Hearing-system quadratic distortion is largely independent of frequency separation between primaries.

Evidence

The fact that the two parts of Fig. 14.2 can he described well by the same set of dashed lines indicates that, in this case, the nonlinearity is independent of frequency distance. This means that the transmission characteristic of the hearing system of such a subject differs relatively little from regularity. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1535