Tweeter CD wave guide

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Robert C White / Rod Elliot have a DIY wave guide design instruction here:

Practical DIY Waveguides - Part 1

(part 2 and 3 of the guide is linked at the bottom)

I have tried to work the formulas into an Excel-spread sheet using the 1500 Hz example in part three, but cannot get the formulas/numbers right: I do not get the same numbers as White / Elliot:

If we make angle α = 90° ...

c = ( 720 + 90 ) / 5 = 162

b = 360 - 324 + 90 = 126

W = 1 / 3 * (( 25,306 (126 * 162 + 90 * 162 + 90 * 126 )) / ( 1500 * 90 * 126 * 162) = 142mm
The horn radius is...

R = ( W - t / 2 ) / sin ( α / 2 )
... where t is throat diameter and α is the radiation angle. Throat diameter is determined by the tweeter used, and must be measured carefully. The overall depth is ...

d = R - (Tan a / 2) ( w - t / 2 )
The radius at points x is ...

r ( x ) = ( t / 2 + ( w - t ) / 2 ) - ( R² - ( m + d - x )² ) ½
... where m = Tan α / 2 ( 0.5w - 0.5t ) and t = throat diameter. x is the centreline distance along the waveguide from the throat in millimetres.


x R x R x R x R
0 18.5 6 25.2 12 34 18 47
1 19.5 7 26.5 13 35.7 19 50
2 20.6 8 27.8 14 37.6 20 54
3 21.7 9 29.2 15 39.7 21 59
4 22.8 10 30.7 16 42 22 71
5 23.9 11 32.3 17 44
Table 1 - Offset (x) for various values of R

Are the formulas above incorrect, or do I just have to look more into my excel-formulas? I get the numbers corret to "w" (expect I get 0,1418 istead of 142), but after that White / Elliot do not sum the formulas for R, d and r(x), so I do not know in which formula (R, d og r(x)) my mistake(s) are.

Have anyone made a spread-sheet for designing CD wave guide / Cd horn?

Best regards

Gisle
 
I think your "mistake" is the number 25,306. In your spreadsheet you should use the number 25306. Notice that in the US they don't use the comma the same way as we do in Scandinavia (or many other European countries). Apart from this detail, your computation looks ok. The following formula (in Excel) gives you the number directly in mm:

= 1000*(1/3)*((25306*(126*162+90*162+90*126))/(1500*90*126*162))

Good luck with the number crunching.

Cheers David
 
Last edited:
I think your "mistake" is the number 25,306. In your spreadsheet you should use the number 25306. Notice that in the US they don't use the comma the same way as we do in Scandinavia (or many other European countries). Apart from this detail, your computation looks ok. The following formula (in Excel) gives you the number directly in mm:

= 1000*(1/3)*((25306*(126*162+90*162+90*126))/(1500*90*126*162))

Good luck with the number crunching.

Cheers David


Aha! Suddenly I blushed... Thanks!

And also thanks to Ed for email.

:)
 
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