# My First Post - Question On Absorption Coefficients....



## LewisCobb (Jan 4, 2007)

Hello - This is my very first post on this forum and although I have spent literally years lurking and posting over on the AVS forums, it's a bit "caustic" over there in the acoustic dicussions so I figured I'd try my hand here instead. This place seems a bit more civilized - especially for DIY people that are trying to learn as much as possible before embarking on their build.

I have many questions but the first one I'd like to ask is on absorption coefficents. What do people use for situations where the material is covered with say GOM 701. Everyone seems to talk about this fabric as being acoustically transparent. Do you for example, treat say bare sheetroc covered with GOM, or bare plywood covered with GOM as the same thing as the bare sheetroc or plywood WITHOUT the GOM on top of it? Or is there some small modification that has to be made to the coefficients that adjusts for this?

I'm starting to craft up an excell spreadsheet of my planned HT room surfaces to get an idea of RT60 ( or should I say "decay" time to be more technically correct) and armed with the knowlege of "Master Handbook Of Acoustics", Bob Golds site of coefficients, and many many hours of reading through these forums, I think I know what I am doing :daydream: I realize that the estimation of the decay .vs. octave bands is not the be all and end all, but it's a start, and I do plan to get the necessary gear such as ETF etc. to do the measurements before/after later on. 

I have been searching around for some spreadsheets that others have done to see if I am on the right track for mine, but have been unsuccessful finding any, so if someone knows of some, let me know.

Thanks !

Lewis


----------



## bpape (Sep 14, 2006)

Hi Lewis - welcome!

You are correct in assuming that GOM over a hard surface like drywall should be considered basically the same as the plain drywall. The GOM has little to no acoustical value worth worrying about. The air in the room is a bigger factor.

As for spreadsheets, the good ones are mostly had developed by people who do this for a living and they don't get posted in the public domain much - sorry. They're a lot of work to do and folks don't usually give them away.

Make sure you include factors for differences like double vs single drywall - large difference in absorbtion and at what frequencies. 

Good luck!

Bryan


----------



## LewisCobb (Jan 4, 2007)

Hi Bryan - wow that was quick. Thanks for the confirmation on the GOM. I understand the reluctance of people letting go of something they put a lot of work into such as these spreadsheets, certainly if it's their livelyhood. I am thinking that what I might do is take my HT design as far as I can, and then submit it to someone that does do this for a living - or at least does it as a sideline for HT enthusiasts. I could be sure then that I am not wasting money somewhere when I do the build but still have the satisfaction that I did as much DIY as I could and did save some money in the process. Is this something that a fellow like yourself does - helping to review and advise DIY HT designs and acoustics as well?

Another couple of quick questions now that I am thinking about them (I really should not be working on this stuff while at work but it's so additictive.....)

1- I see some of the tables list absorption for things like people and sofas. I am assuming these numbers are Sabins - i.e. the area x coefficient already done for you?

2 - I understand that due to the testing procedures involved, the coefficients can be greater than 1 in some specifications. Do you just use the numbers as listed when working out the absorption at each frequency band or do you clamp the number at 1.0 for the maximum? I doubt it makes much difference in the end result, but I am a stickler for details (I'm an electronics design engineer and deal with millions of tiny details daily and it tends to drift over into my everyday dealings in the "real world" !)

Thanks again. I should also thank you for all the well reasoned and practical advice that I have seen you give to countless people on the avs as well as this forum. I am not sure if this is your career or hobby, but regardless you are a real asset to the crowd of people like me that are trying to learn and do a job as best they can within a budget.

Lewis





bpape said:


> Hi Lewis - welcome!
> 
> You are correct in assuming that GOM over a hard surface like drywall should be considered basically the same as the plain drywall. The GOM has little to no acoustical value worth worrying about. The air in the room is a bigger factor.
> 
> ...


----------



## basementjack (Sep 18, 2006)

Lewis, if you're going through the trouble of crafting a spreadsheet, I think there's a $75 program that would do you a world of good - its called CARA - it's a room acoustics modeling program.

http://www.cara.de/ENU/index.html

I have it and it's amazing - frankly, I'm surprised it's $75, and not $995.

In Cara, you draft up your room in 3 dimensions, you select surfaces, such as 1/2 drywall, fiberglass, carpet, etc. each surface you use has an associated Absorbtion coefficient matrix (ie not just one number, but a series of coefficients for each octave.

After your room is layed out, you place your speakers and sub(s)- Cara can do a full surround config with 2 subs or as minimal as a stereo pair.

with the speakers placed in your virtual room, you can move to doing some calculations.
Cara will calculate nearly everything you'd want to measure:
Freq response, 
room modes at every frequency (where cancelations occur due to room size)
even RT 60.

of course once thats done, if you're not happy with the results, you can go back and add or change the room - ie add furniture, add a panel of insulation etc.

- Jack


----------



## LewisCobb (Jan 4, 2007)

Jack - This certainly looks like the ideal thing for fussy characters like me that want to "calculate-build-measure-adjust". Correct me if I am wrong, but this looks like the complement to a program like ETF. i.e. I could see a person first modelling the room with Cara, then build it, then check everything with ETF and make adjustments from there. At the very front end (where I am now) you get your basic room 2D and 3D modelled into things like Autocad and Sketchup.

A few questions - (even though I am sold already on this package !)

Can you enter in your own materials and corresponding adorption coefficients for the octaves if they do not already exist in the program?

What about speakers that are not in the database - is the program still useful (albeit less accurate) in this case? 

How difficult is it to learn the CAD portion of the program where you enter your room? I have my room entered into both Autocad and Sketchup at the moment. Until I found Sketchup, I always felt something was missing from the design process. With Shetchup I can get a realistic view of what the room will look like. It looks like Cara will do the same for me in the acoustic realm. 

Looks like there's going to be a lot of lost productivity at the office next week now that you've put me onto this .....:bigsmile:


----------



## terry j (Jul 31, 2006)

A lot of people use and like ETF, and I downloaded the demo program but in my stupidity I couldn't get it going. Well, to be honest I suspect it was a soundcard issue.

I got REW going however, and it does a very good job too!!:bigsmile: :bigsmile: 

The backup support for it is fantastic too.

Post some of your queries re using the program for your purposes in the BFD FORUM, it would be interesting to see questions that are a little new and interesting and that all of us could use in our own listening rooms.

For example, can we use the REW to estimate the RT times by using the waterfall function???

Should be right up your alley

see ya


----------



## basementjack (Sep 18, 2006)

Lewis, Sorry for the few day delay...

You can add your own materials, with co-efficients. you can also add your own speakers, with a pretty good amount of flexibility.

The cad portion is harder than sketchup, but easier than autocad. I bought the deluxe version with the training CD - I was really impressed by the training - it was well done (Sounds like it was recorded in a studio) and the woman narrating really sounded like she knew the product (did not appear to be reading from a script)

Yes to your ETF question - If you're looking to take it to the max, you'd use CARA to help with design decisions up front, followed up by ETF or similar to take measurements after it's done.

Aside from being a cool thing to play with the value of Cara truely lies in the ability to experiment.

Its priceless if you are building or considering adding soffits etc.., 
Yet I still think it pays for itself if you already have a room and are thinking of adding treatment - $75USD up front is cheaper than buying a bunch of insulation to see if you can fix your problem. You could think of it as a small insurance policy for your acoustics!


----------



## bpape (Sep 14, 2006)

I played with CARA some time ago and only had one complaint. When doing the calculations for the room, you couldn't add in furniture and people (both of which are very good absorbers and must be considered). That may now be resolved. 



Bryan


----------



## basementjack (Sep 18, 2006)

Bryan, you add furniture now. There's no people yet..


----------



## bpape (Sep 14, 2006)

Sounds good - so you could just tweak the coefficients of the seats to compensate for the people in them.

Bryan


----------



## Guest (Jan 13, 2007)

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

can't post an URL but its here-http://forum.studiotips.com/download.php?id=837


----------



## LewisCobb (Jan 4, 2007)

Scott R. Foster said:


> here's a list of Sabine numbers that might be helpful:
> 
> can't post an URL but its here-http://forum.studiotips.com/download.php?id=837


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


----------



## Guest (Jan 14, 2007)

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 :no:


----------



## Guest (Jan 14, 2007)

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.


----------



## Ethan Winer (Jul 21, 2006)

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


----------



## Guest (Jan 14, 2007)

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. :bigsmile:


----------



## bpape (Sep 14, 2006)

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


----------



## Ethan Winer (Jul 21, 2006)

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


----------



## Guest (Jan 15, 2007)

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

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:



> 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
> ...


----------



## Ethan Winer (Jul 21, 2006)

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! :dizzy:

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


----------



## LewisCobb (Jan 4, 2007)

I see there's some "professional sparring" going on in my thread here .... :boxer: 

I have been tweaking my Sketchup model and Scott has suggested that I upload it, or a pic here sometime so that I can get some general feedback as well on my room design, but in the meantime I have another "general" question on coefficients.....

What do people use for a Sabine number for say leather Berkline 090 chairs? I have a row of three and a row of 2. I suspect that the row of three is more like 1.25 "sofas" and a row of 2 is more like 0.8 "sofas" as defined in some of these listings of coefficients that you lads have put me on to, but other than using that approach blindly I thought I'd toss it up here.

Cheers,
Lewis


----------



## bpape (Sep 14, 2006)

Generally, I go purely by the number of seats. Also make sure to allow for whether the seats are full or empty - a large difference there. 

Again, realistically, in a real sized room the difference between 1 and 1.25 'sofas' is lost in the noise in terms of the overall absorbtion in the room in most cases. Going from 3 to 4 people will make MUCH more difference both in the numbers and in reality.

Bryan


----------



## LewisCobb (Jan 4, 2007)

bpape said:


> Generally, I go purely by the number of seats. Also make sure to allow for whether the seats are full or empty - a large difference there.
> 
> Again, realistically, in a real sized room the difference between 1 and 1.25 'sofas' is lost in the noise in terms of the overall absorbtion in the room in most cases. Going from 3 to 4 people will make MUCH more difference both in the numbers and in reality.
> 
> Bryan




ok - thanks Bryan. I guess once I start to run the calculations I will see the effect (or non effect) of such changes.


----------



## Ethan Winer (Jul 21, 2006)

Lewis,

> I see there's some "professional sparring" going on in my thread here <

LOL, not me!

> What do people use for a Sabine number for say leather Berkline 090 chairs? <

If you're wondering for the purposes of bass trapping I'd ignore any contribution from furniture if only because it won't do much out in the room. Maybe a large couch against a wall, so it's near a wall-floor corner, will add a little bass trapping. In the overall scheme of things you want as much bass trapping as possible anyway, so the limiting factor will be budget or visual concerns.

Likewise, a leather covered chair won't absorb much at the higher end of the spectrum. If you want to be truly scientific about this, assuming your intent is to target a specific RT60 time, I'd just measure the room and use those measurements as a starting point rather than calculate based on the room when empty.

--Ethan


----------

