Which formula is correct for re-scaling port level to merge with nearfield?

[fjadouce100]

I have come through 2 formulas for leveling port spl to match nearfield spl.
1. 20log(port diameter/ eff diameter of cone)
2. 20 log(port area/eff diameter of cone)
That makes quite a lot of difference.
What is the correct one?
Any

 

[hifijim]

equation 1 is correct: 20*log(Diameter of port / Diameter of cone)

 

[ernperkins]

Or if you prefer using areas: 10 log(port area/cone area).

 

[fjadouce100]

Tks for inputs.