Alterations to pegboard absorber

Hello, I have been doing some reading on the design and construction of a "pegboard" absorber, as written about on this SAE -page. I also found a local forum discussion on the topic here. Once I have determined my treatment needs, frequency wise, I aim to build a sufficient number of these absorbers for use in my control/mixing room. The pegboard I bough has a white finish on one side and a "rough cut" on the opposite side. I guesse this is the way most of these boards look. Now then. What I would like to do is to alter the surface of the "front" of the pegboard, the white finish. Initially I was thinking about spray painting them, but that quickly got me thinking bad thoughts such as -"Maybe I would alter the width and depth of the holes if I do that." (some might even clog up completely) so I dropped that idea and moved on to -"What if I apply a nicely colored cloth to the surface of the finished absorber?"... Here I am at a loss... After a lot of reading, both printed litterature and online material, I am still uncertain as to how it would affect the actual absorbing feature of the unit. What is your opinion? Would the latter idea work or am I missing some critical factor? Best regards, /Gash

Those aren't pegboard absorbers, they are slot resonators. Re-read Knightfly's post in the 2nd thread there. It'd be hard to build a good resonator out of pegboard.

Quoted from the paragraph on the page, right before the equation, on the page I linked to.

If you place a panel of this over an air cavity like in a panel absorber not only do the little holes act like bottle necks the whole panel acts as a low frequency panel absorber!

Maybe I was not clear in my initial post, as to what on the linked page I was refering to. Am I still to assume that what is being discussed on the SAE-page, in the above mentioned paragraph, is in fact a slot resonator and not a panel absorber? I feel confused. Please clarify, thank you. /Gash

Look at Knightfly's post more carefully:

"In commonly available materials, such as pegboard, the holes are so numerous that resonances at only the higher frequencies can be obtained with practical air spaces. "

I don't think using pegboard will help you with lower frequencies. Run the numbers in the equation, then you'll know for sure. Let us know what you calculate.... I'm interested. I've never built a pegboard resonator so I don't know for sure, I just don't see how it could work at bass frequencies.

jwl wrote:

Look at Knightfly's post more carefully:

"In commonly available materials, such as pegboard, the holes are so numerous that resonances at only the higher frequencies can be obtained with practical air spaces. "

I don't think using pegboard will help you with lower frequencies. Run the numbers in the equation, then you'll know for sure. Let us know what you calculate.... I'm interested. I've never built a pegboard resonator so I don't know for sure, I just don't see how it could work at bass frequencies.

There is another interpretation with the same results. The commonly available materials are so thin that only resonances at mid to frequencies can be obtained. Same results. The only difference is that the second summary implies you could adhere together multiple panels to achieve a useful effective hole depth. The cost of the multiple thinner panels, gluing them together while maintaing hole alignment, etc makes it not very viable solution. Plus BBC's 6" spacing of 1/4" (if I remember the numbers correctly) is a proven design for studio applications. Gash: You seem to be not afraid of mathematics. The best document I know regarding perforated panels and acoustics is the Acoustical Uses for Perforated Metals handbook. Andre

Hello again, thank you for your reply. I ran the numbers according to the following assumptions. Please let me know what you think. What I arrived at seems, at best, "nice". Now then, this how I figured. First to figure out what I am most interested in. I think that in this case that would be the actual depth of my unit. So I decided to solve the equation on "D" This is what I came up with: (no emphasis was put to make the expression as short as possible) D = (w/(f^2/2160^2)) / d "r" was canceled out when solving for D. Ok, so solving for D now. Since the interest had its focus at some low frequency, I started by setting "f" to 100 Hz. I made an estimate that the holes in my pegboard are about 5-7 mm apart, I will assume that as my slat width so I used 5mm in this calculation. So "w" is now 5mm. The thickness of my pegboard is 6mm and as such i set "d" to (1.2 x 6) D = 324 mm :oops: To me that almost makes too much sense. Luckily I am not proficient enough at this to belive in myself yet. Please comment. /Gash

Something went wrong in the equation rewriting. The slot width is needed to know the open area. The document I referenced has direct formulas for circular openings. Andre

Yep, reading it now.

generally speaking you have to make the "pegboard" yourself to get it to the desired frequency range. there are some specialty products but most common pegboard will be a high frequency absorber. there is a stickie with the slat calculation spreadsheet - use the corrected version...

... S i g h So... After I started realizing that something was wrong with my re-write I went back to SAE, re-read that whole chapter and still could not figure out what was wrong. So I opened up Rod's book and re-read chapter nine. Once I got to page 179, at the bottom, I read... - "Be careful if you go this route. Sometime, back a ways, there was an error in this calculation that made its way through the internet." (d*D) + (r+w) should be (d*D) * (r+w) When solving for D, that gives: D= ( r / (f/c)^2 * d(r+w) ) Well... now, at least, the expression is returning some reasonable results and also shows that these so called low-frequency pegboard panel absorbers, as written about in the SAE-pages, do not work within reasonable dimensions. Just like the people replying to this thread have stated before. This is some kind of high-mid range absorber, at best. Usually... I am not one to be sloppy with checking the formal nature of the functions/algorithms I use. But in this case... I guess it was either too late in the evening or I just assumed that all the information on the SAE-pages had the correct information. On a related note I had some thoughts regarding the Helmholtz.xls file that offers a way to calculate the absorption frequency, given some certain panel dimensions. The version I downloaded from SAE has two columns, one for metric units and one for imperial units. What I find strange though, is that the radian value derived from ( c / 2pi ) is set to 2160 in both cases. For imperial (inch/s) units 2160 works out nicely. But for metric (m/s) the value should be 343/2pi = 54.590 It is of course quite possible that the .xls file that I downloaded is an outdated one. Does anyone know if there is an updated version of that .xls file out there or if I should inform someone to change it? /Gash To Glenn: I can not seem to find the spreadsheet you are referring to. Could you post a link? Thank you.

Ok... so what about this page then? Almost at the bottom of the page. Where it says: "Calculate Resonant frequency of Helmholtz Panel Resonator" Has anyone used this calculator before? If so, how reliable is it? Gonna compare some of my own calculations with it now. /Gash

Gash wrote:

Ok... so what about this page then? Almost at the bottom of the page. Where it says: "Calculate Resonant frequency of Helmholtz Panel Resonator" Has anyone used this calculator before? If so, how reliable is it? Gonna compare some of my own calculations with it now. /Gash

I am confused by your question. You have the correct formulas. Just plug them into a spreadsheet. BTW, how is your floor deflection simplification coming along? Andre Edit: You will find this thread of value. Note in the first post the following:

professional acousticians do NOT rely on net calculators, but on there study and books

This faulty Helmholtz equation is not the only questionable item in various pop ups on acoustics, just the most famous. If you are going to use an on line calculator, confirm the equations with test data and compare the results. The last sentence is obviously directed at readers of this thread, as you are doing that already.

AVare wrote:

I am confused by your question.

Current question: - "Is there any reliable (tried and tested) online calculator collection aimed at acoustic construction applications?" And I guesse that, in part, you already answered that question via your quote. I read the thread you referenced... and I have to admit that I am somewhat frustrated that I did not find it on my own.

AVare wrote:

BTW, how is your floor deflection simplification coming along?

I am debating where to cut corners. I have been talking to a good friend and colleague about this and he feels that by cutting the corners I feel are needed, the whole point of the calculation is rendered more or less useless and the effort of an approximation is just as well replaced by a random guesse. I am trying to simplyfy, in order: - The function which approximates the average damping effect of some generic neoprene type. - The function that defines the distribution of mass of the floor frame work. - An apropriate Dirichlet boundary condition for the permanent load, i.e. walls and ceiling - The final diff. eq. for when the whole construction is exposed to stress. (permanent + temporary load) The above topics are, of course, related to eachother in various ways which adds to the task of simplyfying. There are other issues not listed. Problem is, I have to keep up with my work related projects and my own studio planning at the same time. This limits the ammount of time I can spend on the floating floor issue. /Gash

I read your post and there is not too much I can add that is useful. Serious acoustics designers use the equations, which really are not complex at all if you understand the physics they are based on. As far as floor spring deflection goes, now you know why there is no simple way to calculate it. Andre