Tim's Limp Mass Bass Absorbers
Tim Farrant
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#31
9th July 2012
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#32
9th July 2012
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Quote:
Originally Posted by boggy View Post
Yes, and tensioning will raise (change) frequency too...

Question:
Which type of (porous absorber)* you use in this traps?

Thank you very much for sharing your plans.


EDIT: *I mean "fiberglass" (type)
I do not think it's very critical except the fibreglass (or polyester) should not be too dense, otherwise acoustically the absorber depth is reduced. Normal building insulation is perfectly adequate. The Wavebar barrier I use for the membrane is also available with open cell foam on both sides. This is useful for shallow units, as the fibreglass can be eliminated and the foam facing looks after a bit of cavity absorption.

Tim
#33
9th July 2012
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Quote:
Originally Posted by Tim Farrant View Post
I do not think it's very critical except the fibreglass (or polyester) should not be too dense, otherwise acoustically the absorber depth is reduced.
I agree, too dense porous material with high gas flow resistance actually only decrease volume of box at the end.
Quote:
Originally Posted by Tim Farrant View Post
Normal building insulation is perfectly adequate.....
Do you think about cheapest building fiberglass with densities of about 10kg/m3 or something with 40-50kg/m3, both of it aren't too dense, but they are fairly different.
But, of course, people may try both and decide by themselves....
Tim Farrant
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#34
9th July 2012
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Quote:
Originally Posted by DanDan View Post
Sometimes the middle of a boundary is hotter than the corner.
Yes, placing units at a wall/ceiling junction half way along a wall is another good place for absorbers tuned to higher frequencies. Absorbers tuned to the lowest room modes should always be in the wall/wall/ceiling (or floor) corner where possible. I have in the past made absorbers that are also used as coffee tables at the rear of a listening room!

Quote:
I am intrigued about the recommendation for these very small sizes though. Tim, if you could find your info source on that, I would greatly appreciate it.
EDIT, I note the BBC size is not particularly large either.
DD
I cannot for the life me remember, I must be getting old! I was sure I read it in the Acoustic Design and Noise Control books by Michael Rettinger, but I cannot see it there now. Most frustrating. I'll keep looking.
Tim Farrant
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#35
9th July 2012
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Quote:
Originally Posted by boggy View Post
I agree, too dense porous material with high gas flow resistance actually only decrease volume of box at the end.

Do you think about cheapest building fiberglass with densities of about 10kg/m3 or something with 40-50kg/m3, both of it aren't too dense, but they are fairly different.
But, of course, people may try both and decide by themselves....
I would er towards the lighter material in practice.
#36
9th July 2012
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What happens if the cavity absorbent comes in contact with the membrane?
#37
9th July 2012
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Moving on

Well, measurement has moved on a lot in terms of sophistication and availability/price. I encourage you to try REW TIM. It has electronic uses too.
The ability to reveal BIR issues separately from modes is a developing thing here. Not saying the wheel is not already widely used, but lets see what Jeff comes up with in his Gating examples, and in time, lets see a version of that included in my measuring Primer.
Modes are stated as dominant, but tests often show BIR to be much more significant. http://recording.org/studio-construc...en-matter.html
Rebalance would seem wise.
And of course these devices should be equally effective on BIR also.
Newell's full wall ones don't seem particularly sealed, more of a floppy impediment. However I am sure they work too. Size can overcome....
I hope Tim finds the source of the small box vs wavelength. That is important to the DIYer!

Coffee Table! I have done a large acoustic couch. Large open frame filled with fibre. Ideally placed at the back wall floor corner.
Now what could we use for hanging clothes...? A large tall MDF box with a sliding opening........Hmmm.
I have also seen someone try a membrane on springs. And I have tried large tuned and damped bass drums.

The Deadsheets involved can be sourced at or via Revac in the UK
Acoustica Integral (I reckon) in Portugal.

Some of the Portugese products include a useful layer of HF damping foam.

DD
#38
9th July 2012
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Quote:
Originally Posted by DanDan View Post
Newell's full wall ones don't seem particularly sealed, more of a floppy impediment. However I am sure they work too. Size can overcome....
Are you referring to the hanging panels or does Newell also use a limp mass solution? (I should probably just open his book. Sorry for being lazy!)

Alistair
#39
9th July 2012
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OT

Best open the book Alistair. He uses them in many ways. I was just quoting Newell, Munro, and now including SoundOnSound to add some support to the notion that these things are actually widely used.
More so than Helmholtz in my experience. Andre's Invisible Alpha (tm) should not be forgotten. Stud walls of plasterboard can show Alpha of 0.3 or so at LF. A whole wall of 0.3 is very useful.

Hopefully Tim and Glenns designs will encourage some DIY and we will see some GS use them and do the Before and After measurements. I have my own views on the importance or usefullness of measurements in general. e.g. The Sine Wave method provides a simple clear treatment recommendation. So why all the graphs, Cal, confusion etc.?
But it is very difficult to get a Before and After wrong. So IMHO they are as or more significant than Lab results, in terms of practical small room acoustics at a prosumer level.

DD
#40
9th July 2012
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Quote:
Originally Posted by DanDan View Post
So IMHO they are as or more significant than Lab results, in terms of practical small room acoustics at a prosumer level.
DD,

While I would agree that valid test results of DIY projects would benefit people greatly who were considering a DIY approach to their rooms, I would have to add the following.

Not looking to knock head here, but I am going to have to disagree. I know we've touched base on this before....... however I would be remiss if I didn't comment.

Product "A" is not guaranteed to produce the same results in each and every room it is installed in. Therefor the results of field testing (by anyone other than a certified person performing a tests within the requirements of a standard for such tests) is only meaningful to a small extent to that specific space.

It is meaningless in any other context.

This is due to a whole lot of factors - not the least of which is the level of isolation the space has from adjoining spaces/the outside world, but also room geometry, room finishes, room construction, etc.

So how does one compare 50 different company's products that were tested in 50 different spaces around the world to one another and arrive at any sort of meaningful determination of which one produces the best "bang for the buck"?

The most meaningful report is from products compared in spaces which should (at the very least) have the potential for producing meaning repeatable data.

So we have standards to design spaces (to perform tests in) that are meant to create conditions that (as nearly as is possible) are diffuse.

In this manner at least we have a chance at comparing products made all over the world to one another in a manner that allows us to make informed decisions in that regard.

BTW - just for the record - laboratory test rooms also have before tests performed on them - I would assume that data could be requested by the companies having tests performed at those spaces. I cannot see why this would be a problem.

This from ASTM C423:

Quote:
6.2 It is advisable to make measurements in the room when it is empty and in the room when it contains the test specimen under conditions of temperature and relative humidity so nearly the same that the adjustments due to air absorption do not differ significantly.
That sould satisfy your need to see before/after instead of just the test results themselves.

Now, with all of that being said, as I have acknowledged in the past - I am not necessarily opposed to the though of in-situ testing - but if it is to be used in lieu of lab testing by a manufacturer - it really does need to be performed by people certified to perform the tests - and in accordance with the applicable standards for that testing.

Note that the standards for field testing are pretty stringent and should produce meaningful results.

And then the product receives a test report certified by the company who performed the tests the same as if they had the tests done in a lab.

Personally I am not interested in, nor would I consider backing a product and recommending it's use if the only test data I could get on the product was in-situ testing that was done by the manufacturer themselves, or even by their clients........

There are a whole slew of things that can taint test results besides using incorrect methods of obtaining the data.

Respectfully,

Rod
#41
9th July 2012
Old 9th July 2012
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Recommendation

Points taken Rod. But context is everything here.
Obviously you are not going to recommend installing some product or DIY effort in a megabuck new build.
But for prosumer DIUY, I view these in situ measurements as a very useful 'recommendation'. With no disrespect to Tim or anyone else who says 'this works great for me', I am afraid the graphs from real rooms speak way louder to my ear.
Perforated Panel with Porous Absorber trap

Looking through that thread, which I recommend, we see commonalities. We also disappointment, confirm Rod's view that different rooms or placement.....
VPR worked spectacularly well for the OP also. But with a different design. Overall these DIY VPRs seem to work far better in corners than elsewhere. This is contra published wisdom, and agrees with Tim's corner views here. Some of the patented design criteria do not seem to hold up. e.g. They do not need to be mounted on a flat hard wall, nor does the foam need to be bonded to the metal.
While one cannot determine scientific or even statistical certainties from this, I am certainly be encouraged to try these. And it is the in situ results which are fueling that wish.

Sorry for 'shilling' VPR's. I just wish we had similar data on Limp membrane traps. And Slats. I am sure regular GS readers are well aware that I warmly welcome anything which has a go at the LF issue. Anything, including cardboard boxes, or DRC.

DD
#42
9th July 2012
Old 9th July 2012
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Quote:
Originally Posted by Tim Farrant View Post
I have in the past made absorbers that are also used as coffee tables at the rear of a listening room!
Maybe I should give my somewhat crazy idea another chance?

#43
9th July 2012
Old 9th July 2012
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Quote:
Originally Posted by seen-da-sizer View Post
Maybe I should give my somewhat crazy idea another chance?


Not a bad idea actually. The cavity under a desk is an acoustic trouble area with regards to resonance. If it could be turned into an absorber I am sure the idea would go far.
#44
10th July 2012
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Quote:
Originally Posted by Rod Gervais View Post
Product "A" is not guaranteed to produce the same results in each and every room it is installed in. Therefor the results of field testing (by anyone other than a certified person performing a tests within the requirements of a standard for such tests) is only meaningful to a small extent to that specific space.

It is meaningless in any other context.
+1

Tim Farrant
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#45
10th July 2012
Old 10th July 2012
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Quote:
Originally Posted by OpusOfTrolls View Post
What happens if the cavity absorbent comes in contact with the membrane?
Not an issue as long as it's light weight. The cavity absorbent can probably be omitted, I do not believe it's that critical, as the whole system is low Q anyhow.
Tim Farrant
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#46
10th July 2012
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Quote:
Originally Posted by DanDan View Post
I hope Tim finds the source of the small box vs wavelength. That is important to the DIYer!
DD
Found it, it is written in a book called Acoustics, Noise and Buildings by P H Parkin and H R Humphreys, first published 1958. I quote

"The resonant frequency of panels of practical weights and spacings falls within a range of 40 to about 400Hz and is calculated from the formula..." blah blah blah "...above this frequency the necessary requirement that the dimensions of the panel shall be small compared with the wave-length of the sound is unlikely to be met. Apart from this requirement the size of the panel makes little difference to it's absorption characteristics. Measurements show that the position of maximum absorption (meaning frequency) can be affected, in stiff panels, by the elasticity of the panel material and nature of the edge fixing ..... When the material used is one having little or no elasticity, such as bitumen roofing felt, (limp mass) then a practical curve closely approaching the ideal one is obtained. The absorption coefficient is often increased and extended over a broader range in frequency if a porous material is inserted in the cavity behind the panel.
#47
10th July 2012
Old 10th July 2012
  #47
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Source

Kinda regretting mentioning Lab tests now. It always seems to raise hackles.
I am very happy with tests done by professionals in Labs, when the intention and action is to reveal 'small' room performance.
Corner mounting, below 100Hz, and so on. Lab tests done to the Building Acoustics standards however have little or no relevance IMO. In which case in situ strikes me as far better than nothing. And better than calculation.
I am broadly in agreement with this RealTraps - Measuring Absorption
But let's let that OT matter lie, or pursue it in PM.

Thank you very much for taking the trouble Tim. I believe this is one is well worth teasing out fully. If relatively small Bass Traps are somehow better than as large as possible, then I want to fully know about it!

I don't have the book so perhaps you could help me pursue this to conclusion.

Quote:
the necessary requirement that the dimensions of the panel shall be small compared with the wave-length of the sound
Can we find out what this 'necessary requirement' is. It could mean necessary to help scattering or diffusion, to prevent discrete reflections which would occur when the wavelengths ARE comparable. So this could all mean the opposite. i.e. at higher frequencies it would be impossible to make devices which comply with the 'necessary requirement'.

DD
#48
10th July 2012
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Quote:
Originally Posted by DanDan View Post
.......
Can we find out what this 'necessary requirement' is.......
I don't have book too, but I'm pretty sure that this way we can keep sound pressure wavelength (issues) out of equation (at targeted frequencies).
#49
10th July 2012
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We made a limp membrane ceiling in the studio

we used 650 gram m2 tight woven canvas. on 12cm wood. the cavity filled with hemp fibre insulation. on top of that 5cm hemp fibre and to finish it some accoustic transparent cloth. It is pretty broadband and is also effective beneath the resonant freq of 200Hz because of the low q. and in the upper region it works as a normal foam absorber. Pretty neat to shorten the reverb time of any room and reasonably low cost.
#50
10th July 2012
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Quote:
Originally Posted by DanDan View Post
Kinda regretting mentioning Lab tests now. It always seems to raise hackles.

DD
Well, I found your post to the BBC report informative. Then I started reading some other BBC reports. It's interesting to read their progression of thought as they discuss the effectiveness of their various designs and then try to find a way to get away from multiple, smaller, wooden traps to a wall trap design using metal studs.
Tim Farrant
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#51
11th July 2012
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The book does not give a reason for the "necessary requirement", there is not a lot of maths in the entire book, but the absorbers work even though they are small just the same.
#52
11th July 2012
Old 11th July 2012
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Quote:
Originally Posted by DanDan View Post
Kinda regretting mentioning Lab tests now. It always seems to raise hackles.
I am very happy with tests done by professionals in Labs, when the intention and action is to reveal 'small' room performance.
Corner mounting, below 100Hz, and so on. Lab tests done to the Building Acoustics standards however have little or no relevance IMO. In which case in situ strikes me as far better than nothing.
DD,

Don't regret it - please note I mentioned that I would not have a problem with field testing as long as there were controls to assure the tests were providing reliable meaningful results.

Also, when I mention this it it always in the context of manufactured products......

If a manufacturer is not willing to invest in having their products tested by reliable, certified companies, who we can trust to provide accurate data, then (at least as far as I am concerned) their products are not worth serious discussion.

Smoke and mirrors mean little to me - if they have so little faith in their products that they are afraid to have them tested by professionals in the field (this referring to the field of testing - not the location of the test) then why should we give any credence to their claims? Or even consider it?

When it comes to DIY approaches I will review anything - and if it piques my curiosity I will probably build some and test some them myself.

Sincerely,

Rod
#53
11th July 2012
Old 11th July 2012
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Would it be advantageous to incorporate a limp mass into an angled hard-backed cloud?

I am adding a hardbacked cloud to the front (over the Desk) of my studio. This is basically a 2.5 meter x 60 cm frame of 5x2 strapping with a 12mm MDF backing. two layers of 16mm rigid FG will be glued to the front of the frame (interior of the frame is hollow) then wrapped in very thin plastic foil (against particle precipitation onto the mixer over time) and then burlap.
My thinking is to take a piece of some left over novilon (PVC linoleum) and seal the front of the frame with this - then gluing the layers of FG onto this surface as before.
#54
11th July 2012
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Quote:
Originally Posted by Tim Farrant View Post
The book does not give a reason for the "necessary requirement", there is not a lot of maths in the entire book, but the absorbers work even though they are small just the same.
So I will try to elaborate a bit more, than in this post:

Quote:
Originally Posted by boggy View Post
I don't have book too, but I'm pretty sure that this way we can keep sound pressure wavelength (issues) out of equation (at targeted frequencies).
This type of absorber is damped mass-spring type, where air is spring, damper is glasswool and membrane is mass. This isn't correlated with wavelength of absorbed sound energy, it is based on different mechanism. But resonant frequencies of used membranes normally exist, and if we keep membranes and boxes small enough, we will avoid resonances (modes) which is correlated with membrane dimensions, for a three or more octaves. If the box will be big, resonances of membrane itself will mess up expected response.

Stanford University explanation of vibrating systems is possibly good for illustration: https://ccrma.stanford.edu/CCRMA/Cou...g_systems.html
It is only needed to know that we have heavy membrane as mass, instead of "air mass" in Helmholtz tube.
#55
12th July 2012
Old 12th July 2012
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Thanks boggy. At the sizes mentioned I don't think we are in any danger damped membranes resonating significantly.
However, I don't think we would be in such danger with much bigger designs also, due to the limp membrane and the damping in the box. The only designs I have seen using these were by Munro. The traps afaik were about 1200x 600.
I am afraid this 'requirement' will have to remain a mystery, but I am betting on the scattering/diffusion caused by the multiple small boxes required to make up the required absorbent surface area. Again the BBC, found that such an arrangement was sufficient to create a reasonably diffuse sound field in their listening rooms.
As Tim said, varied sizes and depths would all help in this regard.

DD
#56
12th July 2012
Old 12th July 2012
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Quote:
Originally Posted by DanDan View Post
Thanks boggy. At the sizes mentioned I don't think we are in any danger damped membranes resonating significantly.
However, I don't think we would be in such danger with much bigger designs also, due to the limp membrane and the damping in the box. The only designs I have seen using these were by Munro. The traps afaik were about 1200x 600.
O yes, they (membranes) can resonate pretty well and I (from my experience) will take Tim's advice as very reasonable and clever. Limp mass resonant absorbers CAN be pretty small (still working) relative to wavelength of sound wave which they absorb. If you use my wavelet test tone (tone burst), you can pretty easily isolate this resonances in well treated studio... if there isn't anything to mask this resonances... and I don't like surprises....

I suppose too that 1200x600 is still small relatively to wavelength of room modes in studios which is designed by Andy Munro (not-that-small-rooms)

But 2600x1000 is BIG... big enough to mess up absorber's working range

Also, I know that smaller boxes are more expensive, but I can live with this.
NLP
#57
12th July 2012
Old 12th July 2012
  #57
NLP
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Example of limp mass membrane absorbers:
- bitumen limp mass membrane 4mm thick straddling the corner;
- behind of a membrane is higher density mineral wool, not touching the membrane (cut into triangles like superchunks principle);
- membrane edges are airtight glued to walls, well behind is a wooden frame and is useful for distancing mineral wool and membrane, membrane is not strained;
- dimension of membranes: cca. 2,5m X 0,85m;
- dimensions of room: 4,96m X 4,16m X 2,5m:(
- number of membranes: 4 (in vertical corners).

... sometimes I use thinner membrane and different limp mass materials, this is very cheap to build.

Before (empty room)/ after (4 membranes) graphs.
... measurement microphone is not in totally exact position, few cm shifted...
Photo is poor, but for visualization is enough.
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-imag0828.jpg  
Attached Images
File Type: jpg meritev sredina.jpg (72.4 KB, 886 views) File Type: jpg rew sweep sredina samo membrane.jpg (76.2 KB, 2753 views)
#58
13th July 2012
Old 13th July 2012
  #58
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Quote:
Originally Posted by NLP View Post
...Before (empty room)/ after (4 membranes) graphs.
... measurement microphone is not in totally exact position, few cm shifted...
Photo is poor, but for visualization is enough.
Wow. I'd love to see before and after bass decay waterfall graphs for that!

Happy Trails!
MTB Vince
#59
13th July 2012
Old 13th July 2012
  #59
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Quote:
Originally Posted by NLP View Post
Example of limp mass membrane absorbers:
- bitumen limp mass membrane 4mm thick straddling the corner;
- behind of a membrane is higher density mineral wool, not touching the membrane (cut into triangles like superchunks principle);
- membrane edges are airtight glued to walls, well behind is a wooden frame and is useful for distancing mineral wool and membrane, membrane is not strained;
- dimension of membranes: cca. 2,5m X 0,85m;
- dimensions of room: 4,96m X 4,16m X 2,5m:(
- number of membranes: 4 (in vertical corners).

... sometimes I use thinner membrane and different limp mass materials, this is very cheap to build.

Before (empty room)/ after (4 membranes) graphs.
... measurement microphone is not in totally exact position, few cm shifted...
Photo is poor, but for visualization is enough.
In your after graph you have all of sudden created a huge null at 100Hz, why is that?, and you still have a null at 80Hz.

Are you measurering at the listening position?

The mic schould be at the same position when measureing before and after.
NLP
#60
13th July 2012
Old 13th July 2012
  #60
NLP
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"... measurement microphone is not in totally exact position, few cm shifted..."
and of course membranes have limits.
Membranes are only for bass traping.
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