Tapered speaker port

[lee1964]

Hi
I ordered some speaker ports for a smallish desktop design, however on receiving the ports I’ve found the actually taper from approx 41mm at the front to 38 at the rear over about a 100mm length.
So the question is which measurement do I base the calculations off the opening @ 41mm or the rear of the port @38mm or do I take an average between the two?
Thanks for any inout

 

[TBTL]

It is something between 38 and 41 mm.

The final tuning frequency can be checked by performing an impedance measurement or by observing at which frequency the cone excursion is lowest.

 

[GM]

Eff. diameter = [38*41]^0.5.

GM

 

[lee1964]

So would I take the mid point based on the final length,
for arguments sake 39.5mm and base my calculations in that figure?