Hi guys,
I've been reading "the master handbook of acoustics" and to my surprise i seem to be understanding an all lot ;\)

What I can't seem to understand tho is the formula to calculate the percentage of perforation of a panel.

to calculate the freq of resonance for a panel the formula is:


f = 200 * sqr [p/(d*t)]

where:
f = freq
p = perforation percentage
d = depth of air space behind the panel
t = hole lenght

to calculate the percentage there are 2 desctiptions, one in the formulas legenda and one in a picture in the page.

The one in the legenda says to divide the area of the hole (radius square * 3.14) by the area between the center of 4 holes (when the 4 holes are placed in a square pattern) and multiply by 100.

The one in the picture says to divide the diameter of the hole by the distance of the holes centers, square the result and then multiply by 78.5.

to try to understand if they where equivalent or if one of them was a typo i decided to recalculate some of the numbers contained in the tables of frequency included in the chapter but nothing come out the same...

for example, the book shows that a panel 1/8" thick with holes 1/8" in diameter spaced 2.22" apart (vertically and horizontally) is 0.25% perforated.

so i proceed with verifying using the formula in the legenda (hole area/panel area)*100:

1/8" diameter = 0.125" diameter = 0.0625" radius
so the area of the hole is = (0.0625 square)*3.14 = 0.01226 square Inch

now to calculate the percentage = (0.01226/2.2)*100 = 0.55


then i proceeded to do the calculation with the formula in the pic {(hole diameter/hole distance)square}*78.5:

we know the holes diameter is 0.125 so,
to calculate the percentage = {(0.125/2.2)square}*78.5 = 0.25

Now, utilizing the formula in the picture i get the coefficient of percentage described in the table i have tested (0.25), but using a formula that actually gives an accurate percentage of perforation the numbers don't line up.

I'm not questioning the correctness of the authors statemente, i'm just trying to understand the relation between the 2 formulas that give different results.

Thx a lot everyone \:D

Cez

PS. If you have the book is on page 219 ;\)

Edited by killaCez (01/16/08 11:10 AM)
Edit Reason: reviewed