The role of envelope fluctuations in an apparent demonstration of suppression in simultaneous masking

Fasti and Bechly [J. Acoust. Soc. Am. 74, 754-757 (1983)] reported that the threshold of a brief 900-Hz signal simultaneously masked by a band of noise, 100 Hz wide, centered at 1000 Hz, was reduced by approximately 8 dB by the addition of an 1150-Hz tone having a level of 20 dB above that of the narrow-band masker. They concluded that this decrease in threshold was a demonstration of suppression in simultaneous masking. Here it is argued that Fasti and Bechly's results simply reflect the poorer detectability of signals masked by higher-frequency fluctuating maskers (their narrow-band masker) than by relatively flat-envelope maskers (their composite narrow-band plus tonal masker). The results of three experiments support the masker-envelope explanation. In the first experiment, as in the report of Fasti and Bechly, the masker centered at 1000 Hz (Ml) was a narrow-band noise and the masker centered at 1150 Hz (M2) was a tone. Fasti and Bechly's result was replicated. However, thresholds obtained when Ml was presented alone (the Ml-only condition) were more affected by the starting level of the signal within each adaptive track than were thresholds obtained when Ml and M2 were presented together (the Ml + M2 condition). This result paralleled a previous report that starting level influenced performance more with fluctuating than with flat-envelope maskers. For the four of seven subjects who showed learning, there was also more improvement in the Ml-only than in the Ml + M2 condition. In the second experiment, Ml was a tone and M2 was a narrow-band noise. Thresholds were higher in the Ml + M2 than in the Ml-only condition, presumably due to the increased masker fluctuations in the Ml + M2 condition. Finally, in the third experiment, there was little or no difference in threshold in the Ml-only and Ml + M2 conditions for the majority of subjects when Ml and M2 were both tones. Caution is advised in interpreting a release from masking as suppression. The failure to demonstrate suppression in simultaneous masking for the majority of subjects, using an off-frequency signal and stimuli relatively free of the confounding influence of envelope fluctuations, is in contrast to a prediction that suppression in simultaneous masking should be revealed if the masker and signal have different frequencies and thus are not equally influenced by the suppression effect.