Plack C J, Carlyon R P
Laboratory of Experimental Psychology, University of Sussex, Brighton, England.
J Acoust Soc Am. 1994 Jul;96(1):115-25. doi: 10.1121/1.410472.
Thresholds were measured for the detection of differences in the depth of 5-Hz frequency modulation (FM). In the first experiment, listeners detected differences between sequentially presented sinusoidal carriers. The Weber fractions for FM depth decreased from about 0.5 to about 0.3 as the baseline depth was increased from 2.5% to 20%, and were slightly higher for a carrier frequency of 0.5 kHz compared to carrier frequencies of 1, 2, and 4 kHz. In the second experiment, complex carriers were used consisting of consecutive harmonics of 125- and 250-Hz fundamentals (f0's), bandpass filtered between 1375 and 1875 Hz. Performance was worse with these stimuli than with the sinusoidal carriers: The Weber fractions for the 250-Hz f0 ranged from about 0.4 to about 2.0 across listeners, and were roughly invariant with baseline depth. The Weber fractions for the 125-Hz f0 showed a steady decrease from about 3.2 to about 0.6 as the baseline depth was increased from 2.5% to 20%, so that threshold corresponded, approximately, to a constant increase in FM depth, independent of baseline depth. The absolute detectability of the FM may have been a limiting factor for the lower two baseline depths at this f0. In the final experiment, psychometric functions were measured for the detection of simultaneous across-frequency differences in FM depth. Three conditions were tested; in the first of these the two carriers to be compared were 666- and 1500-Hz pure tones. In the second condition the two carriers were complex tones, both with f0's of 250 Hz, filtered between 125 and 625 Hz and between 1375 and 1875 Hz, respectively. The third condition was similar, except that the two (complex) carriers had different f0's of 111 and 250 Hz. In the first and third conditions performance was extremely poor, even when the FM depths of the two carriers to be discriminated were 10% and 50%. Listeners did perform substantially better, however, on the second condition. The implications of these results for the idea that listeners use differences in FM depth to perceptually segregate concurrent sounds are discussed.
