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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
edesilva
768 posts
 
Fer what its worth, and considering I'm not sure what the heck the various terms really mean, I'm with you. I get:

If perf board is 1/4" holes spaced 1", which is what I seem to remember, the "area" of the hole is 3.14*(.125)^2=0.049 sq. inches, so the perf percentage looks like 5%. I dunno what this correction factor is, but figuring my perf board was 1/4", I get t=.25+.8*.25=0.45. D=3.625, so, I get:

F=200*(0.049/(.45*3.625))^.5=34.6 Hz.

Oddly, if you were supposed to enter the "percentage" as a number (i.e., 5 for 5%, as opposed to 0.05--which totally offends my sense of mathematics), I get to 346 Hz.
danish
30 posts
 
I think the book might have a typo on that line. I get 300Hz based on their numbers.

The reason I'm thinking it's a typo is because they have a table of values a couple pages later. It has a list of depths, hole diameters, and panel thicknesses. I made a spreadsheet and it matched the table when I typed in the values.
russno5067 OP
5 posts
 
Yeah, none of the scearios on the table of values on the next couple of pages add up for me! When I work the equation, with the values they provide, I get a different answer than what they give!!! I'm SOOOOOOOOOO frustrated!
I just want a consistent way to figure for this stuff, and it's driving me batty!
I mean, I don't think my math skills are off or anything. Or then again, maybe they are. But I don't think so. Something ain't right, that's all I know. I would like to hear from maybe Rivesaudio or someone on this one personally.
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