Need a Limp-Mass Membrane.

[DanDan]

memphis, despite an odd entrance some nice ideas. I have pondered the designs of panel traps for some time and I come up with pretty much the same intuitive thoughts as yourself. However I have not actually tried and tested my theories. I have stuck with broadband.
I am sure all of us would love to see your ideas tested in some way.
Ethan in particular, walks the walk in this matter. When he says cling film or other light plastic works you can rest assured that he has tested it. Furthermore he must have tested bass performance with the membrane in front and behind, and concluded it works better in front. I would have guessed it would be about equal, given the long wavelength of bass, and it seems I would have been wrong. Test and Measurement absolutely trumps intuition here. FYI I personally don't like the HF bounce of plastic or foil.
If I were to construct a LM trap I would leave a gap between the fibre batt and the membrane. I would also use a membrane intrinsically dampled, eg. lead impregnated vinyl or rubber sheet with a layer of HF absorption on the front
This of course is a classic design, tried and tested. I would point out that since it is actually airtight it does not allow an overall spectrum of sound and is definitely not broadband.
DD

[memphisindie]

Originally Posted by DanDan

memphis, despite an odd entrance some nice ideas. I have pondered the designs of panel traps for some time and I come up with pretty much the same intuitive thoughts as yourself. However I have not actually tried and tested my theories. I have stuck with broadband.
I am sure all of us would love to see your ideas tested in some way.
Ethan in particular, walks the walk in this matter. When he says cling film or other light plastic works you can rest assured that he has tested it. Furthermore he must have tested bass performance with the membrane in front and behind, and concluded it works better in front. I would have guessed it would be about equal, given the long wavelength of bass, and it seems I would have been wrong. Test and Measurement absolutely trumps intuition here. FYI I personally don't like the HF bounce of plastic or foil.
If I were to construct a LM trap I would leave a gap between the fibre batt and the membrane. I would also use a membrane intrinsically dampled, eg. lead impregnated vinyl or rubber sheet with a layer of HF absorption on the front
This of course is a classic design, tried and tested. I would point out that since it is actually airtight it does not allow an overall spectrum of sound and is definitely not broadband.
DD

It was odd. Must have fallen straight off a thread with a shill for the baddies on it.

Ethan and Frank are probably the best resource for data so far. After Ethan's film, I OWE it to him to buy a pane from him one day whether I need one or not.

It's cool, I'm not married to any idea I may have, just dating. Till I build my new room, I can't test or construct. Maybe when the snow melts.

[Ethan Winer]

Originally Posted by DanDan

Ethan in particular, walks the walk in this matter. When he says cling film or other light plastic works you can rest assured that he has tested it. Furthermore he must have tested bass performance with the membrane in front and behind, and concluded it works better in front. I would have guessed it would be about equal, given the long wavelength of bass, and it seems I would have been wrong. Test and Measurement absolutely trumps intuition here.

I did indeed test this in a lab, and here are the relevant numbers for low frequencies with the traps four inches off the floor:

 Freq.    Membrane up     Membrane down
 80 Hz        1.24             1.05
100 Hz        4.83             3.39
125 Hz        6.59             4.15
160 Hz        7.28             3.40
200 Hz       11.97             7.74

--Ethan

[memphisindie]

As I said, Ethan and Frank rock, but, Ethan is faster than Frank. Frank has nicer hair. Ethan has that movie. I like both of them, great guys! One day I'll get to Ethan's room.

[euphonic]

Maybe this on fits into this thread.

I came across this design. Due to the fact it's been developed by the german frauenhofer istitute, I assume it was scientifcal well researched.

IBP - Compound Baffle Absorbers

Any opions on this? Only downpoint, those panels are very expensive compared to Realtrapsor GIK. No to speak of DIY.

[memphisindie]

Have you looked at any of the Armstrong stuff?
You should.

[DanDan]

The German stuff looks interesting. I wish we could find out more, how they are doing this, particularly the membrane material.
Also Armstrong? Link or more info plse?

DD

[euphonic]

afaik the CBA consists of a steel sheet (1mm oder 2,5mm thick) fixed to a damping material (they use caruso isobond 40)

more info:
Übersetzte Version von http://www.pia-alfa.de/de/dat_vprbka.htm
(sorry, the original site is only available in german, but google translations should be sufficient)

[DanDan]

Intriguing, thanks euphonic. Who would have thought, steel?
DD

[NLP]

... patented by freunhofer institute (research by dr.Fuchs).
Licensed to rpg (http://www.rpginc.com/news/reflections/DRV10i1.pdf), renz, faist, burkhardt.... pistonic absorbtion/ mass spring... very old "news"...

[memphisindie]

Originally Posted by DanDan

The German stuff looks interesting. I wish we could find out more, how they are doing this, particularly the membrane material.
Also Armstrong? Link or more info plse?

DD

Manufactured by Armstrong:
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[Brainchild]

Clearly, limp membranes function on a different principle as compared to rigid membranes. How is limpness quantified? Does anyone have a formula for the relationship between limpness and internal absorption?

[SimonW]

Hi all,

Ethan's observations suggest that the paper faced batts are functioning differently from a conventional panel trap, and an MLV membrane may not be the best solution. As I understand it…

The problem of absorbing bass sound waves it is not unlike that of a car shock absorber where one wants to absorb an impact and contain it, controlling the rebound, which would otherwise cause bouncing in the case of car or boominess in the case of a room. It requires an initial compliance to the impact followed by a strongly damped relaxation.

The mass loaded membranes are designed to prevent transmission of sound. Their inertia limits their compliance to the sound wave and so they are reflective at most wavelengths. If set in motion, they would be damped only by the internal viscosity of the membrane.

Cling wrap, on the other hand, has negligible mass and would be very compliant to a sound wave, provided it is limp. If it was stretched tight, it would be elastic and reactive, like a drum skin. By itself, it would be useless for bass attenuation because the membrane would have negligible damping. That is provided by fibreglass in the configuration described earlier in this thread. It seems a very promising combination.

Porous absorption like fibreglass is very compliant to sound, but poorly coupled so it takes a great depth to effectively absorb deep bass. Clearly, adding a highly compliant low-mass membrane to the surface prevents the soundwave permeating through the fibres and transfers energy directly to the surface where it is stored in the compression and expansion of the fibre bed. Apparently, relaxation of the bed is slow and resistive enough to provide adequate damping.

I was surprised that Ethan found that such an arrangement worked as a freestanding panel. I imagined it as the front face of a tube trap. Clearly there has to be a pressure difference from front to back of the panel for it to work. I guess the freestanding panel would be equivalent to a finite baffle loudspeaker mounting. If so, the size of the panel would determine the lower frequency limit. The tube trap would be more like an infinite baffle (limited by its permeability).

Ethan's observation that covering both sides of the panel diminished its performance may be due to the enclosed volume acting as an air spring, becoming more reactive.

Overall the damped membrane seems like a very efficient broadband solution for trapping bass in a small volume with minimum materials. I would be very interested in any further performance figures. I will certainly explore this further when I get my room built. At 8.3 m length, it will resonate at about 22 Hz.

Cheers, Simon.

[SimonW]

After writing the above, I realised that those ideas don’t preclude the use of a heavier membrane like paper or indeed MLV because bass frequencies can easily vibrate 12 mm plywood, however I think that as a general principal, using a membrane rather than a panel should make the rebound and any subsequent resonance easier to damp.
Cheers, Simon

[slowflow]

Reopens an old debate:

-when adding a limp mass membrane to a broadband absorber, as cling film or other light plastic, it is wrong if the membrane is in contact with both fiberglass and the outer fabric?
-It is better than the membrane does not touch the outer fabric?

Thanks!

[Ethan Winer]

Ideally the membrane will be bonded / glued to the absorber. Then it probably doesn't matter if it also touches the fabric.

--Ethan

[slowflow]

thank you!

[Bonemeister]

Originally Posted by Ethan Winer

You asked too many questions for one reply, so I'll just give you the short answer.

Yes, cling wrap type stuff is fine and you should spray glue it to the front surface only.

--Ethan

======================
Sorry. This is not accurate.

For a limp mass or diaphragm to work, it must be allowed to float. More importantly, for it to work on bass frequencies, it must have sufficient mass. Cling wrap will not work as it has no mass. Put a plasitc garmbage bag over your monitor and listen. If bass passes through, then you can pretty much be certain that plastic wrap as suggested by the post is not accurate.

[Ethan Winer]

Originally Posted by Bonemeister

For a limp mass or diaphragm to work, it must be allowed to float.

Not at all, as easily proven by the standard data for Owens-Corning rigid fiberglass with and without the paper "membrane" which is tightly bonded to the fiberglass. Look at Table 1 in my Acoustics FAQ and compare the FRK and non-FRK types at 125 Hz. Yes, one is 703 and one is 705, but the FRK matters more. My Density Report is even more compelling and gives apples-to-apples comparisons.

In this case, the "limp mass membrane" simply enhances the absorption of the porous rigid fiberglass material. A true resonator bass trap will have a much thicker membrane that is not attached. But that type of trap is tuned to a specific center frequency. If the room in question happens to benefit from that frequency, great. If not, then the trap is a poor choice for that room.

--Ethan

[ashebiara]

Originally Posted by Ethan Winer

Not at all, as easily proven by the standard data for Owens-Corning rigid fiberglass with and without the paper "membrane" which is tightly bonded to the fiberglass. Look at Table 1 in my Acoustics FAQ and compare the FRK and non-FRK types at 125 Hz. Yes, one is 703 and one is 705, but the FRK matters more. My Density Report is even more compelling and gives apples-to-apples comparisons.

In this case, the "limp mass membrane" simply enhances the absorption of the porous rigid fiberglass material. A true resonator bass trap will have a much thicker membrane that is not attached. But that type of trap is tuned to a specific center frequency. If the room in question happens to benefit from that frequency, great. If not, then the trap is a poor choice for that room.

--Ethan

Mr. Ethan , in the case of the limp mass aproach, could it work even with none rigid fiberglass?? or does it have to be a rigid fiber glass for the cling wrap to work as it has to in this type of assembly?? i am making basstraps with , mineral wool of 4"thick 2'x4', but the mineral wool that i got it's not fluffy like de R type insulation nor it is rigid like de 703 wich in fact looks very rigid to me.. anyways i just plan to glue the plastic directly to de mineral wool insulation and then put it on the frame and make the whole asembly with the fabric ( burlap in the front/muslin on the back) ...

[Glenn Kuras]

Originally Posted by ashebiara

Mr. Ethan , in the case of the limp mass aproach, could it work even with none rigid fiberglass?? or does it have to be a rigid fiber glass for the cling wrap to work as it has to in this type of assembly?? i am making basstraps with , mineral wool of 4"thick 2'x4', but the mineral wool that i got it's not fluffy like de R type insulation nor it is rigid like de 703 wich in fact looks very rigid to me.. anyways i just plan to glue the plastic directly to de mineral wool insulation and then put it on the frame and make the whole asembly with the fabric ( burlap in the front/muslin on the back) ...

Yes the membrane will work with the mineral wool. It is done that way on mineral wool all the time.

[ashebiara]

Originally Posted by Glenn Kuras

Yes the membrane will work with the mineral wool. It is done that way on mineral wool all the time.

thanks Mr.Kuras i will give a shot, maybe doing this will give back some HF to the room.. i really hope that this aproach really help me in the low end area by getting better absortion performance at least down to 80hz, i am planning on spacing at 2" or 3" from the walls.

[Glenn Kuras]

Originally Posted by ashebiara

thanks Mr.Kuras i will give a shot, maybe doing this will give back some HF to the room.. i really hope that this aproach really help me in the low end area by getting better absortion performance at least down to 80hz, i am planning on spacing at 2" or 3" from the walls.

You can also use paper on the front. I would recommend paper that is used for business cards. It is called 80# card stock or called 100# text stock. Really most papers will work but the card stock is stiff enough to work (looks better) but not overly thick, IMO

[ashebiara]

Originally Posted by Glenn Kuras

You can also use paper on the front. I would recommend paper that is used for business cards. It is called 80# card stock or called 100# text stock. Really most papers will work but the card stock is stiff enough to work (looks better) but not overly thick, IMO

ahh bummer i already glued to the mineral wool cling wrap an then i staple the fabric... but i could take the fabric off and take of the cling wrap and try 80# card stock that you are talikng about.. will it be the same process? i mean should i glue it to the mineral wool and then staple the fabric to the wood frame?

ps. sorry for my english, grammar. i am having a hard time thinking in two languages :( hahahaha

[4 the Max]

I believe that FRK is recommended on non-reflection free zone traps to reflect HF back into the room for a better sound in addition to more bass trapping.

1. Is the cling wrap added over the FRK or is it instead of it?

2. If the FRK is only to reflect HF, would the RFZ traps (which are FRK-faceless) have cling warp on the front to increase the bass trapping?