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11th October 2002, 06:07 PM  #1 
diyAudio Member
Join Date: Oct 2002
Location: bellevue, wa

A problem for a geometry wizard
I need the help of someone who is better at geometry than I am. Here's the problem...
A speaker cabinet design has internal dimensions that are 13"x13"x45"(wdh). Not liking square cabinets, I want to transform this square crosssection cabinet into a circular crosssection cylinder with a flat front baffle. Like a straw sliced end to end. However, I need to maintain the original internal crosssectional area (13"x13"=169"sq) and the flat front baffle width (13"). Ignore the cabinet height. So the problem is, given a chord of length 13" (the front baffle) that bisects a circle, calculate the radius where the larger piece of the circle bisected by the chord (the internal speaker cross section) has an area of 169"sq. Logically we can tell that the angle of the chord must be somewhere between 90 and 180. Any takers? The speaker is the Hammer Dynamics Super 12. Thanks!! Xylenz 
11th October 2002, 06:17 PM  #2 
diyAudio Member
Join Date: Oct 2002
Location: bellevue, wa

One more thing...
The internal baffle width and the external baffle width will be identical (13") so dont worry about compensating for the thickness of the cylinder. All i need is the internal radius. Thanks.

11th October 2002, 08:11 PM  #3 
diyAudio Member
Join Date: Aug 2001

Your inside radius would be 7.974"
assuming the inside of your baffle is 13" 
11th October 2002, 08:11 PM  #4 
diyAudio Member
Join Date: Jan 2001
Location: Cuyahoga Falls, OH

Here's my guess
If I'm reading your question correctly, you want the area of a parabola with a base width of 13" and a total area of 169" sq.
Assuming that, then a quick search on: (And bookmark this folks, its a great site for problems like this!) http://mathworld.wolfram.com/ reveals that the area of a parabola of a base width of 2a and height h is given as: area = 4/3 * a * h or an answer for you would be a height of 16.9". Does that seem correct? 
11th October 2002, 08:36 PM  #5 
diyAudio Member
Join Date: Jan 2002
Location: Vancouver, BC, Canada

I got 7.9708" radius...
__________________
My girlfriend's sub is bigger than mine... 
11th October 2002, 10:06 PM  #6 
diyAudio Member
Join Date: Oct 2002
Location: bellevue, wa

Thanks much!
Im not sure about the parabola thing. Its just a circle. But thanks anyway.

12th October 2002, 12:31 AM  #7 
diyAudio Member
Join Date: Apr 2002
Location: Newcastle, Australia

my results, with working

12th October 2002, 01:42 AM  #8 
diyAudio Member
Join Date: Oct 2002
Location: Mad Dog, Texas

math is fun
if radius = 7.97077469 inches then the area = 169.00000178 sq. inches

12th October 2002, 03:17 AM  #9 
diyAudio Member
Join Date: Apr 2002
Location: Newcastle, Australia

whoops !! error in spreadsheet
I left out a sinØ, doh !!
yep, close as! r = 7.97 Ø=109.26º 
12th October 2002, 03:55 AM  #10  
diyAudio Member
Join Date: Jan 2002
Location: Vancouver, BC, Canada

Re: math is fun
Quote:
I hope you have an accurate ruler!!! A hundredmillionth of an inch!?!? break out the electronmicroscope...
__________________
My girlfriend's sub is bigger than mine... 

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