I've read myself into confusion

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Postby Eric Desart » Mon Apr 30, 2007 9:58 am

:twisted:

Bert is just jealous:
Eriqueze aka Antwerps aka Flemish is an ennobled version of Dutch

Every year there is a  Dutch national dictation on TV for both Netherlands and Flanders. Real heavy, heavy stuff.
Once Bert tried to tease me here at studiotits that for once after x years a Netherlands guy won this dictation rather than a Flemish guy.

:mrgreen:   Bert proud as hell.
I followed that dictation on TV. The guy who won was a Surinamer immigrant in the Netherlands, not even an autochthone.
And there must be somwhere between 2.5 and 3 times more Netherlanders than Dutch speaking Belgians.

:wink:  Their jokes always relate to these dumb Belgians (they always mean the Dutch speaking guys when referring to Belgians, not knowing that French exists, must be something exotic).
But they do love these dumb Belgians.  Every weekend on a huge market here in Antwerp it's invaded by them.  :)  We feel as being in the Netherlands.
.
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Postby Ido » Mon Apr 30, 2007 10:27 am

Eric Desart wrote::....  Every weekend on a huge market here in Antwerp it's invaded by them.  :)  ...


you should be thankfull it's not Israelis (give us a nice spot on the globe, we'll invade it.   Achtoreck, beware.   Belgium I remember as slightly grey, so maybe Eric is safe yet).
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Postby Eric Desart » Mon Apr 30, 2007 10:34 am

Ido wrote:Belgium I remember as slightly grey, ........

:mrgreen:  Yeeeeeeeh, it's grey ...., very grey ......, very very dark grey ......, and wet ...., and windy ...., and the people are terrible .....
Spread the word ......
.
Last edited by Eric Desart on Mon Apr 30, 2007 10:38 am, edited 1 time in total.
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Postby Ido » Mon Apr 30, 2007 10:37 am

apropos books & confusion:
I'm looking for something to calculate slat absorbers/resonaters.
was that the good work of Chris W.?
I checked that in the calc section but only saw file for porous absorber.
can someone please direct me?
Ido
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Postby Ido » Mon Apr 30, 2007 10:39 am

Eric Desart wrote:
Ido wrote:Belgium I remember as slightly grey, ........

:mrgreen:  Yeeeeeeeh, it's grey ...., very grey ......, very very dark grey ......, and wet ...., and windy ...., and the people are terrible .....


yea, and here it's sunny & beautiful, and all the people are wonderful, and there is peace.....   :mrgreen:
(he he, Eric has pride in his Belgium)
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Postby Zaphod » Mon Apr 30, 2007 11:39 am

Hey,
there is an international crysis going on, i see  :mrgreen:
I try never to get involved in my own life. Too much trouble. :mrgreen:
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Postby Ido » Mon Apr 30, 2007 11:39 am

Eric Desart wrote:[....Spread the word ......
.


wise edit  :D .
Eric, is the above a term used in Dutch too?
if not, then you sometimes your english sounds very native/natural  (flattery  :twisted: ).
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Postby Terry Montlick » Mon Apr 30, 2007 3:16 pm

Well, I haven't found a definitive answer as to where the invention of the anechoic wedge occurred, New World or Old. But i did come across an interesting observation regarding ""cutoff frequency" of anechoic designs, which I will now describe.

All anechoic chamber are of course characterized by a cutoff frequency. This is the lowest frequency at which the room may be considered anechoic. The anechoic qualification was (somewhat arbitrarily?) defined by Beranek as the absorbtion of no less than 99% of the sound energy at normal incidence. So if the anechoic wedge has an absorption coefficient of a mere 0.98 at a particular frequency, it fails miserably!

The reason for normal incidence is so that the wedge structures may be measured in an impedance tube, which is an extremely sensitive device capable of very fine discrimination of absorption. In fact, Beranek chose to use as his metric the "percentage sound pressure reflection" rather than the conventional "energy absorption percentage" because the former is more sensitive in the region of highest absorption. The range of energy absorption from 99% to 100% (absorption coefficients of 0.99 to 1.00) maps to the percentage sound pressure reflection from 10% to 0% (remember that you have to square the pressure to get the energy: 10% times 10% = 1%). So a wedge must reflect no more than 10% of sound pressure normally incident upon it to be deemed anechoic.

But the really interesting thing IMO is that for the absolutely best, optimal wedge designs, none could exceed the 1/4 wavelength limit for depth of overall wedge structure. That is, no wedge had a cutoff frequency lower than sound whose 1/4 wavelength was the depth of the total wedge structure (including any air space behind the base of the wedge).

This should be obvious upon reflection, but what struck me was the extreme accuracy to which the cutoff frequency may be predicted based solely on the depth of the absorber. Of course, for inferior wedge designs (sub-optimal geometry, too-high porous material density, etc.) the cutoff frequency did not reach the theoretical minimum.

Regards,
Terry
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Postby J.F.Oros » Mon Apr 30, 2007 5:08 pm

Terry Montlick wrote:But the really interesting thing IMO is that for the absolutely best, optimal wedge designs, none could exceed the 1/4 wavelength limit for depth of overall wedge structure. That is, no wedge had a cutoff frequency lower than sound whose 1/4 wavelength was the depth of the total wedge structure (including any air space behind the base of the wedge).

So this means that Beranek's 1.45m wedges allowed a 60 Hz cutoff for his anechoic room (considering no air space behind them) ?
This also means that you would need at least  4.5m wasted for every wall/ceiling/floor to get a 20 Hz cutoff  anechoic room. That's 9m for every three dimensions only for treatment !  
I don't think someone would build such a room just to measure speakers  :mrgreen:
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Postby Eric Desart » Mon Apr 30, 2007 5:34 pm

Eric Desart wrote:but the largest wedges I personally know for anechoic rooms are guaranteed and measured to 50 Hz (covering the total 1/3 octave down to lower cutoff) having a total mounting depth of 1450 mm


The minimum interior size of an anechoic room without lining for 50 Hz is +/- 9.8 m in all directions.
There is a relation between cutoff, room sizes and maximum size of to be measured objects. (company I was part of designed, sold and build measurement rooms and facilities, produced the doors and all other mechanical related stuff)

The interior size should be (metric):

L >= 2 + a + 172 / fg + 2 * t

a: largest dimension of to be measured object [m]
fg: cut-off frequency [Hz]
t:: depth of lining [m]
Last edited by Eric Desart on Mon Apr 30, 2007 6:11 pm, edited 1 time in total.
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Postby J.F.Oros » Mon Apr 30, 2007 6:04 pm

Eric Desart wrote:The minimum interior size of an anechoic room without lining for 50 Hz is +/- 9.8 m in all directions.
There is a relation between cutoff, room sizes and maximum size of to be measured objects. (company I was part of designed and sold measurement rooms and facilities, produced the doors and all other mechanical related stuff)

Waw, that means they must be over 12-13 meters high inside !  8O
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Postby Eric Desart » Mon Apr 30, 2007 6:13 pm

Flav,

Post crossed, edited mine with the +/- relationship.
Take into account that a lot of that height is below the floor (raster) or net level.

:mrgreen:  Imagine a control room executed as an anechoic room going down to 16 Hz to cover for the lowest musical frequencies (Church organ).
:) You will not have modal problems whatsoever.
:twisted:  These waterfall graphs should look marvelous (almost empty).  The ultimate room ......

Eric
.
Last edited by Eric Desart on Mon Apr 30, 2007 6:54 pm, edited 3 times in total.
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Postby J.F.Oros » Mon Apr 30, 2007 6:45 pm

Eric Desart wrote:Post crossed, edited mine with the +/- relationship.
Thanks for the formula Eric, I see there is even more space to add at the room's minimum dimensions.

Take into account that a lot of that height is below the floor (raster) or net level.
.
I knew about that, but only now I realize it must be kind of scary to walk on that net at 4-5m height, with a forest of menacing pointing objects under, above and all around you !  :mrgreen:
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Postby Terry Montlick » Mon Apr 30, 2007 6:57 pm

Eric Desart wrote:but the largest wedges I personally know for anechoic rooms are guaranteed and measured to 50 Hz (covering the total 1/3 octave down to lower cutoff) having a total mounting depth of 1450 mm

This must reflect the different definitions of "cutoff frequency" as given by Beranek for  normal incidence as applied to just the absortive wall structures, vs that of ISO 3745, which measures the cutoff frequency of the chamber itself according to its adherence to an inverse-square falloff as would occur in free field conditions. ISO 3745 measurements need only be accurate to +/- 1.5 dB at these low frequencies.

Regards,
Terry
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Postby Eric Desart » Mon Apr 30, 2007 7:34 pm

Hi Terry,

I don't know the related US or ASTM standards.

But we have 2 regulations to take into account.
1) For the wedges themselves
2) To control accuracy of the room.

For the wedges we have the same rules as you described. But a 50 Hz wedge must cover the band.

I'll give you the measurement data here of the ones I described:

 Hz          alpha 0                 r
 50             1.00                  0.07
 63             1.00                  0.03
 80             1.00                  0.06
100            1.00                  0.07
125            1.00                  0.06
160            1.00                  0.07
200            1.00                  0.03
250            1.00                  0.04
315            1.00                  0.03
400            1.00                  0.03

Hence we also must be >= alpha 0.99 = reflection factor <= 0.10
alpha = 1 - r2
The r value is also given in order not to loose detail between 0.99 and 1.00
Reduction of reflected sound level: R = -20 * log r [dB]

For the others who don't know (Terry knows of course):
This alpha 0 are absorption coefficients measured in the impedance tube and represent REAL percentages where 1.00 = 100% = everything is absorbed and nothing reflected.
Examples:
50 Hz band above:  alpha = 1.00, r = 0.07 is more exactly alpha = 0.9951 but standard rounded gives alpha = 1.00.
63 Hz band above:  alpha = 1.00, r = 0.03 is more exactly alpha = 0.9991 but standard rounded gives alpha = 1.00.
To better/easier express this detailed accuracy one uses this r factor
Hence you can not compare this with Sabine absorption coefficients measured in a reverb room.
There's no way one can obtain this kind of resolution in a reverb room based on the traditional Sabine approach.

I once measured such wedges (about 900 to 1000 mm = higher cutoff, can't remember exactly without checking) in the reverb room, just to conclude that measuring such large 3D objects isn't that obvious at all in a reverb room.
I measured them screened, but more to get relative measurements to check the effect of different types of filter cloth spanned over the tops of these wedges.
That was at the time that the alleged potential cancerous character of mineral wool fiber was at its top. That's now history. I even don't know if anyone ever used these results, in a real live application, showing that filter cloth could be a valid option.

Hence I assume we follow about the same procedures as you do.
There must be somewhere an ASTM standard as well defining how to check the quality/properties of anechoic rooms.
Also that will show some tolerance.
It should surprise me if there should be much difference with our approach.
A measurement of a couple of wedges in itself still does not guarantee a complete room comprising a lot of variables.
.
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Postby jude3 » Tue May 01, 2007 2:43 am

Scott R. Foster wrote:Jude:

I suggest you read for general background, but focus on specific advice for you project in your posted questions.  Define your project and its limitations and ask how to best to address acoustic treatments.  That method won't obviate your exposure to differeing opinions that may cause confusion, but at least all proper responses will be targeted at your particular needs.


Ok.

What do you suggest for my control room?

It is 8'5" x 12' 1' and 9' high.

The limitation is that is it small.
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Postby Bob » Tue May 01, 2007 4:01 am

About these wedges

I assume they're pyramids and not some other shape. (I've seen tall-thin pyramids in anechoic chamber photos)

You've mentioned their heights. How big are their bases? (please give both measurements -- height and bass width -- and a comment about shape if they're not tall pyramids)


BTW, I have this theory that these tall wedges are not so much absorbing the LF sound (50hz to 100hz), as warping and diffusing progressive bits of it until it bashes it's own self into insignificance. Sort of like what happened with my bathtub QRD diffuser test -- where a clear wave hit the QRD, and next to nothing came back.
Regards
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Postby Scott R. Foster » Tue May 01, 2007 4:30 am

jude3 wrote:What do you suggest for my control room?

It is 8'5" x 12' 1' and 9' high.

The limitation is that is it small.


Sound slike a job for few broadband panels.. maybe a couple of SuperChunks... post a few pics and a simple floorplan let's see what will fit.
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Postby Eric Desart » Tue May 01, 2007 8:36 am

Bob wrote:About these wedges
1) I assume they're pyramids and not some other shape.
2) (I've seen tall-thin pyramids in anechoic chamber photos)
3) You've mentioned their heights. How big are their bases? (please give both measurements -- height and bass width -- and a comment about shape if they're not tall pyramids)

1) Based on what?
For that old historic stuff I'm not sure. Literature discussed seems not conclusive. Tery's info points to pyramids, mine to wedges. If Terry shouldn't have entered his related info, I automatically should interpret my info as wedges. Only Terry's comment and in-depth rereading of my info shows that it leaves some possible room for confusion/uncertainty. Only finding the original publication will probably give me conclusive answers. This isn't that important, but fun to know more correctly.

Further, while they possibly exist I never saw such a room, neither know of it, nor does I know of a pyramid shape used by the main European players with mineral wool versions I know about. I've a list here of 422 rooms/spaces from rather small with high cutoff to huge, anechoic and semi-anechoic. I'm rather sure that most are wedges or other (mostly diverted) designs (mainly for the smaller rooms) but doubt there are pyramids used and if so must be a very minority.
If you read my posts you could see that even in these very early investigations they concluded the wedges to be favorable.
I assume/understood also Terry's info point in that direction

2) Can you link to a couple of these/such pictures (real rooms)?
That's how they standard look like:
http://images.google.com/images?sourcei ... ic%20rooms
I can see a few, and I assume they are designed with the emphasis on higher frequencies, or whatever.
The small look more as an extended version of foam pyramids, not directly famous for low frequency absorption.

3) I gave measurement data of 1 single wedge shaped (further wittingly not defined) type in function of the mounting depth discussion/comments/question marks and its relation with room cutoff.
It also gives a feel for others (combined with Terry's contributions) about how such data is handled and calculated with.

It's not my intent whatsoever to release or publicly discuss exact commercial designs and mounting methods (which are significant part of it, mounting depth and wedge length aren't necessarily synonyms), nor publishing related measurement reports.
Not all data is Public Domain stuff.

:twisted:  I think if I should release you in my office (valid for other people's offices as well), that a month later there should be backups all over the world (US, Canada, India, wherever).
http://forum.studiotips.com/viewtopic.p ... ght=#37810

I had a Dutch friend, owning an industrial acoustic company. When he visited me I turned a lot of pages on my desk and closed my cabinets, standard mostly/often open.
He once asked:  :?  Eric why do you do this, don't we know one another long enough now?
And I answered: :)  J.., that's exactly why I do it. ........

Best regards
Eric Desart
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Postby Bob » Tue May 01, 2007 3:53 pm

Eric:

Starting several posts back you clearly and consistently wrote 'wedges', and somehow, over and over in my mind, what I was thinking was 'pyramids'. oops.

Now that you mention wedges, and I hear 'wedges', I recall lots of images like this one
http://www.axiomaudio.com/archives/22chamber.jpg
with the wedges mounted at 90 degree angles to each other.
Regards
Bob Golds
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