White noise masking of tones is linear but frequency-dependent above 500 Hz

Evidence

Although white noise has a frequency-independent spectral density, the masked thresholds, indicated by solid lines, are horizontal only at low frequencies. Above about 500 Hz, the masked thresholds rise with increasing frequency. The slope of this increase corresponds to about 10 dB per decade, illustrated by the dotted line. At low frequencies, the masked thresholds lie about 17 dB above the given density level. Thus numbers representing the values of spectral density, lWN, indicate that even negative values of the density level produce masking. Increasing the density level by 10 dB shifts the masked threshold upwards by the same 10 dB. This interesting result indicates the linear behaviour of masking produced by broad-band noises. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 407