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micro-perforated absorbers
Old 18th May 2011
  #1
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micro-perforated absorbers

Hi All.

Just wondering if anyone out there has used these and what kind of results they got? how low in frequency where you able to get effective absorption at? if anyone has used them in their studio please let us know which ones and how it all went?

Regards

Anthony
Old 19th May 2011
  #2
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i guess no one.....maybe this is too far an advanced technology

Old 19th May 2011
  #3
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There are a bunch of threads on this forum and resources all over the internet about perforated panels.
Old 19th May 2011
  #4
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I think microperforation is used a lot in industrial applications. I've got no in depth experience myself but the Whealy Pourous Abs. Calc. does microperforated sheets as well.
Old 19th May 2011
  #5
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Micro perforation is usually effective in the midrange and thus not very interesting for studio design where we are mostly interested in either low frequency absorption or broadband (for early reflection points).
Old 27th May 2011
  #6
Gear Guru
Perforated metal

One seems microperforated absorbers in ships, factories, airports, outdoor smoking areas and so on. Usually metal, often stainless steel. I saw a studio in Liverpool with lots of this. There is some rule of thumb. From memory, approximately speaking, if the perforation area is 30% or above, the facing is acoustically transparent. The depth of the absorption behind will determine the lowest frequency of absorption. Expect total absorption down to a frequency where the fibre is 1/4 wavelength, but significant absorption at much shallower, e.g. 1/10 wavelength.

DD
Old 27th May 2011
  #7
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Jens Eklund's Avatar
Why start more of these toxic ”rules of thumb” when this is so easy the learn properly:

Porous Absorber Calculator V1.58
Old 27th May 2011
  #8
Gear Guru
Start

Who's starting what?
The 30% comes from my Institute of Acoustics notes.
The 1/4 and 1/10 come from the Cox and D'Antonio book.
The Whealy does not work for many Intel Mac users.

EDIT, curiosity arose. There is a lot more to it than the simple figure of 30% suggests. Tried running some variations in Whealy, but it is not working stably on this Intel iMac.
However, for the really curious, there are some surprises here http://www.iperf.org/IPRF_ACUSES.pdf

DD
Old 27th May 2011
  #9
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Who's starting what?
The 30% comes from my Institute of Acoustics notes.
...
The Whealy does not work for many Intel Mac users.
Well to bad for you then (why not install a working operating system heh).

Let me then tell you that 30% perforation is more reflective than absorptive above approximately 2-3 kHz (random incidence). Drastically different from your new “rule of thumb”. Again, why start a new “38% rule” (that is apparently causing a lot of bad decisions on placement nowadays)?


EDIT: Noticed that you added an edit, what is surprising in the document you refer to?
Old 27th May 2011
  #10
Gear Guru
Rules of thumb

I take your point about rules of thumb Jens. But perhaps Whealy is using too simple rules of thumb also!
You might want to take a look at the link in my last post. From a brief reading the situation looks very much more complex. e.g. a perforation of only 23% is essentially transparent with a specific size and spacing of holes. I very much doubt that Whealy has this level of complexity in his spreadsheet.
The OS is fine. It's Linux at heart and we don't get viruses and such. The problem with Whealy and many other old and extremely useful Excel sheets seems to be that the recent Mac versions of Excel have gotten rid of Macros.
Don't ya love Microsoft.
DD
Old 27th May 2011
  #11
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Quote:
Originally Posted by DanDan View Post
I take your point about rules of thumb Jens. But perhaps Whealy is using too simple rules of thumb also!
You might want to take a look at the link in my last post. From a brief reading the situation looks very much more complex. e.g. a perforation of only 23% is essentially transparent with a specific size and spacing of holes. I very much doubt that Whealy has this level of complexity in his spreadsheet.
The OS is fine. It's Linux at heart and we don't get viruses and such. The problem with Whealy and many other old and extremely useful Excel sheets seems to be that the recent Mac versions of Excel have gotten rid of Macros.
Don't ya love Microsoft.
DD
I did not use the Whealy sheet do get my numbers. I tried some different models and got a rough trend.

Oh, and the Whealy sheet does not use any macros as far as I know but perhaps I haven’t noticed, it just works but I don’t use it much nowadays.


EDIT: Also:
Quote:
Originally Posted by DanDan View Post
a perforation of only 23% is essentially transparent with a specific size and spacing of holes.
At what frequency? All?
Old 27th May 2011
  #12
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cborg's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
Oh, and the Whealy sheet does not use any macros as far as I know but perhaps I haven’t noticed, it just works but I don’t use it much nowadays.
It does indeed use macros and always has.
(The Mac-version of Office has a few quirks though.)
Old 27th May 2011
  #13
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Jens Eklund's Avatar
Quote:
Originally Posted by cborg View Post
It does indeed use macros and always has.
(The Mac-version of Office has a few quirks though.)
Oh, well as I said, It just works so I haven’t noticed them heh
Old 27th May 2011
  #14
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avare has posted a link to a document on perforated surfaces and their behaviour a couple of times (i don't have the link, sry). Maybe you'll find some answers there?
I've seen it used in windtunnels (pictures only) with some kind of double layer configuration which is supposed to extend the frequency range.
Old 27th May 2011
  #15
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Quote:
Originally Posted by Jens Eklund View Post
Oh, well as I said, It just works so I haven’t noticed them heh
Then maybe you should check your security settings .... heh
Old 27th May 2011
  #16
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Quote:
Originally Posted by cborg View Post
Then maybe you should check your security settings .... heh
I use Excel a lot so that setting was changed years ago.
Old 28th May 2011
  #17
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
The OS is fine. It's Linux at heart and we don't get viruses and such.
This is not a subject that I should debate, but I've been told Mac OS is not Linux based but FreeBSD (and to a smaller degree NetBSD) UNIX, running on the Carnegie Mellon kernel re-write called Mach.

Maybe someone else can confirm or correct this information since I'm not a computer geek, and definitely not in the Mac world.
Old 28th May 2011
  #18
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Relink

Quote:
Originally Posted by cborg View Post
... document on perforated surfaces and their behaviour...
You could try this post about "Acoustic Absorbent Panels with Low Perforation Coefficient." including a link to a paper on the subject.
Old 28th May 2011
  #19
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Thanks G.E! I've managed to miss this even if I read your great thread from time to time.
Old 28th May 2011
  #20
Gear Guru
Performance

micro-perforated absorbers-screen-shot-2011-05-28-16.40.20.png

And yes Lunix it is.

DD
Attached Images
Old 28th May 2011
  #21
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Naturally, if the plate is very thin, this is what you get. If not, you’ll see other results.
Old 30th May 2011
  #22
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Quote:
Originally Posted by Jens Eklund View Post
. Again, why start a new “38% rule” (that is apparently causing a lot of bad decisions on placement nowadays)?
Can you elaborate on that please? Why is it leading to bad placement decisions?
Old 30th May 2011
  #23
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Jens Eklund's Avatar
Quote:
Originally Posted by deve View Post
Can you elaborate on that please? Why is it leading to bad placement decisions?
I often see people placing the speakers about 0,5 - 1 meter away from the front wall in order to meet the "38% rule" and if not treated properly, this is usually not a good place due to SBIR:

https://www.gearslutz.com/board/6124899-post6.html
Old 31st May 2011
  #24
tun
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I was messing around with the Whealy calc the other night, experimenting with various panel thicknesses, radius of perforations, distance between perforations, various flow resistivity figures for the absorbent material, cavity depth etc. See the results below. Not sure how this would translate to the real world (I'm just looking to treat my room as well as I can - I'm certainly no expert) but what the calc threw out was very interesting to me in terms of a design that seems like it would absorb a wide range of lf's very well (as opposed to a hemholtz tuned to a specific lf).

I settled on the following values: 9mm thick panel (based on the knowledge that I can easily get hold of 9mm MDF panels), 2mm radius perforations and 28mm distance between perforations.

The results show that materials with low flow resistivity seem best to cope with a wide band of lf's, with the absorbant material against the panel, and with the air gap between the absorbent material and wall.

Whereas materials with a higher flow resistivity seem like they would be better placed against the hard surface (wall) away from the panel, with the air gap between the absorbant material and the panel. However as the gas flow resistivity increases so does the resonance (see the pink plots in the bottom 2 images). Oh, and ignore the yellow plots!


3825 rayls/m2:



4226 rayls/m2:



6711 rayls/m2:



18740 rayls/m2:



38995 rayls/m2:



Cheers!
Old 10th August 2019
  #25
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This system has some impressive results (PDF in German, but there are drawings and graphs): https://www.akustiksystem.de/de/akus...erforation.pdf
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