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:
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.
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.
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.
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)))
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.
I made up a nice spreadsheet and stuck it on my website.
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.
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
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.
Oh, and for the record, the Master Handbook of Acoustics 4th edition has some MISTAKES it appears, here and there. Happens...
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.
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?
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.
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!)
I don't know if this could help, but the link below may provide some info. It deals mostly with cylinders, but there are many other links on the page that could lead you to something that might help.