Suppression and the upward spread of masking

The purpose of this study is to clarify the role of suppression in the grow th of masking when a signal is well above the masker in frequency (upward s pread of masking);. Classical psychophysical models assume that masking is primarily due to the spread of masker excitation, and that the nonlinear up ward spread of masking reflects a differential growth in excitation between the masker and the signal at the signal frequency. In contrast, recent phy siological studies have indicated that upward spread of masking in the audi tory nerve is due to the increasing effect of Suppression with increasing m asker level. This study compares thresholds for signals between 2.4 and 5.6 kHz in simultaneous and nonsimultaneous masking for conditions in which th e masker is either at or well below the signal frequency. Maximum differenc es between simultaneous and nonsimultaneous masking were small (<6 dB) for the on-frequency conditions but larger for the off-frequency conditions (15 -32 dB). The results suggest that suppression plays a major role in determi ning thresholds at high masker levels, when the masker is well below the si gnal in frequency. This is consistent with the conclusions of physiological studies. However, for signal levels higher than about 40 dB SPL, the growt h of masking for signals above the masker frequency is nonlinear even in th e nonsimultaneous-masking conditions, where suppression is not expected. Th is is consistent with an explanation based on the compressive response of t he basilar membrane, and confirms that suppression is not necessary for non linear upward spread of masking. (C) 1998 Acoustical Society of America. [S 0001-4966(98)03812-0].