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Helmholtz resonator equation help? Anyone?

Don't know if this forum is the right place for this question, as I'm trying to figure out how to build a bass trap with this formula. But thought I'd try.
Anyways, does anyone know or know how to work the helmholtz resonator formula, for constructing a perforated box?
I seem to be getting conflicting results with my mathematical formula proceedures for some reason. Don't know why. Also, does anyone know where I can find a "plug in" or whatever to simplify this process? I'm a bit "puter ill-literate", sadly.

I have the following formula, but am getting conflicting "right answers" when I try to work it. Here's what I'm using from F.Alton Everest's Master Acoustics Handbook:

F = (200)x square root of (P/(D)x(t))

In which F = target frequency,hz
P = perforation percentage
t = effective hole lenght, inches, with corretion
applied
= (pannel thickness) + (.8)x(hole diameter)
D = depth of air space, inches.

Now, this same book says that if you use common peg board, with a .18 inch (3/16ths) thickness, and same hole diameter, with 1" spaced holes on center, at a depth of 4"(er, 3.625" on 2x4 studs), your reasonant fequency is around 420hz. I seem to get anything but that when I calculate!
Maybe someone has some input here? I know it's an "off the beatten path" post at that.
Any input will be greatly appreciated!
Please no PM's, just respond here for all to see
russno5067
danish
30 posts
 
I made up a nice spreadsheet and stuck it on my website.

http://www.dyacoustics.com/Links.html

I'm not sure what math you are using :), but I get the same answers as the book for the table.

It's the 420Hz in the text that I think is a typo. The same spreadsheet that matches all the values in the table comes up with 300 Hz, not 420.

Take a look, let me know what you think or if you have problems with the file. You need to have Excel to run it.
danish
30 posts
 
Careful with the formula they sent you. It is correct of course, but has been rearranged to solve for the air space depth instead of the frequency.

Let me know if you get my spreadsheet to work.

Here's a sample calculation (first line in the table).
hole size = 1/8" (0.125), spacing = 2.22, thickness = 1/8", depth = 3 5/8" (3.625)

using the square configuration:
perf % = 78.5*(0.125/2.22)^2 = 0.25%

t (effective hole length) = 0.125 + 0.8*0.125 = 0.225

f = 200*(0.25/(0.225*3.625))^0.5 = 110 Hz
flrnlamb
508 posts
 
Ok, it appears the 3rd collumn of examples down on their comparison chart, is ALL WRONG! Everything else seems to calculate out, using the formula, figuring for frequency of resonance.
Now all I have to do is plug in to figure for "airspace depth", and I should be good to go! We'll test it out.
Thx folks
Oh, and for the record, the Master Handbook of Acoustics 4th edition has some MISTAKES it appears, here and there. Happens...
flrnlamb
508 posts
 
Ok, it's official! It appears my mathematics has developed a sever case of rust! Either that, er I'm just not good. Probably the later (lol)
I can't seem to figure "backwards", trying to figure for "DEPTH", rather than from the forumula in the book, which figures from "frequency".
I'm trying to do this by hand first, so I can see it work.
Anyone want to confirm how to break down the forumla
F=200 x the square root of (P)/(D)x(T), so I can figure back for D? So, would it be D = (P)/(T)x((200/F)squared)?
I don't seem to be doing this right.
I really need to figure for depth, rather than from Frequency.
Thank you
flrnlamb
508 posts
 
Can anyone take the first formula I had which is:

F = 200 x the square root of ((P/(D)x(T))

And then, while knowing the other variables, switch the equation around to solver for D, rather than F?
Thanks
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