The allowed vibration modes are identified by the lowercase letter u followed by two quantum numbers. The first quantum number designates the radial symmetry, identifying the number of nodes (minus 1) counted out from the center. The second designates the azimuthal symmetry, identifying the number of nodes (minus 1) counted around the perimeter. Thus, the u1,1 mode has two radial nodes (one at the center, and one at the edge), and two azimuthal modes (one at each end of a diametrical neutral stripe.)
These paintings depict a shapshot of a membrane disturbed by one of these vibration modes. A background color signifyies the neutral plane of the undisturbed membrane. Relatively warmer colors identify areas where the membrane has moved above the neutral plane, while cooler colors signify movement below that plane.