Punita G. Singh
Albert S. Bregman
Dept. of Psychol., McGill Univ., Montreal, PQ H3A 1B1, Canada
Perceptual judgments of the presence or absence of tones embedded in modulated noise maskers were obtained using a six-point rating scale (1=clearly absent, 6=clearly present). The maskers comprised four simultaneous noise bands centered at 500, 1000, 1500, and 2000 Hz, with bandwidths of all bands either 12.5, 25, 50, or 100 Hz. The bands were derived either from the same noise source by filtering and transposition, or from different sources, and were subsequently multiplied by same or different (phase shifted) modulators. A sine tone at the center frequency of one of the noise bands was used as the signal. A single noise interval was preceded by a cue tone that was either present or absent in the noise at a fixed S/N(approximately equal to)-13 dB SPL. The cue and noise intervals were both 400 ms long, and separated by 1 s of silence. Rating scores from ten normally hearing listeners were generally higher for tones embedded in maskers with common envelopes, indicating less masking and improved perceptual clarity of the tones. Common origin of noise source did not lead to improved audibility of the tones. There was no significant effect of tone frequency. However, bandwidth affected judgments significantly. For the 12.5- and 25-Hz bandwidths, the difference between modulation conditions was reduced, while false alarm rate increased substantially. These narrow-band noises apparently sounded tonal even in the absence of the signal, thereby receiving higher ratings. For the 50- and 100-Hz-wide maskers, the effect of modulation pattern was enhanced, and there were fewer false alarms. Similarity of amplitude envelopes within a masker has been invoked as a simultaneous grouping cue facilitating segregation of a concurrent signal. It seems that sequential grouping factors such as similarity of perceptual attributes (e.g., pitch or timbre) of the cue and masker also play a role in signal detection. [Work supported by NSERC.]