibrates.
What I found out is that the flow resistivity of the material inside play a big role in the tuning…..probably also the material used for the structure but the absorber is very important, I have two more traps 50x90 12cm deep filled with 8cm of 60kg/m3 rock wool and 2cm of 100kg/m3 rockwool, same MDF for the panel……well the tune is only slightly higher.
So, I will do measures of course, as soon I feel a little better :(
but in the mean time I’ve been reading that it could be possible to add mass to the front panel just attaching something on the front panel itself.
Any suggestions/thoughts?

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Membrane tuning

Quote:

Originally Posted by PaulRain ➡️

… in the mean time I’ve been reading that it could be possible to add mass to the front panel just attaching something on the front panel itself. Any suggestions/thoughts?

I suggest some posts from other threads:

Basically it works but it doesn’t help the quite bad predictability of membrane-absorbers.

thanks for the links.
In my case the magnetic foils are useless because they don’t stick on MDF Smile smiley
I was thinking to use more of the same MDF, maybe attached with double adesive tape…
I suppose the best place for them is the center of the panel.

even if I still didn’t make any of the things I planned to do in the last few posts Smile smiley I already started thinking and searching something more to add.
I don’t want to give up the limp membrane thing because everywhere I read about it looks like is great.
but reading this:
https://gearspace.com/board/bass-tra…limp-mass.html
I suppose I got it all wrong…..seems like it’s not suppose to be a sealed device….I still have some materials around, the smelly rubber 4,5kg/m2 which I could use for this porpouse…
I could use a laminated chipboard 140x60cm I have in the garage……
I mean I don’t need to invest much money on this.
But I need to make it right.
one question:
is the membrane just hanged somehow and no big worries?
better as close as possible to the wall?
sorry to keep bothering you kind people.
Paolo

actually. looking at these designs:
Primacoustic Acoustic Solutions
looks like there is nothing at the back plus the membrane is even smaller than the actual structure
there are also most of the specs….the membrane is 4,88 Kg/m2 so I suppose the rubber I have could be good enough.

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While it is important to have good writing skills, that is only part of writing a good post. Obviously you possess these skills, but you also have a special flair for informative writing. Well done.

Source	https://gearspace.com/board/
Author	laagman
Keyword	Absorber
Category	Module Absorber
Tags	#Membran Absorber