THE ROLE OF SPREAD EXCITATION AND SUPPRESSION IN SIMULTANEOUS MASKING

This experiment was intended to clarify the relative role of spread of excitation and suppression in simultaneous masking, for masker freque ncies just below and well below the signal frequency. The experiment h ad two stages. In stage 1, growth-of-masking functions were measured i n simultaneous masking for a 2200-Hz sinusoidal signal and a sinusoida l masker with frequency of either 1800 Hz or 500 Hz. Straight lines fi tted to these data were used to determine masker levels that would giv e 10, 20, and 30 dB of masking. In stage 2, thresholds for detecting a brief 2200-Hz signal were measured using forward masking. It was reas oned that the threshold of the signal would give an indication of the amount of excitation evoked by the masker in the frequency region of t he signal. Three forward maskers were used: (1) a 2200-Hz sinusoid at 10, 20, or 30 dB sensation level (SL); (2) a 2200-Hz sinusoid at the s ame levels as in (I) together with a sinusoid with frequency 500 or 18 00 Hz at a level just sufficient to mask the 2200-Hz sinusoid. We refe r to this as the ''combined masker,'' (3) a 500-Hz or 1800-Hz sinusoid at the same levels as in (2) above. The 1800-Hz combined masker produ ced slightly less forward masking than the 2200-Hz masker (1), which m ight be explained in terms of suppression or as perceptual cueing. Bot h the 1800-Hz combined masker and the 1800-Hz component alone (3) gave significant amounts of forward masking (up to 18 dB), indicating that these maskers produced substantial excitation at 2200 Hz. This is con sistent with the idea that the simultaneous masking of the 2200-Hz com ponent in stage 1 was produced by spread of excitation rather than by suppression. The 500-Hz combined masker produced much less forward mas king than the 2200-Hz component alone, indicating strong suppression o f the 2200-Hz component of the combined masker by the 500-Hz component . However, both the 500-Hz combined masker and the 500-Hz component al one produced some forward masking. This is not consistent with the ide a that masking of the 2200-Hz component in stage 1 (simultaneous maski ng) was produced solely by suppression. (C) 1997 Acoustical Society of America. [S0001-4966(97)01710-4] PACS numbers: 43.66.Dc, 43.66.Mk, 43 .66.Ba [JWH].