Masking by harmonic complexes in budgerigars (Melopsittacus undulatus)

In humans, masking by harmonic complexes is dependent not only on the frequ ency content of the masker, but also its phase spectrum. Complexes that hav e highly modulated temporal waveforms due to the selection of their compone nt phases usually provide less masking than those with flatter temporal env elopes. Moreover, harmonic complexes that are created with negative Schroed er phases (component phases monotonically decreasing with increasing harmon ic frequency) may provide more masking than those created with positive Sch roeder phases (monotonically increasing phase), even though both temporal e nvelopes are equally flat. To date, there has been little comparative work on the masking effectiveness of harmonic complexes. Using operant condition ing and the method of constant stimuli, masking of pure tones by harmonic c omplexes was examined in budgerigars at several different masker levels for complexes constructed with two different fundamental frequencies. In contr ast to humans, thresholds in budgerigars differed very little for the two S chroeder-phase waveforms. Moreover, when there was a difference in masking by these two waveforms, the positive Schroeder was the more effective maske r-the reverse of that described for humans. Control experiments showed that phase selection was relevant to the masking ability of harmonic complexes in budgerigars. Release from masking occurred when the components were in c oherent phase, compared with a complex with random phases selected for each component. It is suggested that these psychoacoustic differences may emerg e from structural and functional differences between the avian and mammalia n peripheral auditory systems involving traveling wave mechanics and spectr al tuning characteristics. (C) 2000 Acoustical Society of America. [S0001-4 966(00)06203-8].