[roscoeiii]

The beaming of the woofer is gradual with increasing frequency and highly dependent on the diameter of the cone.
My personal formula, rule of thumb is to use the number 8658 divided by the actual cone diameter in inches. Cone diameter only, not including the surround. The number you end up with would be the approximate frequency where beaming would just begin to get noticeable.

Neat trick AE. Thanks for the tip.

So for the OP's 7" SEAS L18 woofer, that  beaming frequency would be 1237 Hz (assuming 7" is the actual diameter as described by AE. And general rule then becomes this: the larger the woofer the lower the beaming frequency. Here is a list of common woofer sizes (which may or may not equal the diameter needed for AE's calculation to work) and their beaming frequency based on the approximation given by AE above:

5"= 1732 Hz
6"= 1443 Hz
7"= 1237 Hz
8"= 1082 Hz
9"= 962 Hz
10"= 865 Hz
12"= 722 Hz
15"= 577 Hz

[Æ]

Neat trick AE. Thanks for the tip.

It's kind of an empirical figure, one I developed for myself, tested it out and it works fine for me.
Lets say you have a 6½" woofer with an actual cone diameter of 4½" (I do consider the minor part of the surround that is glued to the cone as part of the cone) 8658 divided by 4.5 = 1924Hz. Anything above that frequency is going to start to beam. As the frequency gets higher and higher, the beaming becomes more pronounced.

You're welcome.

P.S. I derived the number 8658 from 2 divided by (over) pi, x 13,600. I use 13,600 for the speed of sound in inches. If you perfer to use a slightly different speed of sound, OK, not much difference either way.

[roscoeiii]

So sounds like actual cone diameter will generally be less than the diameter a woofer is usually listed at (which would be measuring what?)

[Æ]

So sounds like actual cone diameter will generally be less than the diameter a woofer is usually listed at (which would be measuring what?)

Well, it kind of depends on the manufacturer and how they list their measurements. Thiele/Small parameters probably should, likely do include half of the surround when computing bass alignments. But with my personal formula which has nothing to do with Thiele/Small, I measure only the actual cone diameter, straight across.