Five tones replicate narrow-band noise masking while single tones distort it

Evidence

The data displayed in Fig. 4.11 show very clearly that a single masker tone is an inappropriate approximation of a narrow-band noise masker. Two tones produce masking effects relatively close to those produced by narrow-band noise, provided the distance between the two tones is chosen in such a way that the tone frequencies correspond closely to the lower and higher cut-off frequency of the narrow-band noise. However, there still remain differences of up to 7 dB between the two masking curves. An approximation of the narrow-band noise by five tones produces an almost identical masking curve. The remaining differences are within the accuracy of measurement. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 458