Narrow critical bandwidths at low frequencies reduce hearing sensitivity via noise fluctuations

Evidence

This masking index amounts to about - 2 dB at low frequencies and decreases at high frequencies to -6dB. The masking index, av , is outlined in the upper diagram of Fig. 6.17 on an enlarged scale. The value of -6 dB for the masking index is the value that would be expexted from the model describing justnoticeable differences in Sect. 7.5.1. This value, however, is reached only at high frequencies where the critical bandwidth is so large that the fluctuation of the noise is not audible. Parts of the noise that are assumed to correspond to the frequency selectivity of our hearing system as being critical-band wide, are effective in that frequency range in a way similar to stationary sounds without fluctuations. At low frequencies, however, the critical-band width is only 100 Hz; at this bandwidth, strong fluctuations of the noise falling into the critical band are produced. These fluctuations reduce the sensitivity of our hearing system for the test tones. Consequently, the masking index, av , is increased to -2 dB. The dependence of critical bandwidth on frequency is the reason that the masking index, av , depends on frequency and that the density level of uniform exciting noise does not show the same frequency dependence as the density level of uniform masking noise. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 990

Categorization

• DEFINE: Explains what the masking index (av) is and why it shifts from -2 dB to -6 dB.
• XREF: Connects to the reader’s psychoacoustics reading on critical bands and just-noticeable-difference models.
• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 990