My First Post - Question On Absorption Coefficients.... - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 01-13-07, 01:34 PM
Scott R. Foster
Inactive

Posts: n/a
Re: My First Post - Question On Absorption Coefficients....

here's a list of Sabine numbers that might be helpful:

Old 01-13-07, 04:09 PM Thread Starter
Lewis

Join Date: Jan 2007
Posts: 14
Re: My First Post - Question On Absorption Coefficients....

Quote:
Scott R. Foster wrote: View Post
here's a list of Sabine numbers that might be helpful:

Thanks Scott - Didn't see this source in my travels.

I have another question with respect to these coefficients. I have been assuming that the GOM fabric is of no effect to the underlying material as Bryan explained earlier, but what do you do in the case where you have two materials over top of each other that are non zero coefficients across the octave band? Is there a way to compute the cummulative coefficient betweeen the two materials at each frequency? Or are you just as well off to use the coefficient of the material on the top and ignore the one underneath?

Lewis
LewisCobb is offline
Old 01-13-07, 11:42 PM
Scott R. Foster
Inactive

Posts: n/a
Re: My First Post - Question On Absorption Coefficients....

Lewis:

A simple additive model may mis-lead in the two layers of different materials example you cite for a number of reasons, and I am unware of a a straight forward way to make such a calculation.

Ignoring the second material would I reckon be pretty dicey as well. In some cases I suppose the 1st layer would essentially mask the 2nd, but there are also circumstances where that is not at all the case.

If confronted with such a question I would post a query at the forum.studiotips.com and hope one of the acoustic engineers or designers that read that group have some familiarity with the "sandwhich" in question.

Sorry I couldn't be of more help
Old 01-13-07, 11:45 PM
Scott R. Foster
Inactive

Posts: n/a
Re: My First Post - Question On Absorption Coefficients....

Lewis:

If you are familiar with Excel I can send you a spreadsheet I have used for such calcs... but I warn you up front its a kludge... you work it and it works you.

drop me an email if you are interested.
Old 01-14-07, 11:08 AM
Senior Shackster

Ethan Winer

Join Date: Jul 2006
Location: New Milford, CT, USA
Posts: 251
Re: My First Post - Question On Absorption Coefficients....

Lewis,

> what do you do in the case where you have two materials over top of each other that are non zero coefficients across the octave band? <

This is simpler than you may realize. Most absorbing materials are more alike than different. So if you add two inches of 703 rigid fiberglass to 2 inches of acoustic cotton, the result will be similar to 4 inches of either rigid fiberglass or acoustic cotton.

--Ethan

Focal Press Book: The Audio Expert
Ethan Winer is offline
Old 01-14-07, 12:44 PM
Scott R. Foster
Inactive

Posts: n/a
Re: My First Post - Question On Absorption Coefficients....

Ethan:

There are several reasons that approach may mislead.

If Lewis already has such a construct in place, with a two porous absorber materials with essentially the same absorptive properties stacked into a "sandwich" - then the additive approach you propose may well provide a reasonable estimate - FWIW its as good as any other approach I could propose for rough estimating.

But, save for the narrow case of stacked porous materials of similar absorptive performance, I believe such an additive model is more an exercise in guesswork than the "calculation" that Lewis asked about.

For example, if the materials have even modestly different gas flow resistance properties then sound propagating through medium "A" will hit an impedance jump as it transfers to medium "B' - which could make the "sandwich" behave very differently than an additive model would predict.

In my opinion, the case is an illustration of why folks should NOT make sandwiched porous absorbers - it being easy and inexpensive to avoid the practice [just use a solid construct of one material - that being the mineral fiber which your local market provides at best price] - there being a number of potential pitfalls to sandwich building [such as a large impedance jump] that could result in unwanted and occult peaks in what would otherwise be a device which performed smoothly across the band.

If Lewis does have a significant square footage of a layered materials in his room - I suggest he put his efforts into measuring the room versus guessing at the acoustic performance of sandwiches - YMMV.
Old 01-15-07, 08:29 AM
HTS Senior Moderator

Bryan Pape

Join Date: Sep 2006
Location: Wildwood, MO (St. Loui
Posts: 5,209
Re: My First Post - Question On Absorption Coefficients....

Another case where it can be misleading is when you do something like putting 1" 703 or Linacoustic over a single layer of drywall over 16" studs. Just using the 1" numbers would show little to nothing at the bottom end. However, even with 703 on top of it, the drywall still has some absorbtion in the 125 and 250Hz range. Normally this wouldn't be a big deal. However, when you figure that you have all 4 walls and the ceiling using drywall, it can be a significant difference in the ending solution

In a case like this where the materials are very different and not both velocity absorbers, you can kludge it by adding say 75% of the drywall coefficients to the numbers for the 703. It's not perfect but it will get you in the ball park. Normally this wouldn't be a big deal. However, when you figure that you have all 4 walls and the ceiling using drywall, it can be a significant difference in the ending solution. How much it impacts in depends on how much of the % of surface area of the room is done this way. If it's just a panel here and there, probably not a big deal. If it's the brute force method of using 1" all around the room from the floor up and the whole front wall, then it's a significant amount.

Realistically, most of the coefficients should only be relied upon to the first decimal place anyway. IOW, the difference between a .22 and a .25 is really trivial and may in fact be incorrect and even reversed depending on the labs used, temp, altitude, humidity, placement, etc. during the testing.

Bryan

I am serious... and don't call me Shirley.

Bryan Pape
GIK Acoustics
bpape is offline
Old 01-15-07, 01:39 PM
Senior Shackster

Ethan Winer

Join Date: Jul 2006
Location: New Milford, CT, USA
Posts: 251
Re: My First Post - Question On Absorption Coefficients....

Scott,

> if the materials have even modestly different gas flow resistance properties then sound propagating through medium "A" will hit an impedance jump as it transfers to medium "B' <

Do you have any evidence or data to support that conclusion?

--Ethan

Focal Press Book: The Audio Expert
Ethan Winer is offline
Old 01-15-07, 03:56 PM
Scott R. Foster
Inactive

Posts: n/a
Re: My First Post - Question On Absorption Coefficients....

Here's a good place for you to start... you'll find all sorts of interesting stuff in there.

An Introduction to Acoustics
S.W. Rienstra & A. Hirschberg
Eindhoven University of Technology

Take a look in section 4.4 "Reflection at discontinuities" for the sub-section titled "Jump in characteristic impedance".. it starts on page 86

But I warn you this stuff is math heavy - here's a an excerpt:

Quote:
The factor R between G1 and F1 is called the reflection coefficient and the factor
T between F2 and F1 the transmission coefficient. We observe that if ρ1c1 =
ρ2c2 the acoustic wave is not reflected at the contact discontinuity. Inspection of
(4.44a,4.44b) for ρ1c1 = ρ2c2 also shows that the only solution is F1 = F2 and
G1 = G2. This corresponds to results obtained already in section 3.2 when considering
harmonic waves.
Old 01-15-07, 04:52 PM
Senior Shackster

Ethan Winer

Join Date: Jul 2006
Location: New Milford, CT, USA
Posts: 251
Re: My First Post - Question On Absorption Coefficients....

Scott,

> Take a look in section 4.4 "Reflection at discontinuities" for the sub-section titled "Jump in characteristic impedance".. it starts on page 86 <

That doesn't really explain by how much absorption will be harmed between two absorbing materials like rigid fiberglass and cotton, or two densities of rigid fiberglass. That's why I wondered if you had any evidence or data, as opposed to pure theory and math formulas. As we both know, theory often misses the forest for the trees with small room acoustics!

For example, I remember well the months-long denial of quarter-wave boundary interference - basic comb filtering - by some folks who were otherwise very knowledgeable about acoustics. That happened in two different forums as I recall, which prompted me to finally make a video proving the point. Another time I recall an expert in one of the audio newsgroups insist that large changes in response were impossible at low frequencies due to the long wavelengths involved. That too has since been proven wrong, as further evidenced by the graphs in my Believe article I showed today here in a different thread.

Of course, it's not that the theory is wrong! Rather, it's the misunderstanding or misapplication of theory. Likewise for those who claim EQ can reduce modal ringing by a meaningful amount over large distances in a small room. In theory one could show it's possible, but when it comes time to prove the point not one of the vendors has been able to produce a waterfall plot.

So in that light, do you happen to have any data showing how the absorption changes with disparate materials?

--Ethan

Focal Press Book: The Audio Expert
Ethan Winer is offline

 Bookmarks

 Tags absorption , coefficients , post , question

Message:
Options

## Register Now

Random Question
Random Question #2

User Name:
OR

## Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.