MASKING BY SINUSOIDALLY AMPLITUDE-MODULATED TONAL MASKERS

In experiment 1, masking patterns were obtained with a tonal masker th at was sinusoidally amplitude modulated (SAM) at a rate of 8 Hz and a depth (m) of 1.0. The signal was centered at a masker peak or masker v alley. Masker frequency (f(m),,,) was 750, 1350, or 2430 Hz, and signa l frequency (f(s),) ranged from 0.8 to 1.62f(m),,. Thresholds were gen erally higher for a signal in a masker peak than in a masker valley. T he magnitude of this peak-to-valley (PV) difference was governed by f( s)/f(m),, rather than by f(s),, and was largest for f,(s)>f(m).,,. The PV differences were smallest at the lowest f(m),, at least when f,(s) >f(m).,,. Ln experiment 2, growth-of-masking functions were measured ( f(m),=1350 Hz, f(s)=1.44f(m),). The masker was modulated at a depth (m ) of 1.0, 0.75, or 0.50. These thresholds were compared with those obt ained with an unmodulated masker in forward or simultaneous masking. T he comparisons suggest that thresholds for a signal at a peak of an 8- Hz SAM masker are due to simultaneous masking, while those in a valley are due primarily to forward masking when m=1.0 or simultaneous maski ng when m=0.75 or 0.50. For these masker depths, the PV difference fir st increased but then decreased as masker level increased from 60 to 9 0 dB SPL. This was a consequence of the slope of the masking function for peak placement changing from a value, greater than 2.0 to a value of 1.0 at the highest signal levels (an effect that was also observed with the unmodulated simultaneous masker), a result that may be unders tood in terms of basilar membrane nonlinearity. (C) 1998 Acoustical So ciety of America. [S0001-4966(98)05202-3].