Four frequency regimes present in the spectra. Region I is the lower cut-off due to the acoustic cut-off frequency (below 4 mHz). Region II is the broad peak (4−17 mHz). Region III is the gradient decline (17−200 mHz). Region IV is the upper cut-off (above 200 mHz). This spectrum was generated for a flat chromospheric profile of depth 1.8 Mm with a white noise velocity source. The line indicates the gradient of the power in region III, which is –1.15 in this case.