Need a Limp-Mass Membrane.
 

Ok- I can't find an answer via searching (though, there are a lot of limp-mass membrane threads that end in: "get FSK").

A. First of all, when adding a limp mass membrane to a broadband absorber, should one glue it at all or just sandwich it between the glass and the outer-cloth? I cant see a limp mass membrane being very limp after adding glue to its entire surface area.
Is this why RealTraps doesnt use FSK?

B. What would be the best limp-mass membrane material? Are we talking like some super thin plastic (like cling wrap)?

C. Why only put the limp mass membrane on one side and not all the faces of the trap?

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

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

--Ethan

Yes, I have a history of asking too many questions. I'll try to hold back. kfhkh

First of all: Why doesn't a bunch of spray glue reduce the amount of "limpness" in the membrane?

I'm gonna have to start charging you pal! heh

As for the spray glue, try and and you'll see it doesn't really get stiff.

--Ethan

LOL, NICE ONE!!!!!!!!thumbsup

First of all: Why doesn't a bunch of spray glue reduce the amount of "limpness" in the membrane?[/QUOTE]
If you have about a 1/4" thick layer of glue it would reduce limpness, but it may be more glue than required...HA

Ok, I'm not familiar with spray glue so maybe it'll seem obvious if I try it.

Alright- so why not cover every surface with a limp mass membrane?

I can't give you a scientific answer as to why that happens, but we once made a batch of MondoTraps with membranes on the front and back as an experiment. When we measured them in the lab they performed worse.

--Ethan

Ah, I see.

Ok, well as far as helping the absorber get better performance in the bass frequencies, is something like cling-wrap the right thickness/material? Do different materials/thicknesses affect the bass response?

Better yet- do you know of any books that cover this sort of information so I don't have to keep buggin' ya? kfhkh

Thanks for all of the help, though.

can you list the test, withe the results?

Glenn

The only way to know for sure is to build 4 to 8 traps each way and test them. The first year we built MiniTraps we used FRK fiberglass and that worked great. Then we spent a few weeks experimenting with different materials and came up with a plastic that's even better. For $100,000 I'll gladly reveal to you the results of all our tests and materials. heh

I have never seen this mentioned in any book. I will say that many reflecting materials can do the job. I've used FRK paper, various thicknesses of cardboard, various thicknesses of plastic, and so forth. All are better than plain rigid fiberglass.

--Ethan

Yes I can but I won't. heh

Kidding!

I'm pretty sure I mentioned this in the past: The day we did that experiment I was not at the lab. Doug was there, and he called me to say the extra membranes gave an absorption curve that was "screwy" in the midrange. That's all I know. Doug and the lab guys didn't even plot the data - they just looked at it on the computer screen all agreed it was a failed experiment. So Doug and his helper took the traps apart on the spot and removed the extra membrane, then they ran the tests again.

--Ethan

Ok, that's understandable. Trade secrets and such :)

Is there any scientific explanation for how something as thin as cling wrap can help the absorption of bass so significantly?

I have some theories. One theory is that a slab of rigid fiberglass acts somewhat as a membrane all by itself. In this case I mean the type of "membrane" used in a wood panel bass trap. Rigid fiberglass is pretty heavy and massive. So adding a layer of thin non-porous material on top might help that behavior. But I'm not a physicist, nor a materials scientist, nor even a college graduate. Hell, I barely made it out of high school, so don't listen to me! heh

--Ethan

Oh man that one will be played back on you...hehheh

I think your theory is pretty much on. I have some ideas we are going to try in the next few months.

Glenn

Screw that - I'm proud to be a successful pro in more than one field mostly self-taught! kfhkh

--Ethan

You're a pretty smart guy, college or no.

I do have some background in physics, though that's not what I have my undergrad degree in. When we talk about porous and non-porous layers, we're really using a shorthand that makes a statement about the way molecules are structured in terms of their proximity to one another, the way they're shaped and the way they're networked, and that has implications for the way they behave when stimulated. That behavior has implications for the way we perceive that stimulus reaction in terms of energy release in the form of heat, light or whatever. It seems reasonable to me that it's as simple as adding a layer that, though thin, has a much tighter molecular structure than rigid fiberglass; more molecules more closely related to one another equals more heat released equals slightly less energy transfer to the 703, but still enough to excite a reaction that means energy attenuation to us.

What's going on on the other side of the 703 is a little harder for me to grasp, but it might have something to do with the reduced amount of energy transferred to a limp membrane on the other side after having passed through a limp membrane an a second absorption layer...I wonder if it's acting as an insulator in a bandwidth-specific way.

I don't have any math on this...I'm not that well educated in physics...just some thoughts. Feel free to correct me. howdy

Frank

Thanks Frank. As I always say, knowing a lot of facts is not the same as being smart. Knowing facts gets you on Jeopardy. Being smart means you'll vote for the right candidate in the upcoming US presidential election. kfhkh

Agreed, and that's why I emphasized "non-porous" above, which is my intuitive way of understanding what you described more technically.

My guess is the fiberglass absorbs the membrane's vibration in much the same way the fiberglass in a wood panel bass trap absorbs the wood's vibration. In this case the membrane is much more tightly coupled to the fiberglass, so there's more damping.

Or something like that. heh

--Ethan

I don't know about you guys, but I'm penciling in Glenn's name.

That makes some sense.

Well I can see Alabama from my back yard so yea........................ hidz

LOL! Nice one, Glenn. heh

I forgot to add

U Betcha!!!!!!!!! wink wink. heh

Stop it you guys, you're killing me! heh heh heh

Oh god. Don't get me started on potential Head of "Skate"..

Alright Ethan, I'm going to take your advice and test a few prototypes myself.

Is there any way do an accurate test with just one customized panel (so I don't have to make so many)?

Maybe moving the mic closer to the corner the trap is in?

There's something going on here that I just don't understand...

In previous threads, it had been suggested that my broadband absorbers would benefit from a more breatheable fabric perhaps, than the canvas/upholstry fabric I had been using. And also, that the 2" wall panels that I have should be faced with something more breathable than carpet.

If the above statements are true, than shouldn't a completely non-porous material like plastic be far worse than say canvas or carpet?

Just trying to wrap my head around all this.

Sadly, No. The fewer traps you make, the "lower down in the noise" the results will be. I suggest at least four traps for a smallish room. That should be enough to see a real difference. Here's how I did that:

Density Report

--Ethan

anybody?

Sorry John, I missed the post above. The answer is pretty simple, actually: the plastic is just a whole lot thinner and its structure is such that it transfers enough energy to the rigid fiberglass/mineral wool to be useful rather than detrimental. Comparing the two from a physics standpoint is apples and oranges.

Make sense?

Frank

I think so...

so the fact that the plastic isn't breathable isn't exactly what makes it benefical, it's the thin-ness of a boundary that effects the fiberglass on one "membrane"...allowing the surface to act as a complete unit?

because my covering has too much mass and doesn't utilize the benefits of a membrane, not only am I not recieving the benefits of a "surface as a unit", but also not allowing the airflow to reach any of my insulation optimally.

SO what if I were to utilize a membrane under the covering I already have?

"less confused but trying to spend less money",

-John