This is one of my favorite demonstrations of resonance in an "enclosed space." The space is, of course, the plate. The lines are formed where the plate is NOT moving, so the salt/sand is moved away from the moving portions of the plate to the stationary locations. As the frequency changes and the associate wavelengths change (determined by the mass and elasticity of the plate material), the patterns move toward the next resonance. Since this is a 2D "room," the more complicated modal patterns in a 3D room are not present, but there is plenty of complexity in 2D for us to enjoy.