Bass Traps - Diaphragmatic resonators & Limp Mass

[RKrizman]

I've seen one of those and I seem to remember that the limp mass hung free of the absorption. Read their description again.

Doesn't mean it wouldn't work to attach it as well.

-R

[johndykstra]

Yeah, you're right, I misinterpreted the photo.

[SaSi_SiDi]

I was at a car body work shop this morning. There was a lot of noise and also cold as the workshop had large access openings.

As soon as they closed the openings with free hanging clear plastic (looked like PVC) stripes, both the cold wind AND the ambiend sound level decreased. The difference was that hearing a discussion was problematic before and it became much easier after the "curtains" were drawn.

I wonder if bitumin sheets might work better or worse.

[SampleSmith]

I'm curious as to whether this sort of design could be used to really hit a specific frequency, and I'm thinking along the lines that the Q might be affected by the location of the membrane.

For example, I need to suck up a huge amount of 58 Hz in a local jazz club. If I made a 'classical' rectangular bass trap 57" deep, and used a limp membrane near the back of it, where the pressure is highest, would I get more efficiency out of it(?) or do I need to get it where the particle velocity is highest?

Buried in the question is the notion that the Q would be higher, the further the membrane is away from the opening -has anyone played with that?

Thanks!
Keith

[Bonemeister]

Hi Guys - and thanks for the support.

As I run several brands, I simply do not have the time to get on line as much as I would like. So pardon me for not being as active as I should.

First and foremost, I want to make sure everyone knows that I am not an acoustician. There are tons of specialists out there that are well educated that I think can answer SampleSmith better that I ever could. There is also a ton of misinformation out there and it is very difficult to find factual information that is not produced for commercial interest. We are no different and just as guilty. Good news is that we are working on a web site that will hopefully help with factual tests. In fact, we just hired UBC (University of British Columbia) to do some scattering tests on our diffusers and sound scattering devices. Measuring bass traps and diffusers is difficult.

Anyway... a bit more:

For a membrane to work on bass, it must have sufficient mass. To employ a light weight plastic is no different that placing a plastic bag over you head and talking. I can assure you that sound will pass... for about 5 minutes until you pass out.

We sent some number of bass traps - I think 15 or 20 - to Riverbank Labs and had one of our guys fly there, assemble the bass traps and then take them apart after the tests were done. Doing this is expensive. These independent tests presented astonishing results. From my perspective, a hanging (floating) membrane must act just like a microphone diaphragm. It this is attached to the acoustic panel, or if the membrane is in fact a rigid device, then it will have a very specific Q and resonance. The diaphragmatic membrane does not follow the same rules… it seems to center itself where the energy is found. This was the precise result we found when the MaxTraps (and FullTraps) tested at Riverbank.

Our new web page should be up in a few weeks and there you will find comparative charts using the MaxTrap as the baseline and then other commercial products. I am confident that you will find our measured results to be quite impressive when compared against ‘claimed’ results. This is 100% due to the suspended membrane.

You can build your own by simply hanging some loaded vinyl in a corner.

Have fun!

Peter Janis

[Ethan Winer]

Excellent Peter, great to see you here. I have only one comment:

Originally Posted by Bonemeister

For a membrane to work on bass, it must have sufficient mass. To employ a light weight plastic is no different that placing a plastic bag over you head and talking.

Even a very light weight membrane can have a surprisingly large effect on bass absorption. For example, the FRK paper on Owens-Corning rigid fiberglass is thinner than a paper grocery bag, yet it increases bass absorption nearly four-fold at 125 Hz! Here are the relevant numbers from the OC data for 705 and 705-FRK at 125 Hz, both two inches thick:

705: 0.16
705-FRK: 0.60

I've had even more improvement using very thin plastic. The key is how the membrane is bonded to the rigid fiberglass. This stuff is not always intuitive!

--Ethan

________________
The Acoustic Treatment Experts

[SampleSmith]

Thank you Peter and Ethan,
Well, um, you do leave me in a bit of a quandry -no worries.

Your disagreement on the amount of mass required provokes me to thinking (read guessing) about the mechanics of transmission versus those of absorption, and also the amount of energy we're dealing with, which must be very small.

At first blush, what Peter said had me thinking about mass as it is applied to soundproofing. Certainly, when these tiny sound pressures are applied over the entire surface of a wall, a fair amount of energy is acccumulated and the amount of mass required to absorb it (i.e. abate transmission) could become substantial. Considering that the wall itself has it's own masses and resonances, one soon has a network of attached acoustic systems that may weigh hundreds to thousands of pounds and the only way to dampen it is to add more mass or buttress the wall (possibly creating more problems).

If one intercepts (a smaller cross-section of) the soundpressure while it is still airborne and lacks the mass of a vibrating wall or other structure to give it inertia, then the amount of energy to be arrested is back down to something managable and Ethan's observations make a lot of sense.

To me, it begs another question. Looking at Fig 9-20 in Everest's Master Handbook [4th ed.] we see a horizontal section through a bass trap such as I was referring to in my previous post. If I have his drift right, Everest is analysing the FG of the front cover and any interior FG in terms of friction offered to the movement of air.

I'm beginning to wonder if the premise is incorrect; that the FG is really acting as a (somewhat) limp mass with low density and very high surface area, and that the attachment of any membrane to it simply provides a more contiguous surface on which the wavefront can act, thus deforming a considerable amount of the attached FG and absorbing an amount of energy proprtional to the deformation. Perhaps the friction is the active mechanism with no backing and limp mass, the mechanism with the backing?

Maybe I'm chasing myself down the rabbit hole. I should know my way around, down there by now, It's always a nice place for a short cheap vacation, but it wastes so-o-o much time -not yours too, I hope.

Any thoughts?

Getting back to my first question, I take it that the corner-straddling designs are really dependent on this 'frequency-adaptiveness' (if I may) of the MLV.
Budget permitting, I may see if I can build the 1/4 wave trap and experiment with the MLV usage and placement. The trap, as I envision it will run floor to ceiling (almost 13 ft high) in a curtained-off corner of the room, so hopefully I'll be able to compare results with ETF.

Keith

[Ethan Winer]

Originally Posted by SampleSmith

Looking at Fig 9-20 in Everest's Master Handbook [4th ed.] we see a horizontal section through a bass trap such as I was referring to in my previous post. If I have his drift right, Everest is analysing the FG of the front cover and any interior FG in terms of friction offered to the movement of air. I'm beginning to wonder if the premise is incorrect; that the FG is really acting as a (somewhat) limp mass with low density and very high surface area

That's a Helmholtz absorber, and I don't have much experience with those. But I'm quite certain that the fiberglass inside the tuned cavity works via porous absorption as Everest states.

--Ethan

________________
The Acoustic Treatment Experts