Temporal Resolution Within the Upper Accessory Excitation of a Masker

Professor Eberhard Zwicker observed that the dynamic range of masking period patterns for amplitude-modulated maskers was larger for frequencies above the masker (in the upper "accessory excitation" where no physical masker energy exists) than for frequencies near the masker (in the "main excitation" where masker energy exists), which implies that temporal resolution of acoustic envelopes is substantially better within the upper accessory excitation than in the main excitation. His finding has significant implications for speech perception in fluctuating-masker environments, but unfortunately it has received little attention since his original report. The present research investigated temporal envelope resolution in normal-hearing subjects by measuring masking at the peaks and valleys of a 500 Hz 100% amplitude-modulated tonal masker as a function of test frequency and stimulus level. Modulation rate was 4 Hz. Masked thresholds were measured using amplitude-modulated signals with envelope peaks that corresponded with envelope peaks and valleys of the masker. The results confirmed Zwicker's earlier observation. Peak-to-valley masked-threshold ratios averaged 14.4 dB for signals within the main excitation, compared to average ratios as large as 33.4 dB within the upper accessory excitation. Slopes of the growth of masking support the interpretation that masking during envelope peaks is associated with simultaneous masking, for both the main excitation and the upper accessory excitation. Masking during envelope valleys appears to be determined by non-simultaneous masking in the upper accessory excitation, but behaves more like simultaneous masking in the main excitation of the masker.