Internal nonlinear distortion products, not poorer frequency selectivity, explain noise maskers’ extra low-frequency masking

Evidence

The frequencies of the five-tone complex are indicated by vertical lines. The threshold of a pure tone masked by the five-tone complex is given as a solid curve. A comparison between frequency and level of the difference tones on the one hand, and the masked threshold on the other, suggests that the masked threshold in the frequency range of 1300 to 1700 Hz is due to the difference tones which also produce masking. Therefore, it can be assumed that the frequency selectivity of the ear remains the same, irrespective of whether a narrow-band noise or a tone is used as the masker. However, the internally produced nonlinear components, either difference tones or difference noises (in the case of narrow-band noise as the masker) change the physical stimulus into an internal stimulus which is broader and therefore produces more masking at the low-frequency side of the masker. At the high-frequency side, this effect does not appear to playa role because masking is already spread much more towards higher frequencies. Therefore, it can be assumed that the frequency selectivity measured with a pure tone as the masker, although level dependent, is the largest that is possible. Masking produced by a narrow-band noise is somewhat less selective at the low-frequency side. Due to the appearance of distortion products (in this case continuous spectra) it is almost level independent. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 463

Categorization

• DEFINE: Clarifies the mechanism by which internal difference tones and difference noises widen the effective internal stimulus for noise versus tone maskers.