18DD round

What makes an 18DD go down to a greater depth than the 11DD let's say? I understand when it's a mono, the cone is larger with the bigger coil, hence more depth, but with a DD the pulse is sent down the center, so what determines the depth that the coil will go down to? And for that matter, if the center is the sensitive area, is there any depth difference between using an 18DD and an 18x9DD elliptical (for example) as both have the "sensitive legth" in the coil (both being 18)?

Depth appears to be based on diameter, for example, a 14" round coil goes deeper than a 14"x9", no official results published that I"m aware of.

hi CostasDee
18" round coil has a bigger send and recieve loops than a 11" round coil (one side sends the other recieves)
cheers