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Two faced hanging baffles...
PostPosted: Tue Aug 30, 2005 2:19 am Reply with quote
intercessor
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Hi,
If I were to hang 2" baffles from a ceiling would each face be 1" or 2"?



P.S. Does anybody have a good suggestion for absorbing 250hz?

Thank you,
intercessor
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PostPosted: Wed Aug 31, 2005 9:37 pm Reply with quote
Jim Mobley
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Hi Intercessor,

You should make both sides 2 in. Also, for accurate predictions of room RT, you should plan on doing an AURA calculation or a long Ray Tracing, as I think that Sabine RT will be inaccurate.

Let me explain. These baffles are measured in a reverberation chamber hanging in free space. The RT is measured both with and without the panel and then using Sabine's formula, (a = .05(V/RT)) the number of Sabines of absorption (one Sabine = one square foot of perfect absorber) in the panel is calculated. If for example you have a baffle that is 2 feet by 5 feet (10 square feet) and at some frequency, say 1khz, there are nine Sabines of absorption, then the absorption coefficient would be 9/10 or 0.9. (Sabine was a Yankee and I don't feel like doing metric conversions; sorry everyone who's not American.)


Here's the problem. In EASE, if you put the same 2 x 5 foot baffle in the model and make it two-fold face you now have 20 square feet of active surface area, so your Sabine calculations will be off. If you are only going to do statistical acoustic calculations, you'd get closer by putting in just half of the baffles. Remember, for the statistical formulas, Eyring and Sabine, distribution doesn’t matter, you could put in just one face with the area of all the baffles and get the same results as inserting all of them. Distribution does matter for Ray Tracing and AURA.

Stefan will correct me if any of the above is incorrect. Shocked

In most of the venues where these types of baffles are used, statistical acoustical calculations aren't the best in any case. These venues are typically not mixing, have lots of specular reflections and flutter echo, and lack an even distribution of absorption. In situations like this, AURA is the preferred method of calculation, with Ray Tracing being second best.

If your model is just for sound system design or you are using EASE Jr., then I'd suggest just putting in half of the baffles and realize that your RT times are based on assumptions that may not be correct. If you have the full version of EASE, or even better, AURA, then you can get more reliable RT times using Ray Tracing or AURA calculations.

As for absorbing 250Hz, try some of the vendors specializing in studio acoustics, they'll have LF absorbers.

_________________
Best regards,

Jim Mobley
Sr. Application Engineer
Renkus-Heinz, Inc.
19201 Cook St.
Foothill Ranch, CA 92610 USA
TEL: +1-949-588-9997
FAX: +1-949-588-9514
www.renkus-heinz.com
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PostPosted: Wed Aug 31, 2005 11:21 pm Reply with quote
Jim Mobley
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Location: Foothill Ranch, California, USA




Reading over this, I may not have been clear, if you are going to do a Ray Tracing or AURA calculation, then you should use two-fold faces and model the baffles full size in their correct locations. This will be the most accurate.

_________________
Best regards,

Jim Mobley
Sr. Application Engineer
Renkus-Heinz, Inc.
19201 Cook St.
Foothill Ranch, CA 92610 USA
TEL: +1-949-588-9997
FAX: +1-949-588-9514
www.renkus-heinz.com
mailto:jim@renkus-heinz.com
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PostPosted: Thu Sep 01, 2005 10:46 am Reply with quote
Peter Patrick
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Jim Mobley wrote:
Reading over this, I may not have been clear, if you are going to do a Ray Tracing or AURA calculation, then you should use two-fold faces and model the baffles full size in their correct locations. This will be the most accurate.
I'm not argueing Jim but I would like to point out that one baffle manufacturer I called about this could NOT assure me that the tests were conducted with the baffles hanging in the Reverberation chamber. He could NOT say for certain that they were not placed on the floor of the chamber.!

This, I am sure you will agree, makes a huge difference.

So I suggest that the designer checks the data with the manufacturer to ascertain the measurement method and veracity of the data.

Best Regards
P.J.P.

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PostPosted: Thu Sep 01, 2005 12:20 pm Reply with quote
intercessor
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We have Full 4.1 without AURA. Just for clarification, would I want to use 2" on both sides if using Ray Tracing or Local Decay from a seat?
Since this is my first time using hanging baffles, if I used the 2" fiberglass panels in the database would that be accurate?

I might have asked this in a different topic, how accurate are RT Ray Tracing calulations compared to using the Local Decay on a seat?

Thank You,
Andrew
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PostPosted: Thu Sep 01, 2005 1:22 pm Reply with quote
Peter Patrick
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intercessor wrote:
We have Full 4.1 without AURA. Just for clarification, would I want to use 2" on both sides if using Ray Tracing or Local Decay from a seat?
Since this is my first time using hanging baffles, if I used the 2" fiberglass panels in the database would that be accurate?

I might have asked this in a different topic, how accurate are RT Ray Tracing calulations compared to using the Local Decay on a seat?

Thank You,
Andrew
Andrew,
I believe you will find the panels in the database are measured on the floor of the reverberation chamber.

My best guess at how to model that as a baffle is to go to the material database and open the 2" fibreglass material and save it as a new product ... say FG Baffle.
Then halve all the values for absorption and save it again.

In the model you can hang it anywhere and call it a 2 sided face and make both sides the baffle material you just saved.

Best Regards
P.J.P.

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PostPosted: Thu Sep 01, 2005 4:58 pm Reply with quote
Jim Mobley
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Location: Foothill Ranch, California, USA




Andrew and Peter,

I also called a baffle manufacturer/supplier, Acoustical Solutions, and they said that their baffles were hung in the reverb chamber away from barriers. I didn't call MBI, as they were closed at the time of posting, but their numbers are similar.

Peter, don't you think that for Ray Tracing and AURA, where Sabine and Eyring statistical calculations don't apply, that two-fold faces with the normal surface values, i.e. 2" Lapendary or whatever, would be more appropriate? After all, if a ray or particle strikes one of these faces, shouldn't the energy loss be that of the material and not half of it?

Stefan?

_________________
Best regards,

Jim Mobley
Sr. Application Engineer
Renkus-Heinz, Inc.
19201 Cook St.
Foothill Ranch, CA 92610 USA
TEL: +1-949-588-9997
FAX: +1-949-588-9514
www.renkus-heinz.com
mailto:jim@renkus-heinz.com
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PostPosted: Thu Sep 01, 2005 10:50 pm Reply with quote
Peter Patrick
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Hmmmmm,
Now you've given me food for thought!

Does this mean we need to change the product to 1/2 values if it's FG tested on the floor to obtain a print out for RT60 then change it back to full values to do a Ray Trace ???

So what is the origin of some of the items in the main database such as "Drape Thk" and "Drape Thn" .... what's the correct method of modelling a drape 100mm from a 5/8" Gypsum board wall.??

This has caused me some grief just recently in a model of an enclosed stadium. It has FG under the roof sheeting. How do I model the FG plus some diaphragmatic absorption from the sheeting ?? Ok I figure I will have to guess what the sheeting might be doing and add that to the FG material absorption coefficients and make a single material called "FG under tin roof" or something.

So do I add "Drape Thk" absorption coefficients to "GYP5/8" and create a new material called mabe "Drapes in front of a wall" or perhaps a double sided face 0.1m from the Gypsum board wall ... do I then use half values of Drape Thk to get the RT60 then full strength Drape Thk to do a Ray Trace? Would that then give the correct reflection strength for a phon going through a thick drape, bouncing off a Gypsum board wall then returning into the room through the drape ???


My head hurts!

P.J.P.

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PostPosted: Fri Sep 02, 2005 10:55 am Reply with quote
Emad El-Saghir
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Hi Peter,
Most of currently available absorption data of materials describes the random-incidence coefficient measured under a given set of conditions during the measurement, such as termination behind the surface material and its thickness. These conditions are not known in every single case, but more importantly is that the absorption values are valid only under these conditions. If you want in your model to change the air gap behind the panels or increase the thickness of the fibres, then you are stuck with the question, how far the available absorption curves should be modified Confused .
Certainly, you can "qualitatively" estimate the new absorption curve at different frequency ranges, but it is not that simple to have a new curve with a reasonable degree of confidence in the values.
This shows that we increasingly need the new method of characterising the absorption data using the acoustic impedance and complex wave number of materials. Going in this direction, we can determine the absorption characteristics of "assemblies" of different isotropic homogeneous materials cascaded together at any thickness and even at any angle of ray incidence. The measurement of the impedance and wave number is not common and not straightforward, but so was the case with loudspeaker phase data, when it was first introduced in EASE.
Best regards,

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PostPosted: Fri Sep 02, 2005 11:28 am Reply with quote
Peter Patrick
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Thank you Emad,
My head still hurts!

P.J.P.

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PostPosted: Fri Sep 02, 2005 12:47 pm Reply with quote
intercessor
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Just a couple thoughts,
A material is tested laying on the floor or on a wall and an absorption curve is calculated. Now, if this is say 2" thick fiberglass and I decided to apply this data to a two-fold surface hanging in space, I would assume that the data could be directly applied to each surface without modification. However, realistically does sound absorption double with double the surface area? Or should the data be modified to a certain percentage say 50%, 75%...etc. and then added to each face or, better yet does the program calculate these variables or does it assume double material surface equals double absorption?

Vis-versa, If the absorption curve is achieved by testing the material hanging in space and then that data was to be used on a free hanging two-fold material I would assume that the data would be halved, because the curve was achieved by testing two sides not one. Again I don't know how sound absorption in reality or EASE for that matter handles doubling surface.

Main thought... Does sound absorption have a one to one correlation to the surface area or is it some other mathematical constant. Twice the material = 2x sound absorption or is it more like 2x material = 1.66x sound absorption?

I imagine that if there was a constant it wouldn't actually be so given the numerous variables encountered in an actual building as opposed to a sound chamber?

Sorry for the rambling...
Andrew
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PostPosted: Fri Sep 02, 2005 3:22 pm Reply with quote
Emad El-Saghir
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Quote:

Let me explain. These baffles are measured in a reverberation chamber hanging in free space. The RT is measured both with and without the panel and then using Sabine's formula, (a = .05(V/RT)) the number of Sabines of absorption (one Sabine = one square foot of perfect absorber) in the panel is calculated. If for example you have a baffle that is 2 feet by 5 feet (10 square feet) and at some frequency, say 1khz, there are nine Sabines of absorption, then the absorption coefficient would be 9/10 or 0.9. (Sabine was a Yankee and I don't feel like doing metric conversions; sorry everyone who's not American.)

Hi Jim,
I think it is important to keep in mind that absorption coefficient is a property of surfaces. The measurement process in the reverberation chamber is based on calculating the change in the equivalent absorption area (A) due to introducing the sample in the room relative to the equivalent absorption area with the empty room.
In your example, if you are measuring a freely hanging 2ft.x5ft. baffle, then you are adding to the reverberation chamber 20 sq. ft. of additional area not 10, and if this results in a change in the absorption area of 9 units at a given frequency, then the absorption coefficient at that frequency is 0.45.
This value is true for the “surface” of the baffle, regardless what the intended installation is.
If you are measuring the same 2ft.x5ft. baffle, but placed on the floor, then you are adding to the reverberation chamber 10 sq. ft. of the sample material, and taking away 10 sq. ft. of the floor, but since the floor is supposed to be reflective (reverberation chamber), you can assume you are just adding 10 sq. ft. of the sample material. This should result in a change in the absorption area of 4.5 units at the same given frequency.
Best regards,

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PostPosted: Fri Sep 02, 2005 3:37 pm Reply with quote
Bruce
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Andrew,
The way that Emad does it is what I use, that is, I try to get the abs. data from a measurement of the hanging baffle in the Rev. room. Typically, as Jim indicates, this is presented as a reduction in Sabins for the sample size, then you divide by the surface area to get the abs. coeffient at each frequency. (MBI gives their data as a reduction in Sabins, so I have used that to create my own data for them too in the same way.) If the only data available is from the material in a Type A mounting, then you have a problem to know what it's effect will be when not mounted on a surface. I would expect the multiplier (2x or 1.66x) would be frequency dependent also.

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Bruce C. Olson
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PostPosted: Fri Sep 02, 2005 8:55 pm Reply with quote
Jim Mobley
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>>> "In your example, if you are measuring a freely hanging 2ft.x5ft. baffle, then you are adding to the reverberation chamber 20 sq. ft. of additional area not 10 ..."

Considering that the specified absorption coefficients exceed 1.0 for many of these products, I don't think they're doubling the surface area. I could be wrong. It would be nice to have a really good report on how these things are measured.

_________________
Best regards,

Jim Mobley
Sr. Application Engineer
Renkus-Heinz, Inc.
19201 Cook St.
Foothill Ranch, CA 92610 USA
TEL: +1-949-588-9997
FAX: +1-949-588-9514
www.renkus-heinz.com
mailto:jim@renkus-heinz.com
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PostPosted: Fri Sep 02, 2005 11:31 pm Reply with quote
Emad El-Saghir
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Jim,
Yes, it is true that most manufacturers describe the absorption of such products in terms of "Sabins per baffle". My point was how to import this into what EASE can correctly interpret. That is why, we need to consider that the Sabins per baffle (what the measurement says) are resulting from the two sides of the baffle (what EASE understands).
Best regards,

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