Excitation slopes are steeper toward lower frequencies than higher ones

Evidence

For uniform exciting noise, only main excitations occur, and hence we have the same figure as on the left. For the narrow band noise, however, we have to introduce the slopes of excitation, which are quite steep towards lower critical-band rates corresponding to lower frequencies but flatter towards higher critical-band rates. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1255