Check for the dispersion relation of sound waves. Upper panel: amplitudes of the radial velocity at two different frequencies for ℓ = 0. Circles mark radii where our routine detects peaks in the amplitude. The radial wavelengths are then estimated from the distance of neighboring peaks. For the blue line, the frequency matches a standing mode and it shows well defined, distinct peaks. The red line corresponds to a frequency which is between standing modes and shows small-scale incoherent oscillations with many peaks. Lower panel: resulting wave numbers from the simulation are compared to the expectation for sound waves k_theo = 2πf/c_sound. We find the blue line in good agreement with theory for radii above the convection zone while the red line considerably differs.