If you take consider the wave equation on a disk $D$, then if we use Dirichlet boundary conditions, it means the wave function is fixed at $0$ on the boundary of the disk, and if we consider the eigenvalues of the system, this can be visualized physically as a vibrating drumhead.
If we use Neumann boundary conditions then the normal derivative of the wave function is fixed at $0$. How can we visualize this physically? The eigenvalues for the Dirichlet boundary conditions on a disk represented a vibrating drum. What physical phenomenon do the eigenvalues for the Neumann boundary conditions on a disk represent?