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 hea ring and hearing-impaired listeners using maskers consisting of harmon ic series with components summed in positive or negative Schroeder pha se. Each task was carried out with the signal set at 60, 70, or 80 dB SPL. For listeners with normal hearing, positive Schroeder-phase compl exes masked tones and sentences less than negative Schroeder-phase mas kers. Ln the two experimental tasks, to achieve the same amount of mas king, positive Schroeder-phase complexes had to be set as much as 12-1 5 dB higher than negative Schroeder-phase complexes. Large phase effec ts were observed on both tasks at all three test levels. The two maske rs were more nearly equal in effectiveness in the presence of cochlear damage. The findings support an interpretation that involves differen ces in the shape of the basilar-membrane waveform generated by each ma sker and active cochlear processing which enhances the internal signal -to-masker ratio for signals presented in the positive Schroeder maske r. This spectral enhancement appears to require nonlinear active gain that is characteristic of normal auditory processing at moderate prese ntation levels. The results of the sentence recognition task suggest t hat group differences observed in tone detection transfer fairly direc tly to speech perception under masking. (C) 1998 Published by Elsevier Science B.V.