MembraneCircle and MembraneHexagon are rather similar, being triangular waveguide meshes of a drum-like membrane. You input some excitation, such as a pulse of noise, and can adjust the tension and loss while it plays.
They're named after the shape made out of triangular meshes. Obviously you can't make a circle out of triangles, but it tries. At the moment MembraneCircle is a bit bigger than MembraneHexagon, using more waveguides and therefore more CPU.
These UGens are by Alex McLean (c) 2008.
My stuff is here:
I also recommend the following thesis:
and these websites: