Masking of tones and speech by Schroeder-phase harmonic complexes in normally hearing and hearing-impaired listeners

Tone detection and sentence recognition were measured for normally hearing and hearing-impaired listeners using maskers consisting of harmonic series with components summed in positive or negative Schroeder phase. Each task was carried out with the signal set at 60, 70, or 80 dB SPL. For listeners with normal hearing, positive Schroeder-phase complexes masked tones and sentences less than negative Schroeder-phase maskers. In the two experimental tasks, to achieve the same amount of masking, positive Schroeder-phase complexes had to be set as much as 12-15 dB higher than negative Schroeder-phase complexes. Large phase effects were observed on both tasks at all three test levels. The two maskers were more nearly equal in effectiveness in the presence of cochlear damage. The findings support an interpretation that involves differences in the shape of the basilar-membrane waveform generated by each masker and active cochlear processing which enhances the internal signal-to-masker ratio for signals presented in the positive Schroeder masker. This spectral enhancement appears to require nonlinear active gain that is characteristic of normal auditory processing at moderate presentation levels. The results of the sentence recognition task suggest that group differences observed in tone detection transfer fairly directly to speech perception under masking.