Temporal resolution within the upper ‘‘accessory excitation’’ region for maskers with fluctuating envelopes.

Abstract

During Zwicker’s numerous investigations of the time structure of masking, he observed that envelope masking period patterns were larger for frequencies above the masker (in the upper ‘‘accessory excitation’’ region) than for frequencies near the masker (in the ‘‘main excitation’’ region). For example, the peak-to-trough ratio of the masking period pattern for a 4-Hz 100% amplitude-modulated masker was only 12 dB when the probe tone was within the main excitation region, at 1000 Hz, while the ratio was as large as 36 dB when the probe tone was within the accessory excitation region, at 3200 Hz [E. Zwicker, Acustica 36, 113–120 (1976)]. In the present research, masking at the peak and within the trough of 500-Hz amplitude-modulated maskers was investigated as a function of test frequency and masker level for modulation frequencies from 4 to 128 Hz in normal-hearing and hearing-impaired ears. Peak and trough masked thresholds were measured using amplitude-modulated signals with phasic and antiphasic envelopes. The results from normal-hearing ears confirm Zwicker’s finding that temporal resolution can be up to three times better in the upper accessory excitation region than in the main excitation region. Masking during envelope peaks remains constant while masking during envelope troughs increases with modulation frequency. This finding, and the slopes of the growth of masking, support Zwicker’s interpretation that masking during the envelope peaks and troughs are associated with simultaneous and nonsimultaneous masking, respectively. Peak-to-trough ratios in the upper accessory region are considerably reduced in hearing-impaired ears, which has important implications for processing speech signals in the presence of maskers with fluctuating envelopes. [Work supported by NIDCD.]