Limp Mass Vinyl in Broad-Band Absorber?

[1ManGarageBand]

Hi--

I've built 5 framed broad-band absorber panels with 2'x4'x2" rockwool.

Wouldn't hanging a sheet of 2'x4' limp-mass vinyl loosely in the space behind the rockwool increase their efficiency significantly?

Thanks --David

[tINY]

One would think - but I haven't seen much data....



-tINY

[bpape]

Almost impossible to say what it would do. If the cavity isn't sealed and it's not coupled to the absorbent, it will likely do nothing but reduce the effectiveness from a few hundred hz up.

Bryan

[jwl]

I've never seen a bass trap or absorber design using MLV. I don't think it would do anything helpful either.

[andrebrito]

It might act as a membrane absorber.

[bpape]

IF you stretch it tight, it could act as a membrane certainly. However, you can use something much thinner and less expensive to serve the same purpose depending on how you want it tuned and what kind of depth of cavity behind you have to play with.

Bryan

[Brainchild]

This seems to be a major lacuna in the popular knowledge around the boards: what is the absorption frequency;efficiency of damped mass as mass.

[bpape]

Impossible to say by itself. There are way too many other variables at play including:

- Is it over a sealed cavity? If a cavity, what is the depth?
- Is it bonded to the absorbent or spaced?
- What is the density of the material it's bonded to?

Bryan

[1ManGarageBand]

Hi, and thanks for the replies--

I was only thinking of hanging something like 1/8" Auralex SheetBloc 430 loosely from the top of the frame behind the rockwool; not stretched or sealed or anything. My thought was that those little vibrating air molecules would bump into it and change some of their energy to heat and attenuate/absorb more sound (especially in the more energetic lower frequencies) making the broadband absorber panel more efficient.

I don't understand how it could hurt; I just wondered of it would make it work significantly/noticeably better.

Thanks again,

[DanDan]

Hi David, questions pop up now and again about hanging sheets in one form or another.
I think it best to assume that if your plan had value it is more likely than not that someone has thought of it and tried it. Philip Newell uses hanging deadsheets, but as part of large complex systems. These techniques seem to go back some time to Tom Hidley. You might be able to take a peek in Philip's book at Amazon.
The BBC had modular Bass Traps with a sealed front of LMV in the form of Asphalt Roofing Felt. They have a remarkable performance in the frequency area intended. However since it is sealed, higher frequencies are unaffected. It's a tradeoff.

DD

[memphisindie]

Use the search, there is a very very descriptive thread about this.

[1ManGarageBand]

Hi memphisindie--

I've searched "limp mass vinyl, limp mass vinyl and 'broadband absorber', LMV, vinyl, LMV+absorber" with no related results. Would you advise me on how to narrow/focus the search?

Thanks,

[memphisindie]

I'll give you a synopsis based on your post. LMV is for stopping "through transmission" not attenuate in the same room. If you put it in an absorber, it will reflect mids instead of absorb bass. It will not make a broadband absorber so much as a mids reflector.
If you use plastic wrap on the front of an absorber "stuck" to the front, it works better as a bass absorber without reflecting as much mids. Counterintuitive yet true according to Ethan and Frank.

[bpape]

Right. If you bond something like FSK to fiberglass or mineral wool, what you have is a damped membrane. That will give a hump to the absorption in the bottom depending on the material it's bonded to and the thickness while reflecting upper mids and highs. This allows more bass treatment in the room without overdoing the high frequency absorption and making things too dead.

Bryan

[tINY]

Originally Posted by memphisindie

I'll give you a synopsis based on your post. LMV is for stopping "through transmission" not attenuate in the same room. If you put it in an absorber, it will reflect mids instead of absorb bass. It will not make a broadband absorber so much as a mids reflector.
If you use plastic wrap on the front of an absorber "stuck" to the front, it works better as a bass absorber without reflecting as much mids. Counterintuitive yet true according to Ethan and Frank.

If it reflects mids, where do the highs and lows go? Transmitted to somewhere? Absorbed?

I'm sure there is a way to use limp membranes for bass trapping, but it may be a lot more complicated and design intensive.




-tINY

[memphisindie]

Originally Posted by tINY

If it reflects mids, where do the highs and lows go? Transmitted to somewhere? Absorbed?

I'm sure there is a way to use limp membranes for bass trapping, but it may be a lot more complicated and design intensive.

-tINY

The highs get absorbed first, the mids get reflected because they are louder, can't get through, and the waves aren't diffused in the glass, and the low end waves have the same problem with really long wavelengths.
Really, it's only designed to stop through transmission, it's just not made to be a bass attenuator. If you did make a trap with it it would be as big and deep as any other trap without it, only the traps without it would be cheaper since they didn't have the added cost. Ethan and Frank's tests showed that a much thinner and lighter membrane worked much better than MLV for attenuating bass without reflecting highs and mids in the process and worked much better than MLV overall in that application.
HOWEVER,
I wonder if the back of an absorber could use them in loose strips or a shaped centermounted piece behind the absorber that doesn't seal at the edges and leave a few inches of open space.

[1ManGarageBand]

Originally Posted by memphisindie

HOWEVER,
I wonder if the back of an absorber could use them in loose strips or a shaped centermounted piece behind the absorber that doesn't seal at the edges and leave a few inches of open space.

This is my question exactly!

[memphisindie]

Originally Posted by 1ManGarageBand

This is my question exactly!

Well, it took you long enough to ask, and someone came out with a newer smaller trap, ETHAN, but, I'd buy his.

[1ManGarageBand]

Thanks again for your reply, memphisindie--

You probably think I wasn't paying attention, but I really just gave up on the idea. I'm getting ready to build some more absorber panels and the idea came up again.

Whats the "better trap" that Ethan developed? And whats the answer to my question???

[memphisindie]

PM Ethan and ask him, he "might" describe it but it's new. I think one of his partners said something about it. Frank maybe.

[Millo 3.1]

This is the same as that basic design you're talking about:

Primacoustic - Acoustical Solutions

Limp-mass membrane BEHIND the absorptive material... I guess different purpose from the membrane IN FRONT of the absorptive material.
WARNING: I'm just learning about these concepts, and not too deeply, SO I REALLY DO NOT KNOW WHAT I'M TALKING ABOUT. I only posted that because I wondered about this myself, and then saw this design... The first time I read the term "membrane" being used for these panels I thought that their purpose was to convert the compression-wave energy into motion/heat while resonating, but then I thought it REALLY needs to NOT sound as it er... resonates. Then I read what Ethan, Glenn and others meant by their membrane (facing, stuck to the material, not hanging freely), and how the idea I had of a "membrane" was that limp-mass thingamabob that the Primacoustic Full Trap thing includes in its design. Is it worth it for a DIY project? Heck, I got not clue. Notice it's just 3" of fiberglass, but the whole casing is 8" deep (if I remember correctly). I guess you'd need to compare it to the data on the 3" fiberglass panels, but it wouldn't be unfair to compare its effectiveness, or efficiency to the thicker ones if you consider the extra space, material and labor cost. One might as well simply increase the thickness of the panels, but apparently the limp mass membrane makes thinner traps way more efficient--you can check the specifications on the left-hand side of that webpage I linked to. There's an asterisk there on the absorption data chart that I think is important.

Hope it helps, as I'm truly clueless. LOL!!! Cheers!

[PaulP]

Originally Posted by Millo 3.1

The first time I read the term "membrane" being used for these panels I thought that their purpose was to convert the compression-wave energy into motion/heat while resonating,

I think that's correct.

...but then I thought it REALLY needs to NOT sound as it er... resonates.

In a sealed box the air behind the membrane acts as a damper to deaden/
absorb the vibrations. The BBC panel absorbers work like this.

Then I read what Ethan, Glenn and others meant by their membrane (facing, stuck to the material, not hanging freely),

In these cases it must be the fiberglass that's damping the vibration but
I've wondered about this because in the BBC documents they say not
to let the membrane touch the insulation.

Paul P

[Millo 3.1]

I wonder if it can still make a 6" or thicker absorber more efficient... I assume if you put it straddling a corner, the thicker the fiberglass/mineral-wool front the further back you'd have to place the membrane, so the narrower it'll be. I would assume this design becomes less and less effective the narrower the membrane is. However, I do plan on eventually placing two 6" 2x4's on my back wall, with at least 4-6" air gap. I wonder: If I build the typical wooden back-frame and hang a limp mass encased in it, will it make it more effective? These panels would go over a low-profile, 13"-deep, 4-ft wide bookcase I had made precisely w/ plans of installing these bass absorbers on that back wall, over it. It seemed like the only place where I could place this very necessary bookcase in my room w/o really messing things up. I also wonder how heavy such a design would be, LOL!

[Millo 3.1]

Originally Posted by PaulP

In these cases it must be the fiberglass that's damping the vibration

Yeah, I think that's it. Who knows, according to those tests Ethan made (you know, the ones w/ the 703 vs the 705 plain and vs the facing) it works, though. I assume that the transferring of energy from that facing directly to the fiberglass is precisely what makes it work--faster and more efficient transfer than through air? [Sorry, it's been a long time since high-school physics and calculus (not that they would've helped understand this, LOL!), and I'm a lowly musician, so it's hard for me to wrap my mind around these things. It's still fun reading about it and planning it, LOL!] I mean, I'm not sure why Ethan doesn't sell his famous "better bass trap" as a RealTraps design but I assume it's because the ones he makes w/ his proprietary facing are as efficient, or more efficient, and most likely more cost-effective for both the manufacturer and the customers. That article is old, and I imagine he and everyone else in the field has learned enough to favor the facing design for most broadband applications. Again, I'm guessing, sorry if I'm way off... also, I'm discussing this w/o intentions of questioning the validity of anyone's opinions, tests or products as anyway I don't have the knowledge or authority to do so... saying this just in case, so that no one gets personal about it. Thanks!

[PaulP]

Originally Posted by Millo 3.1

However, I do plan on eventually placing two 6" 2x4's on my back wall, with at least 4-6" air gap. I wonder: If I build the typical wooden back-frame and hang a limp mass encased in it, will it make it more effective?

This should be fairly easy to test. Just run Room EQ Wizard with and without.

Paul P

[memphisindie]

Sounds interesting. I think the limp membrane Ethan and Frank are talking about is like Saran wrap thin, so, it's 'OK" as a damper alone, it moves freely but not thick enough to stop low freq through transmission, fine, so, paste it on th front of the fiberglass and together they damp the bass, it's the combination. It attenuates up to highs quite a bit, fine too.
The MLV mineral impregnated limp laying vinyl membrane is very heavy, 1/8" thick, and is really a commercial roofing material. You could use it, but, it would have to lay limp/loose. tighten it and it becomes a reflector.

I have built some very large (4' x 8') absorbers for a commercial stage in a noisy loud warehouse club, they worked beautifully, canvas covered, with paint (original art), 2" off the wall, 2 x 4 frames, 2" of 7lb fiberglass sheets, 1/2" of homesote backer. These things worked wonders. The canvas and the fiberglass destroyed bass overloading for the stage, mounting it off he wall took al the mud off the stage and did not retransmit it to the audience. The PA does the bulk of the work now, even drums and cymbals make it into the PA well. I had a design that was easy to build for further back and we used them too.
The artwork in the place is excellent, so, I had to make that equally important to the design. The owner went for it and they are working perfectly, everyone is very happy.

[RKrizman]

Primacoustic makes something that incorporates a limp mass into it's bass traps.

Primacoustic - Acoustical Solutions

-R

[memphisindie]

Originally Posted by RKrizman

Primacoustic makes something that incorporates a limp mass into it's bass traps.

Primacoustic - Acoustical Solutions

-R

Look up at post #21, haha.
Hey, you ever use one of those anywhere?

[RKrizman]

Originally Posted by memphisindie

Hey, you ever use one of those anywhere?

I heard a set of them used in a very small room. Seemed to do the job of evening out the low end.

-R

[1ManGarageBand]

Originally Posted by 1ManGarageBand

Hi--

I've built 5 framed broad-band absorber panels with 2'x4'x2" rockwool.

Wouldn't hanging a sheet of 2'x4' limp-mass vinyl loosely in the space behind the rockwool increase their efficiency significantly?

Thanks --David

Thanks for all the responses!

So the answer to my original question would be "yes"?

Anyone disagree?