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4th March 2004, 03:45 AM  #1 
diyAudio Member
Join Date: Dec 2003
Location: Houston, TX

Calculating driver Sd
The Sd(M2) values given in the The Loudspeaker Design Cookbook are wrong. can somebody post the formulas for Calculating SD in either
cm2 or m2? thanks 
4th March 2004, 05:44 AM  #2 
Wizard of Kelts
diyAudio Moderator Emeritus

I do not have the Loudspeaker Design Cookbook.
There are two ways of calculating it. One way is to measure just the cone. Famed audio author David Weems recommends including the surround on just one side. One thing you can do is to go to a site that offers woofers in cm², like www.peerless.dk click "Enter a world...." Another is to take these estimations I find that are pretty close, based on Weems' formula: 15"=132 sq in=825 sq cm. 12"=84 sq in=525 sq cm 10"=56 sq in=350 sq cm 8"=32 sq in=200 sq cm 6.5"=24 sq in=148 sq cm To get from sq in to sq cm, just multiply by 2.5². These are close enough for most purposes. Another way is to take the diameter of the basketnot the cone, the entire baskedsubtract 1.5" from the diameter, and use that as the cone diameter. Thus, a 10" basket will have a cone of 8.5" (8.5/2)² x 3.14=56.7 x 2.5²=354 sq in. Below 8" subtract 1" instead of 1.5" from the basket diameter. Works well enough for most purposes.
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4th March 2004, 07:09 AM  #3 
diyAudio Member
Join Date: Dec 2003
Location: Houston, TX

Thanks. Actually 1 sq in = 6.451599999999999 sq cm = 0.00064516 sq m
which would make both the cookbook and weems calculating formulas wrong.Most definitely cookbooks. http://www.loudspeakers101.com/PortForm.htm 
4th March 2004, 07:33 AM  #4 
Wizard of Kelts
diyAudio Moderator Emeritus

Okay, so I used 2.5 cm/inch instead of 2.54 cm/inch. That's within 3.2 percenta negligible difference.
What do you need this measurement for? Why must it be so exact?
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4th March 2004, 07:36 AM  #5 
diyAudio Member
Join Date: Dec 2003
Location: Houston, TX

I'm writing a program so I wanted to know which formulas are accurate.

4th March 2004, 07:49 AM  #6 
Wizard of Kelts
diyAudio Moderator Emeritus

Okay. Now I understand.
The "formulas" I gave above where just quick rules of thumb meant to be close enough for practical work. Do you want any of Small's papers on Direct Radiators or Closed Boxes? I've got them on my hard drive. Lots of real formulas there.
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4th March 2004, 08:01 AM  #7 
diyAudio Member
Join Date: Dec 2003
Location: Houston, TX

Thank you but not necessary.
The program I'm writing is basically nothing fancy or complicated. It's meant to replace a calculator for people who still have to do their own old fashion way measurements Like myself ;) It calculates normalized box dimensions. Port length. Internal volume. T/S parameters. 4 different compensation filters. Complex impedance. And couple of extra things. No graph or anything because I find graphs useless. The entire code is based on my entire speaker builder collection from 80's and bullock and cookbook. 
4th March 2004, 10:35 AM  #8 
diyAudio Member
Join Date: Nov 2003
Location: Brighton UK

For Sd some aproximation must be made because
the surrounds contribution is not exactly known. I think using half of the surrounds area is as good as any. If you want to be exact for phase plugs and porous dust caps you should also substract the the CSA of the coil. sreten. 
4th March 2004, 11:37 AM  #9  
diyAudio Member
Join Date: Feb 2004
Location: Stockholm

Quote:
If you are writing a program then it should be possible to enter the Sd value, and that's it. ? 

4th March 2004, 01:23 PM  #10 
Wizard of Kelts
diyAudio Moderator Emeritus

That's true. What are you trying to calculate Sd from? The diameter of the stiff part of the cone? Vas? Qts?
What formulas are the Loudspeaker Design Cookbook giving and how do you know they are wrong? Most programs let the user enter Sd and calculate the other parameters from that, like Bullock and White's BoxModel.
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