Strictly speaking the Sd should be the radiating surface area, cone and whizzer together. However, for purposes of box design, the whizzer cone can be ignored, leaving only the back side of the cone to consider. The calculation is the area of the cone extended to a point, minus the "imaginary" part of the cone. It's common practice to take the diameter of the base at one half the surround diameter, in this case 6.5". The truncated (imaginary) portion of the cone has its diameter taken from the diameter of the dust cap: 1.5". The height (depth) of the cone taken to its imaginary point is 2.5" and the height of the imaginary portion of the cone beneath the dust cap is 0.5". Now, the surface area of a cone is : S = π × r2 + π × r × √ (r2 + h2) But this includes the base, which is the π × r2 + part. We can ignore that, as we only care about the area of the conical portion. So, the main cone extended to its imaginary point is: pi * radius * square root of (radius squared plus height squared) OR pi * 3.25 * sqrt (3.25^2 + 2.5^2) OR 41.84 in^2 For the portion below the junction of whizzer to main cone, we use: pi * .75 * sqrt (.75^2 + .5^2) OR 2.12 in^2 Subtracting the imaginary portion from the whole cone taken to its imaginary point is 41.84 - 2.12 = 39.72 square inches. A truly accurate Sd would require consideration of the additional area of the ribs, plus a calculus estimation of the surround's contribution varying by increasing radius and curvature up from the cone to the edge of the surround. But chances are the difference would be so slight as to make that calculation an academic exercise.
Bottom line, I'd call it 40 in^2 based on measurements taken from the one on my desk.