Posted by Jeff Aiken [ 68.105.157.45 ] on September 24, 2004 at 00:37:18:
In Reply to: Re: 8 Pi Tuning posted by Wayne Parham on September 23, 2004 at 18:14:49:
Hi Wayne, thanks for the reply. I did make a mistake in that I calculated the driver volume to be .75ft^3 instead of 75in^3.
So, here's the internal volume I calculate, now (sorry so lengthy, my geometry is a little rusty ;).
Using the 8 Pi external dimensions I've subtracted 1.5" from each dimension (2*.75" material) to arrive at the internals.
1) I calculated the front "cube" portion of the cabinet first:
24.5"W x 30.5"H x 9.6"D = 7173.6in^3 or 4.15ft^3
2) I calculated the rear "trapezoid" portion as (A=h/2(b1+b2))*Height:
(5.25/2(12.5+24.5))in^2 x 30.5"H = 2962.3125in^3 or 1.714ft^3
3) Add front and rear portions and get 5.864ft^3 total internal before subtracting mid-horn or bracing.
4) I estimated the mid-horn flare w/mounting plate to be about 1.12ft^3 and the driver to be .043ft^3 totaling 1.163ft^3.
5) Subtract 1.163 from 5.864 and total volume remaining is 4.701ft^3.
6) I only subtract 5% for bracing giving 4.466ft^3 total internal volume.
According to WinISD, using 3 ports (1.625x3.5 as the plans suggest) this tunes a 4.466ft^3 cabinet to 62hz with a F3 = 50hz and a 5.4dB peak at 70hz with the Alpha 10. To get a flat response to 40hz WinISD says the ports would each have to be 4.43" long.
Am I still calculating something wrong? Is WinISD's vent calculation inaccurate (seems I read that somewhere)?
Wayne, if you're bored (you have to be if you've read this far ;p) and enjoy toying with the mathematically challenged I still appreciate your help and now your patience. ;^> Sorry, if I'm beginning to aggravate. Just trying to grasp things before I make the plunge.
Thanks,
Jeff
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