A new concept for a membrane bass trap

[JamesBowers]

Hello Audiophiles! My name is James - I've been working with a talented and insightful acoustic designer and together we've come up with an idea for a membrane corner bass trap. I'm including 3 graphics of the concept, and would be very interested in your feedback. Here's the idea...

- The trap stands 4' tall, about 2'6" wide and stick out about 2'6" from the back wall. The back is open to the room corner. I intend to stack 2 (8' high) in 2 of my 4 corners - other 2 corners are too small with windows and doors in the way and will receive a more generic treatment.

- At the front we plan to use 4" thickness of mineral wool - 2" of 6lbs/ft3 at the very front, backed with 2" of 2.5lb/ft3. We feel the variation of density may tend to cut back on the natural resonance of the panels, small as that resonance may be.

- Behind this is a 1.5" air space, followed by a mass loaded vinyl membrane hanging free from a top bar. It is about 2'5" wide and 48" high.

- Then another 1.5" airspace followed by the same mineral wool treatment as on the front of the trap, with the 2.5lg/ft3 on the inside , followed by the 6lb/ft3 at the very back, then there's an open back into the corner of the room, and we may loose pack insulation between there and the room corner.

It seems some of the most effective products from RealTraps, GIK and possibly ASC all use a membrane - also an interesting video on youtube of guys from sound-on sound mag constructing a big bass trap in a vocal booth. The big question is, some folks put the membrane at the front with the 703 or such behind - I imagine some high freq reflections off the membrane back into the room with this scenario. Others put the membrane behind the 703 - maybe for mid and high absorption. Or maybe there are other implications to which order they are in, like Q or just plain bass sucking power.

Can't wait for your input! James

[John White]

Awaiting test results.

However, I would be interested to compare this design to just the same amount of mineral wool placed in as similar position as possible.
Even better, How about comparing the same material, similar position, but with the membrane in front of the mineral wool in order to preserve some high frequencies.

Typically, membrane absorbers are sealed in some way. Is this?

[DanDan]

Well done, but, perhaps this is the wheel.
Check out the details of the Primacoustic MaxTrap on their site.
You device is bound to work but how well, who knows. I reckon the membrane should be sealed to the boundaries. Also it would probably be better placed further out from the apex of the corner.
DD

[JamesBowers]

Trying to figure out to make the frame accommodate experimentation...without radically bloating the budget; let me know if you have an inspiration! One reason we setled on having the front absorber in front of the membrane is that we have a lot of diffusion going on in the rear and rear side walls...

[JamesBowers]

Thx 4 your reply...
We have looked at the primacoustics. Their membrane is behind - Realtraps is also behind the front porous absorber. if you look at a youtube video with the SOS magazine team building a bass absorber for a vocal booth you will see them put the porous at the back, and the membrane at the front - mind you, they also include 2 layers of wool carpet underlay.

YouTube - ‪The Studio SOS team build a vocal booth - Part.2‬‏

I've found no other reference to using this, let alone on both sides of the MLV! One other manufacturer - can't remember who, has the mlv at the front - again probably to not suck up mids and highs.

[JamesBowers]

As far as I've been able to tell there are two "interpretations" to sealed and tensioned or hung and loose... there is much conflicting theory out there... as I understand it, A "diaphragm" is anything that is supposed to resonate - particularly at a certain frequency. This is I think supposed to be sealed. Then there is a "membrane" which is NOT supposed to resonate, and should not therefore be sealed. Teach me!

[DanDan]

James while I admire your effort and sketchuping skills, I do think you are picking up information incorrectly.
The membrane in the Max is close to the front of the trap, closer than yours.
Primacoustic Acoustic Solutions
I can't tell if they finally include a layer of HF absorbent in front of it or not.
I don't know which RealTrap design you are referring to. However my understanding is that RealTraps have a membrane bonded to the front of the fibre panels in the case of MiniTraps and MegaTraps. HF MiniTraps have the membrane on the back. The membranes fully bonded to rigid fibre have a different type of behaviour to yours and Prima. I believe they enhance the slightly resonant behaviour of the rigid fibre. The two work together as one.
Your membrane should vibrate and be damped by the nearby front and back fibre. I reckon if it is not sealed then LF energy will pass around it, less vibration , thus less absorption.

DD

[audiothings]

I reckon if it is not sealed then LF energy will pass around it, less vibration , thus less absorption.

+1

watching this thread... please take before/after measurements and share with us.

[Sebg]

There should be at least 3 sets of measurements:

1. Before
2. After
3. After, but with the membranes removed...

Also, have you looked at the Modex? It uses a diaphragm that is not sealed.

Typically, "membrane absorbers" are resonant devices. Hence the membrane is used to enclose a fully sealed volume of air. The interaction of the membrane and the trapped air has a resonance at a certain frequency and this resonance can be used to remove energy from the room at that frequency. An unenclosed membrane (diaphragm) will not have quite the same effect. Not that it won't do something.

[audiothings]

Also, have you looked at the Modex? It uses a diaphragm that is not sealed.

are you sure? it looks sealed to me... were those test details always there or have they added it recently?

[Sebg]

The product photos clearly show the sides as perforated and the product description talks about sound diffracting around the edges of the plate and being absorbed.

RPG Diffusor Systems

edit: the Modex is a resonant absorber, based on the fact that the steel plate and the foam behind it are a "mass-spring" system. The foam also dampens plate resonances.

In the case we are considering, the membrane is "blowing in the wind". It wouldn't be a resonant absorber but it may work better than having fresh air - let's wait for the tests (I confess I have long wanted to test this configuration but have always had other projects to get through first).

[Glenn Kuras]

With a build like that the only way to really know is to send it off for testing at the lab. You can take before and after in your room but that still will not tell you "how" effective it really is.

[Dange]

Just a design point, you need to tie down the mechanisms that are working within the bass trap. Otherwise after testing you won't know why it works so well/fails completely. Just having feelings that things might do things won't really help.

If you have an initial idea of what is going on mechanically and acoustically then you can optimise and improve the design. If the volume behind the membrane is sealed then this would be quite easy to model using a transfer matrix. (Maybe Soundflow can do this..... Jens?). A good starting point would be here - Acoustic absorbers and diffusers ... - Google Books

I'd like to offer more insight but am really quite busy at the moment, maybe others can help

[Dange]

Originally Posted by Sebg

The product photos clearly show the sides as perforated and the product description talks about sound diffracting around the edges of the plate and being absorbed.

RPG Diffusor Systems

edit: the Modex is a resonant absorber, based on the fact that the steel plate and the foam behind it are a "mass-spring" system. The foam also dampens plate resonances.

In the case we are considering, the membrane is "blowing in the wind". It wouldn't be a resonant absorber but it may work better than having fresh air - let's wait for the tests (I confess I have long wanted to test this configuration but have always had other projects to get through first).

The Modex Corner (RPG Diffusor Systems) and Modex Plate (http://www.rpginc.com/products/modexplate/index.htm) are two different products working by different concepts.

The Modex Corner is most simliar to the concept proposed in this thread (if the propsed has a sealed rear volume)

[Rod Gervais]

Originally Posted by JamesBowers

Trying to figure out to make the frame accommodate experimentation...without radically bloating the budget; let me know if you have an inspiration! One reason we setled on having the front absorber in front of the membrane is that we have a lot of diffusion going on in the rear and rear side walls...

James,

I have a question......... is this for a one shot deal (2 or 3 whatever) in your room - or is this for commercial purposes?

The reason I ask this is because if it is a one shot deal for a space you're building then I have to seriously wonder why you are experimenting as opposed to sticking with tried and true methods......

If this is for a commercial venture then all of the speculation in the world is meaningless..... only lab testing is going to determine whether the product is effective in the long run.

Believe me when I tell you that for every one of these devices (meaning variations on room treatments) that are tested and prove to make sense - there are hundreds tested which do not.......

If it could be modeled then at least you would have an indication of possible success - but even the models that look promising do not always prove effective......

Good luck,

Rod

[audiothings]

Originally Posted by Sebg

The product photos clearly show the sides as perforated and the product description talks about sound diffracting around the edges of the plate and being absorbed.

RPG Diffusor Systems

edit: the Modex is a resonant absorber, based on the fact that the steel plate and the foam behind it are a "mass-spring" system. The foam also dampens plate resonances.

In the case we are considering, the membrane is "blowing in the wind". It wouldn't be a resonant absorber but it may work better than having fresh air - let's wait for the tests (I confess I have long wanted to test this configuration but have always had other projects to get through first).

I am aware of what you are talking about... it is the modex plate, based on the Franhaufer institute's relatively recent research. It not the product I am referencing. What I am talking about is the modex corner, which I think, is sealed. Any clarity on this is appreciated. I can't find it on RPG's site now, but there is (or was) also a version of it simply called the "modex", which is (or was) a more generic, non corner based version. Here is some info on a supplier's site. They call it "The First Model Control System", and the pic is very suspect... but my memory ain't so bad...

[G. E.]

Originally Posted by JamesBowers

... Can't wait for your input!... ]

The function of the membrane is to "convert" air pressure to air velocity (at its resonant frequencies). To make full use of the membrane and it's resonances absorbent needs to be placed as close as possible to the membrane because that's where the air is at maximum velocity. The membrane itself has very little potential to absorb (by inner friction) because you want to choose the membrane's material so that maximum resonance occurs (membranes with lots of inner friction don't resonate easily).

[Glenn Kuras]

Originally Posted by Rod Gervais

James,


Believe me when I tell you that for every one of these devices (meaning variations on room treatments) that are tested and prove to make sense - there are hundreds tested which do not.......

If it could be modeled then at least you would have an indication of possible success - but even the models that look promising do not always prove effective......

Good luck,

Rod

+10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
I HAVE THE BILL TO PROVE IT!!!!!!!!

[JamesBowers]

thanks all for your replies - we've decided to go with a conventional approach. Too much conflicting info out there! All the best, James.

[localhost127]

Originally Posted by G. E.

The function of the membrane is to "convert" air pressure to air velocity (at its resonant frequencies). To make full use of the membrane and it's resonances absorbent needs to be placed as close as possible to the membrane because that's where the air is at maximum velocity. The membrane itself has very little potential to absorb (by inner friction) because you want to choose the membrane's material so that maximum resonance occurs (membranes with lots of inner friction don't resonate easily).

thanks for the clearest explanation ive came across yet - that helps

[JamesBowers]

I've been getting a lot of conflicting info re having mass loaded vinyl held in place and functioning as a resonator, as opposed to hanging loose as a membrane. please follow these links that advocate it hanging loose and comment. As far as I can tell, these are two completely different approaches, the resonator approach seems frequency specific to the MLV's resonant frequency, which will not help create broadband absorption. Here are links advocating having the MLV hang loose.

First, a video from Primacoustics...

YouTube - ‪Primacoustic® MaxTrap

Second, Sound on Sound mag guys building a limp mass with MLV hanging loose..

Studio SOS: Building a DIY Vocal Booth

Cheers, and again thanks for all the great input so far!