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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

Showing 4 responses by danish

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
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.
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
 
Which comparison chart are you referring to? I'm assuming the table starting on page 223? I tried a few random lines in their examples and they worked out correctly.

D = (40000/F^2) * (P/T)

If you have Excel you could easily set this up using the "goal seek" function to get the result you want.

Let me know if you need any help.
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