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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 3 responses by russno5067

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.
russno5067 OP
5 posts
 
I had another acoustical consulting company GRACIOUSLY send me a "plug in formula" for an Excel spread sheet. Not knowing how Excel spread sheets function, it's a little greek to me. Here's what they sent me.

C4 (the internal depth inside box between back plate and front skin, inches)=(((78.5)*(POWER((B4/E4),2))/(D4+(0.8*B4)))*(POWER((200/A4),2)))

Where:

B4 = diameter of the hole you drill (inches)
E4 = center-to-center hole spacing pattern (inches)
A4 = target frequency in Hz
D4 = thickness of the front plywood skin (essentially the throat length of the drilled hole) (inches)

I'm told that the following traslation applies for the "POWER" thingy!...lol:

POWER(x,2) = x squared

POWER((x/y),3) = (x/y) cubed, etc, etc

K,here's the deal...when I plug either of these formula configurations in with the variables, and compare the charts in the MASTER HANDBOOK OF ACOUSTICS no less, I get completely wrong answers from what they have!!!!...double, doh!!! This has been nothing short of down right pissin' me off! (lol)
For instance, using the chart in F.Alton's Master Acoustics book, some of their chart examples bellow give:

Air Depth Hole Diam. Panel Diam Perf% Hole spacing Freq rez.
3.625" 1/8" 1/8" 1.25% .991" 248

3.625" 1/8" 1/4" 1.25% .991" 199

3.625" 1/4" 1/4" 1.25% 1.98" 199

If I plug in the variables in these equations, I DON'T GET THEIR ANSWERS! And this is supposed to be the MASTER HANDBOOK! Or maybe I'm just a mathematical numbskull!? Lol
Again, plese post to audiogon for all to see, if anyone has answers here.
russno5067 OP
5 posts
 
Thanks for the help guys. I think I was just lazy with most of this. I think I got the forumulas figured out to solve for any variable.
Flrnlamb? If you need any equation help, er whatever, let me know. I'll shoot you over what I came up with anyway.
I'd check em out thouroughly though before trying however, as my math is "suspect" at best.(lol!)
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