The masking index varies with frequency, distinguishing UEN from uniform masking noise.

Evidence

The difference between uniform exciting noise and uniform masking noise arises because the masking index, av , is different at low and high frequencies. Because this effect strongly influences the frequency difference of the critical bandwidth as defined by Fletcher, it may be discussed here in some detail. Let us start with uniform exciting noise with a critical-band level LG = 40 dB. The overall level of this noise, which includes 24 critical bands, is therefore (40 + 10 . log 24) dB = 54 dB. This uniform exciting noise will be used as masker. The threshold of sinusoidal tones masked by this noise is indicated in the lower part of Fig. 6.17 as L T , the level of the sinusoidal tone necessary to be just-audible. The difference between the critical-band level, L G , and masked threshold, LT, represents the masking index HiBt~ • ---=:’-:----=’: ,~~~~l 0. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 984

Categorization

• DEFINE: Defines the masking index as the difference between critical-band level and masked threshold, and explains why frequency dependence of that index separates UEN from UMN.