Modal power spectrum of the vertical velocity component extracted at a height of z = 200 km above the photosphere, from a 12 h long simulation with C-PMLs placed at the upper and lower boundaries(plotted on a linear scale). The horizontal axis is the normalized wavenumber, the vertical is frequency, and regions of high power are the normal modes that appear in the calculation. The symbols overplotted on the power contours are the theoretically expected values of the resonant frequencies, computed using the MATLAB boundary value problem solver bvp4c.