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#121
23rd July 2012
Old 23rd July 2012
  #121
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Quote:
Originally Posted by NLP View Post
I do not have nothing new to say McTwins.
No problem here with that. I will not continue to bother you with your measurements file anymore. I can not help if others do.

Thanks
#122
23rd July 2012
Old 23rd July 2012
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So, can someone sum up for the non-acoustician:

what benefits can I expect to gain having several of these in a small to medium room, and how to they compare to the standard dense-foam-corner-straddling bass traps?
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#123
23rd July 2012
Old 23rd July 2012
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Clean

This thread was very welcome IMO. The recommendation of a particular LF trap, it's practical design, and very decent measured results, all good.
Until McT intruded. I see nothing but distraction and hot air from Post 100 onwards. In fact there are a few before that which add nothing of value.
I am about to remove any my own redundant ones.
I of course recommend the same to all involved.
If we all do that, perhaps Tim could oblige by making all the deleted posts disappear entirely. We have used this technique before to rescue the SBIR thread.

DD
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#124
23rd July 2012
Old 23rd July 2012
  #124
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I agree.
#125
23rd July 2012
Old 23rd July 2012
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Standard

Quote:
So, can someone sum up for the non-acoustician:

what benefits can I expect to gain having several of these in a small to medium room, and how to they compare to the standard dense-foam-corner-straddling bass traps?
That question pops up all the time. e.g. What is the most efficient type of Bass Trap? Until someone does a comparative test we will never know.
From experience and other's related experiences, there are a few things I am pretty sure of.

1 The 85cm wide SuperChunks are extremely powerful. In one case three of them almost halved the length of a 35Hz mode.
2 The smaller typiclly 60cm corner straddling panels are remarkable value for money. Their absorption seems to usefully peak at 80-100Hz, however I would expect little or no action at 35Hz.

The results shown by NLP here seem very similar to what I would expect from SuperChunks alone. Undoubtedly HF will be reflected off the Membrane but unfortunately there is no conclusive evidence of increased LF absorption due to the Membrane. Elsewhere, way back, a GS tried cardboard, plastic sheet, and other light facings on a SSC. There was no measureable improvement in LF absorption, but there was an increase in HF decay time.

DD
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#126
23rd July 2012
Old 23rd July 2012
  #126
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With porous superchunks only I could never so effectively absorb so much energy till 25Hz.
As I said in an earlier post: Tim's Limp Mass Bass Absorbers
But I understand that this must be to prove in a right way, I'm sorry that I did not archive these measurements (only superchunks) in the future I will.
Food for thought:
even in very big control rooms, many acoustic designers loves to integrate membrane absorbers in their designs, even if they have enough space available for passive absorption.
Some membranes are large as the whole wall.
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#127
23rd July 2012
Old 23rd July 2012
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Depending on design concept used; we do everything we can to keep mid and high frequency energy in order to reintroduce it to the sweet spot, terminating the ISD-gap. In order to do this, you want to use as little velocity based absorption as possible (that otherwise absorbs the entire range above the LF cutoff frequency of the device). Pressure based absorbers also normally consumes less depth than velocity based absorbers if you need to absorb very low frequencies.
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#128
23rd July 2012
Old 23rd July 2012
  #128
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Yes.
But if! you have enough space, knowledge and experience, can only with passive insulation absorb even lowest modes of the room, and then return "healthy" (diffusive) reflections with diffusers.
But ok we are back to OT.:(
#129
23rd July 2012
Old 23rd July 2012
  #129
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Concept

I don't see anything around 25Hz on the info here. Nor is there in my test.
I do not wish to dwell on this too much, so unless someone wants to see my Before and Afters for some good reason, I will leave them off.
Simply put. 35Hz went from 850mS down to 480mS.
I have no doubt aht NLP's devices work, ditto Tim's.

However, we simply have no evidence to show that any type of facing enhances the LF behaviour of a SSC.
In fact we have some tests which suggest the opposite.
On the other hand we do have two tests on large SSC's. MLV and no MLV.
Remarkably similar results.

It would be wonderful if someone does got to the trouble of trying some MLV on large SSC's in the same room. It is worth noting that the test I referred to, where a GS tried cardboard and plastic, there was only one SSC.
It would be nice to see focus on topic, I am sure the last thing anyone wants is another debate on historical CR design concepts.
DD
#130
23rd July 2012
Old 23rd July 2012
  #130
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Quote:
Originally Posted by ciro View Post
Thank you for the test/files, Boggy!
Ciro, you're welcome!!!

Quote:
Originally Posted by DanDan View Post
That question pops up all the time. e.g. What is the most efficient type of Bass Trap? Until someone does a comparative test we will never know. .....
Most efficient type of Bass Trap? For which particular purpose?
Anyway, bass trapping depends on budget, type and size of room (space), room LF behavior, acustician's knowledge, owner/customer preferences.

Something which work "best" for one particular case, doesn't mean that will always work "best" (for anyone).
(IMHO)
Quote:
Originally Posted by DanDan View Post
..........I am sure the last thing anyone wants is another debate on historical CR design concepts.
DD
I agree, of course, but please, be aware that this topic is easiest to "discuss"

AFAIK, bass trapping is common for ALL acousticall ways (historical or not) of CR design... All known design principles assume that boundary interferences (below 150Hz) are fully resolved in entire room, even if this isn't always explicitly described (how and why) in publications (books, scientific papers, etc.) and even if discussed techniques are pretty old (I mean, there was enough time to describe it in detail ), and even if customer may invest 60-80% of acoustical treatment budget in only bass traps.

Any room where we have negligible boundary interferences below 150Hz (even "totally" dead at end, but with hard wooden floor), already sounds good enough for work (because we already hear a true bass response), whatever way of "reflection management" we choose later.

(IMHO, again)
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Better control room needs:
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-a broader and thicker wall panels
-more super chunks in all corners
-a binary diffuser slats over proposed treatment
-to not expect "sensational" response
-good luck!
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#131
24th July 2012
Old 24th July 2012
  #131
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Quote:
Originally Posted by ben_allison View Post
So, can someone sum up for the non-acoustician:what benefits can I expect to gain having several of these in a small to medium room, and how to they compare to the standard dense-foam-corner-straddling bass traps?
Ben

I researched this concept of what I have presented here years ago and I gave it a try in a commercial studio (I was the house tech, not acoustic designer) and they worked really well. I have since used the small traps effectively in many studios. The key is to make several different depths based on the major axial modes of your room and mount them at corner intersections. If you take the time to anaylsis which mode is which ( ie, across the room, down the room) you get a better idea where to place them. It's not that critical but the more effort you put in the better the results. I will post the following again, as the BBC tests are flawed.

Quote:
Originally Posted by Tim Farrant View Post
From the thread mass loaded vinyl as front of bass trap... thoughts? on 12th Jul 2012

Thanks DD

It is interesting to note the calculated design freq of the limp membrane in the BBC test was 22Hz, however the test frequency only goes to 40Hz. Obviously they should have constructed a more shallow enclosure for a higher design frequency so they could actually measure it - ooops!! I would suggest the peak they are seeing at 56Hz is approx twice the design freq of 22Hz.
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#132
24th July 2012
Old 24th July 2012
  #132
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I'll try and post some examples of how I would set up various sized rooms. I am always pretty busy, so please bear with me.
#133
24th July 2012
Old 24th July 2012
  #133
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BBC

BBC tests flawed?
Sacrilege! Your electrolytics will dry up and shrivel.
Auntie will need smelling salts.

;-)

I must look at that test again. If I remember correctly the BBC used to use Asphalt Roofing felt for some time. The research leading to the VLF module was partly an effort to find something better. I believe this was because the original material was extremely variable in substance. e.g. huge variations in density. Making calculation impossible. Also sagging, or becoming brittle over time.
It seems odd that they did not try something more similar than hardboard. Was Vinyl common in those days? Nevertheless, the (cheap) hardboard fronted devices performed very well, best in the tests, and afaik are probably still working away happily in hundreds of studios.

It is IMO worth noting that they also measured essentially Flat (Alpha 0.85) absorption down to 50Hz, using modules which had NO fronts.
1 foot deep boxes simply filled with fibre.

Overall, it seems to me that deep light fibre absorbs very well (Alpha 0.85) to as low frequencies as space will allow. And that the space required is nowhere near 1/4 wavelength of the lowest frequency. Blocking access to the fibre will undoubtedly restrict absorption at higher frequencies. A Low Pass Filter if you will. (Not a bad thing, addressing tonal balance, called Anti Carpet by the Beeb).
However I not convinced at all of actual enhancement of LF absorption. i.e. Absorption over 100%.
Sure we can see resonant peaks, but one could also view such graphs as the the removal of absorption outside of the pass band. Hardly 'efficient'.
Although I still have hopes for VPR, I think it wise to remember that lunch still costs money.
Active absorption perhaps.

NLP's membrane fronted SSC's perform similarly to mine at LF.
Tims membrane fronted devices perform to 50Hz we are told.
BBC ones do the same without membranes but also absorb higher frequencies.

BTW, I have seen tests which clearly show a 45% increase in absorption of a fibre panel, by fronting with Space Couplers.
This does not happen when they are placed in front of SSC's.
Go figure.

DD
#134
24th July 2012
Old 24th July 2012
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Quote:
Originally Posted by DanDan View Post
......... Was Vinyl common in those days? ....
I don't know for vinyl flooring material, but linoleum was invented in 1855., and according to Wikipedia it is pretty durable...

Linoleum has similar density as vinyl, as from my post in other thread:


Quote:
Originally Posted by boggy View Post
birch plywood ~700kg/m3
bitumen ~1000kg/m
PVC (soft) ~1200kg/m3
linoleum ~1200kg/m3
steel ~7500kg/m3 (!)
lead ~11300kg/m3 (!!)
#135
24th July 2012
Old 24th July 2012
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There were a series of BBC research reports which appear to reflect their efforts to find effective, but economical treatment. The 1994-12 report is pretty clear that they were looking to save money. Somewhere along the way, they also mentioned that one of the prior membrane materials was degrading over time.

But even the 1995 reports are now about 17 years old. One has to wonder what their current research shows.
#136
24th July 2012
Old 24th July 2012
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Outsourced

Quote:
But even the 1995 reports are now about 17 years old. One has to wonder what their current research shows.
Indeed. And one wonders what do they use in new installations.
I get the sense that their R&D is very diminished or nearly gone.
We don't see White Papers from BBC sources at the AES etc.
Also I have seen industry news saying that outside consultants did the acoustics at the new Broadcasting House.

I also wonder if Munro or any of his people are around here. They are actively using Limp Mass Membrane traps.

DD
#137
24th July 2012
Old 24th July 2012
  #137
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I am not sure if anyone said this, and if so I apologize, but . . . .

He said "limp mass". he. he he.
#138
25th July 2012
Old 25th July 2012
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I have a few tons of depleted uranium lying around, will that work?
#139
25th July 2012
Old 25th July 2012
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Quote:
Originally Posted by 7thbass View Post
I am not sure if anyone said this, and if so I apologize, but . . . .

He said "limp mass". he. he he.
I was just going to say! How effective at low end absorption is a flaccid penis?
#140
25th July 2012
Old 25th July 2012
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how did I miss this?!?!?

fantastic information Tim,

thank you!
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#141
26th July 2012
Old 26th July 2012
  #141
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Here's an example for you. Thanks to John Brandt for his room mode calculator!!
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-modeexample1.jpg   Tim's Limp Mass Bass Absorbers-limpmassexample1.jpg  
Attached Files
File Type: pdf limpmassexample1.pdf (455.0 KB, 533 views)
#142
26th July 2012
Old 26th July 2012
  #142
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So is tuning just down to box volume? Would it be possible to make a tunable absorber by having some sort of sliding rear panel inside the box (assuming edge sealing can be taken care of)?
#143
26th July 2012
Old 26th July 2012
  #143
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Quote:
Originally Posted by DanDan View Post
BTW, I have seen tests which clearly show a 45% increase in absorption of a fibre panel, by fronting with Space Couplers.
This does not happen when they are placed in front of SSC's.
Go figure.

DD
DanDan,
I experienced this myself but wasn't sure if it was my brain or a real fact.
Have you links to these studies ? I'm really interested in.
#144
26th July 2012
Old 26th July 2012
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Quote:
Originally Posted by Bilou View Post
DanDan,
I experienced this myself but wasn't sure if it was my brain or a real fact.
Have you links to these studies ? I'm really interested in.
Ditto here. And my brain can't help but bring up a parallel with Sir Boggy's MyRoom diffusers. I wonder if they have a similar effect?


Edit: I'm not spamming- honest, but I also can't help thinking Ponoko might be a good way to go for such a device.
#145
26th July 2012
Old 26th July 2012
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I built several limp mass absorbers for my room, a concrete bunker in the basement. With the very stiff walls and room dimension 786 x 419 x 225 cm (about 25,8 x 13,7 x 7,4 ft), I suspected, before any building took place, there should still be large problems in the roughly 80-90 Hz area if only resistive absorbers were used. Axials => L 87 Hz, W 82 Hz, H 77 Hz plus 6 tangentials within 77-93 Hz.

The Danish acoustician Jan Voetmann wrote an AES paper in the 80:ies for a “SLAM Absorber” (Soft Layer Acoustic Membrane. Tool have a reference to this one in his latest book.) This one consists of a closed box with front of 3 mm thick natural rubber sheeting which has a surface weight of 3 kg / m². In the box common house insulation is added behind (not touching) the rubber membrane. Width of the box is 1200 mm and height 1800 mm, depth can vary depending on target frequency. The SLAM absorber is thoroughly described on Voetmanns home page (voetmann akustik - Home) but only in Danish (http://www.akustikjav.dk/assets/Ress...bsorbenten.pdf ). In the document there are direct references back to the BBC version with its roof felting and that the SLAM is a further development of the BBC one, avoiding the roof feltings drawbacks. It is said that the SLAM is very effective in the 60-100 Hz range with an absorbtion factor up to about 80%, where a normal membrane absorber may have only 10-20% (It does not say what “normal” means, my guess is with a front of plywood, masonite etc.)

Below is a diagram with measured (= not simulated) absorbtion which I received together with a sketch of the box itself from Jan Voetmann after some mail correspondence. (mysteriously the alfa sign has transformed into a pair of scissors ... )

To tame the 80:ies in my room Voetmann adviced 5 boxes 1200x1800x180 mm = 10,8 m² (116 ft²). As the total wall area is 52 m² this equates to about 20% wall surface treatment.

I built 2 versions of his proposal, one against length of the room, about 4,8 m² and another versus the width, about 6,5 m². Nice thing about them is that they do seem to work well as predicted and also that they were easy to integrate into the room (hide from sight).

I took some pictures of the construction and can upload them if there is any interest.
Attached Thumbnails
Tim's Limp Mass Bass Absorbers-slam1.gif  
#146
26th July 2012
Old 26th July 2012
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Adhoc, were you going to include a diagram of the measured absorption and sketch of the box? I am curious to know the thickness of the frame material and also how the floppy rubbery sheet is installed - e.g. is it pulled taut or is it say fixed along one the top edge first and then allowed to hang loose while the other three edges are fixed?
#147
26th July 2012
Old 26th July 2012
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Quote:
Originally Posted by Sebg View Post
Adhoc, were you going to include a diagram of the measured absorption and sketch of the box? I am curious to know the thickness of the frame material and also how the floppy rubbery sheet is installed - e.g. is it pulled taut or is it say fixed along one the top edge first and then allowed to hang loose while the other three edges are fixed?
(?) I see the diagram on the screen right in front of me, now after I logged on again after lunch. Don´t you see it?

Voetmanns sketch is on another computer with a crashed hard disc, don´t know if I can rescue it. Anyway, the box is a framework of 22 mm thick particle board. Outside dim is 1200x1800 mm. The natural rubber membrane was on a 1250 mm wide roll and was stapled to the front. (The rubber can still be bought from a Danish company in the flooring business.) At the middle there is a wooden cross bar for reinforcement of the framwork, it will also make stapling of the rubber to the frame a little bit easier.

A backing lid, also of 22 mm particle board, is nailed to the back of the frame.
45 mm common house insulation is attached to the inside of the box, not touching the membrane. (I belive one should avoid a thin lightweight backing as it may introduce its own resonance, bouncing back from the absorber into the room.)

All sides and joints should be carefully sealed with caulking, strips of window sealant etc, to secure an airtight box.

"Limp membrane" to me means the rubber sheeting should not be streched taut like a drum skin, more like you wrote: Attache / nail it along the top side, with 3 sides loose, pull down slighly as left and right hand side is stapled to the frame towards the bottom.
#148
26th July 2012
Old 26th July 2012
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Thanks for your description!

Edit: I don't see the sketch either. Is it a .PNG image? Gearslutz seems to dislike those. If it is anything other than a jpg you may want to consider changing it to one for assured ....erm... forum attaching?

Quote:
Originally Posted by Adhoc View Post
"Limp membrane" to me means the rubber sheeting should not be streched taut like a drum skin, more like you wrote: Attache / nail it along the top side, with 3 sides loose, pull down slighly as left and right hand side is stapled to the frame towards the bottom.
So you have it attached only along the top and the two bottom corners, correct?

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#149
26th July 2012
Old 26th July 2012
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#150
26th July 2012
Old 26th July 2012
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Adhoc - I see the graph now but I did not see it earlier on when I made my post. Thanks very much. That link to your construction was great as well. edit: the photographs seem out of sequence but I think from your written description it is possible to see what you did.

I wonder if the grade of the rubber makes much difference? Do you have a link or some physical properties of the rubber sheet? Natural rubber is around 1,000kg/m^3 so 3mm thick = 3kg/m^2 but I wonder what Duro (hardness) it is.

KAsmira - the rubber sheet was fixed and sealed on 4 sides to make the enclosed volume airtight - it was just a brief discussion about how tight the membrane should be when it is fixed.
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