Surface/Cone Area Comparison

http://editweb.iglou.com/eminence/eminence/pages/params02/params.htm

The above should help.

 

This is not complete because it doesn't take the area of the voice coil into account which is a greater proportion of the area for smaller speakers than for larger. However, having said that I use the square of the nominal speaker radius to give me a rough guide for comparing. Obviously I am leaving pi out, but it is not necessary for a rough comparison.
8" speaker = 16
10" speaker = 25
12" speaker = 36
15" speaker = 56
Of course there are lots of things in speaker design besides the size of the cone that influences how much air it will push and how clearly you can hear the music.

 

What you are looking for is the piston area of the driver. This is the "Sd" component in the Thiele-Small specs published for the driver.

You can download my spread sheet, and sort by driver nominal size, then by Sd, to get an average of 10" drivers, etc. The Sd spec varies between drivers depending on the cone surround.

 

Keep in mind that the piston area (Sd) of the driver is *not* independent of the driver sensitivity. Instead, the driver's sensitivity (among many other parameters) is related to the Sd (though not exclusively in any way shape or form).

IOW: You cannot compare how loud two drivers will be by comparing the piston area alone.