Can you tune panel traps accurately?

brookster · Post by **brookster** » Wed May 26, 2004 6:32 am

That's about it really. Is there a rough guide to panel size x frame depth x fibreglass position/depth with which you can approximate a responsive frequency/frequency range?

Cheers guys

AVare · Post by **AVare** » Wed May 26, 2004 12:38 pm

170/((m/d)^0.5)

m=mass (lb/ft^2)
d=depth of cavity(inches)

The absobent material increases the peak of absorption and widens the peak.

Have fun!

Edit: when I first wrote the reply, I wrote the equation completely wrong. It is corrected.

brookster · Post by **brookster** » Sun May 30, 2004 11:48 pm

Thanks AVare!

cadesignr · Post by **cadesignr** » Mon May 31, 2004 3:02 am

To what extent does SIZE of the panel affect the coefficient of the membrane absorber. I understand, the larger the size, the more mass of the panel, but nothing I have read tells me how size relates to absorption. For instance, which has more influence on the coefficient of the absorber, increasing depth, panel thickness(density), or square footage of panel? Also, how or what determines the coefficient, as I read in Mr. Everests handbook that "1/2" drywall on studs spaced 16" O.C. offers an absorption coefficient of 0.29 at 125 Hz and even more at 63 Hz"......(per square foot?)
How is this conclusion arrived at, as the formula for the resonance of the membrane absorber is given, but nothing that indicates or tells how to calculate the absorption at a given frequency? I am trying to determine exactly what size has to do with the coeffiecient as I have read here as well as other places that small panel absorbers do little. If the panel density, and depth of the void behind it are the governing factors of frequency, what does overall dimensions have to do with it, and in turn, how does the panel density and depth affect the absorption coefficient?
Instead of increasing size, would altering either the depth or the panel density have any bearing on the coefficient? Maybe skull density has some bearing here.

fitZ

Ethan Winer · Post by **Ethan Winer** » Tue Jun 01, 2004 12:24 am

fitZ,

> To what extent does SIZE of the panel affect the coefficient <

In theory the size will have no effect because absorption coefficients are by definition independent of size. The raw unit of absorption is the Sabin, and that increases as the absorber material's size increases because you have more material. Absorption coefficients are calculated by dividing the number of Sabins by the surface area of the material. So no matter how large or small a surface you consider, the absorption coefficient remains the same. That's why it's called a coefficient.

For more on this see the sidebar "Measuring Absorption" in the Acoustics FAQ, second in the list on my Articles page:

www.ethanwiner.com/articles.html

--Ethan

AVare · Post by **AVare** » Tue Jun 01, 2004 12:52 am

Ethan gave a good response. Where the size starts to have an effect is in the damping of the panel at the edges.

In very broad terms, the larger the unsupported section of the panel, the greater the absorption peak and closer to theoretical frequency.

cadesignr · Post by **cadesignr** » Tue Jun 01, 2004 2:52 am

Ah yes, skull density, age, and brain info absorption coefficiency....

Somewhere along the line, I mixed up Sabines and coefficients. I must have had a stroke
Ok, then what determines the absorption (in SABINES?)? This is why I asked:
Here is a quote from you Ethan on another thread.....

"Also, there's little point in making small panel traps. You need to cover a lot of surface area for any bass trap to do much, and it's hardly any more effort to make one that goes from floor to ceiling versus only a few feet high. "

Ok Ethan, I read your article, and it STILL doesn't answer my question, so could you PLEASE tell me why, and if panel density and depth and size have no bearing on the absorption, what does? I don't understand how size relates. Maybe this will explain my question....
Does a panel absorber double its absorption when you double the area?

BTW, thanks for the answers too. I DO appreciate it. Acoustics is NOT intuitive.
fitZ

cadesignr · Post by **cadesignr** » Tue Jun 01, 2004 4:11 am

Ok, something just occured to me that REALLY doesn't make sense. Ethan, you say that a panel absorber, such as your company makes, requires that it be mounted to a stiff wall. I don't know the construction of your panels, and what the difference is between a wall and your panel, but Mr. Everest states for 1/2" drywall fastened to studs 16" O.C., there is an absorption coefficient of .29 @125 hz. Ok, thats a standard wall, 3 1/2" deep, 8 feet tall. It has NOTHING that it is mounted to, being a panel of drywall on BOTH FACES. So why does that work, but your panels will not, unless mounted to a wall? Does not the low HZ vibrations simply transfer to the wall and through it? Just as a single TWO leaf wall does? What am I missing here? What makes your panels absorb BETTER than a sheetrock wall? What if your wall, was not a structural wall, but simply a wall of studs, 48" on center, with sheetrock as a panel. Maybe built in FRONT of and up to the existing REAL wall, whereby the ACTUAL wall was the back of the panel absorber, just as you state. Does MORE density(mass?) increase or decrease the absorption coefficient? How do you determine the difference in absorption between a 4'x8' panel absorber with sheet rock, or mdf, or ply, or whatever, and your product. I haven't seen the specs on yours yet. I guess what I'm saying is ...Where do you find the coefficient ratings of different materials used as a panel absorber, just as you find 703 ratings? Or do they exist? Some must, because Everest stated the one for a standard sheetrock wall, which is the same as a panel absorber, no?

Ethan Winer · Post by **Ethan Winer** » Wed Jun 02, 2004 6:46 am

fitZ,

> could you PLEASE tell me why, and if panel density and depth and size have no bearing on the absorption, what does? <

Suppose you have a 2x2 foot throw pillow and it has 2 Sabins of absorption at 1 KHz. By extension, two throw pillows will have 4 Sabins of absorption at that frequency. Again, a Sabin is an absolute unit of absorption, so the more absorbing material you have altogether, the more total absorption exists in the room. However, each pillow has an absorption coefficient of 0.5 - computed by dividing the 2 Sabins by the 4 square feet (2x2) of surface area. So each pillow has an absorption coefficient of 0.5, and two pillows have the same value because the Sabins are doubled but so is the surface area.

> Does a panel absorber double its absorption when you double the area? <

Yes, because you have twice as much surface area! You're thinking too hard about this.

It really is all just common sense and simple math.

--Ethan

Ethan Winer · Post by **Ethan Winer** » Wed Jun 02, 2004 6:56 am

fitZ,

> say that a panel absorber, such as your company makes <

Just to clarify, my company used to sell wood panel bass traps, but we discontinued those about a year ago once we developed MiniTraps which are better and also cost less to make and ship. Wood panel traps do attach to a wall, and probably interact with the wall in some regard. But MiniTraps are completely different and work on a very different principle.

> What makes your panels absorb BETTER than a sheetrock wall? <

Again, assuming we're really discussing wood panel traps which are meant to mount on a wall:

A sheetrock wall absorbs by flexing, but it flexes only a little compared to the front surface of a panel trap. So where a wall can absorb maybe 0.3 (coefficient) over a small range of frequencies, a panel trap built properly can absorb very close to 1.0 and over a wider range. It works better than the bare wall because it's between the sound waves and the wall, and so it absorbs the waves before they reach the wall.

> Where do you find the coefficient ratings of different materials used as a panel absorber <

You build them and test them in a lab, as I did. That's how I know a panel trap can have an absorption coefficient very close to 1.0. I have no idea how Everest came up with his figures but I suspect a lab, or at least lab grade gear, were used.

--Ethan

cadesignr · Post by **cadesignr** » Wed Jun 02, 2004 10:26 am

You're thinking too hard about this. It really is all just common sense and simple math.

Your assuming I have common sense Ethan

Ok, thanks much Ethan, your answer about your panels absorption coefficient helps. With out knowing that, how would anyone know for sure that it was doing anything period, as no one would have thier own traps tested. But this STILL doesn't totally answer my question, but oh well. I'll find it sooner or later. I've burned enough cells for the day. Ha!

fitZ

Ethan Winer · Post by **Ethan Winer** » Thu Jun 03, 2004 2:02 am

fitZ,

> this STILL doesn't totally answer my question <

If only I understood what you're asking. I'm not holding anything back, really!

--Ethan

cadesignr · Post by **cadesignr** » Thu Jun 03, 2004 11:45 pm

f only I understood what you're asking. I'm not holding anything back, really!

No problem Ethan! Its a matter of me not understanding how panel density and depth affect the absorption coefficient of a membrane absorber.
In other words, if I had a 2'x8' unit, 4" deep, with a 1/2" ply face, it has an absorption coefficient of xSabines at xHz. If I change the 1/2" panel to 1/4", the frequency changes, correct? But now, will the coefficient change also? It would seem so, but thats why I ask, as there is no way to calculate the total absorption of homemade panel absorbers in a room, unless you know the coefficient, correct? And there is no way to calculate the coefficient of a single panel, correct?
Only by tests, would one know, therefore, the only way to calculate the total absorption, would be to purchase, TESTED units. So for those who follow the doctrine of calculating the absorption of everything in a room to determine RT-60, home made units cannot provide any means for doing so. Not that I would. Its just another rabbit hole for me, and I seem to question things I do not understand. Thanks for the info you supplied. It DID help.

fitZ

Ethan Winer · Post by **Ethan Winer** » Fri Jun 04, 2004 4:07 am

fitZ,

> not understanding how panel density and depth affect the absorption coefficient of a membrane absorber. <

Ah, got it - now I see what you're after!

I don't think either of those affect the actual amount of absorption. At least not with a simple relationship between the few values used in that formula. If all else is equal, then a panel trap will absorb 100 percent at its center frequency, and less at either side of that center.

Things that can reduce the 100% absorption are losses in the panel and the lack of an air seal. This is getting to the bounds of my understanding, so I can't offer too much else. Maybe some of the "real" acousticians / physicists here can comment on what else affects the amount of absorption.

I'll also mention that the fiberglass inside the box affects the total absorption, while it simultaneously affects the trap's Q, or bandwidth.

Finally, the total absorption can be reduced if the panel is too small for a given thickness, or anything else that prevents it from flexing fully. For example, if you use 1/2 inch plywood and the panel is only six inches wide, the center of the panel will not deflect nearly as much as if the panel were two or more feet wide.

--Ethan

cadesignr · Post by **cadesignr** » Fri Jun 04, 2004 11:35 pm

For example, if you use 1/2 inch plywood and the panel is only six inches wide, the center of the panel will not
deflect nearly as much as if the panel were two or more feet wide.

Voila! Ok, then size DOES have a bearing on it. That is exactly what I was trying to find out. There must be a ratio of edge length to panel thickness or something like that. But I've beat this dog enough. Thank you for your persistance Ethan.

fitZ