Keldryn
Adventurer
Oops. Didn't go back and read it carefully...Thunderwave creates a horizontal line (or wall) of force, 15' long (and 15' high), which then travels 15' away from you, hitting and pushing any creatures it strikes. Note that you're not at the centre of the cube, you're at the edge. The force doesn't radiate from the centre of the cube.
Cheers!