I kind of had the same "Dubya tee ef, mate?" reaction to the thumper tower . . . no little pop up to tell me a bit about it.

I still can't decide if I like it or not -- it would've been nice to have a little bit of an explanation.  At the same time, you're more likely to learn how the tower best suits your style of play by figuring it all out as you go.

In the meantime . . . here's what David had to say on GDS tower changes and additions in the IGN interview:

"The Thump Tower, which emits shockwaves of damage against anything that strays too close. The Vortex Towers (the black holes) are a little different this time in that you don't ever buy them... they are pre-positioned on some maps for you."

I would imagine that upgrading the thumps increases their damage, rate of fire, and splash area; however, I havent spent much time using them in game yet.  Also, keep in mind that upgrading to level seven on almost any tower in the game is not always the best thing to do depending on what you're trying to accomplish with the tower.  The level seven jump often creates a drastic change in firing rate and/or range compared to its level six counterpart, and can sometimes hinder your efforts.

Lastly . . . here are some grabs from geoDefense to give you an idea of tower basics.  I do HIGHLY recommend the previous suggestion of downloading geoDefense Lite (or why not just buy the real thing -- especially with the update coming out soon) to practice the tower mechanics.