Fourth-order distortion products are nearly inaudible and appear only above 85-90 dB.

Evidence

Distortions of the fourth order are almost inaudible. They can be produced and cancelled by adding the additional tone only at levels above 85 to 90 dB. In this region, the stapedius muscle is already active and may strongly influence the results. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1551