THE ROLE OF FREQUENCY-MODULATION IN THE PERCEPTUAL SEGREGATION OF CONCURRENT VOWELS

Two experiments investigated the effect of frequency modulation on the identification of vowel sounds presented concurrently with interferin g vowels. In experiment 1, identification, thresholds were measured fo r each of five target vowels, masked, in each trial, by one of ten mas king vowels. Both target and masking vowels were synthesized using inh armonically spaced frequency components. Inharmonic spacing was used i n order to prevent powerful grouping processes which exploit fundament al frequency from dominating the results. The target vowels were synth esized with sinusoidal frequency modulation on each frequency componen t which was either coherent (same phase) or incoherent (random phases) . The masking vowels were synthesized with components which were eithe r modulated in the same way as the target vowel or were unmodulated. I dentification thresholds were lower when the masking vowel had no modu lation. The effect occurred for both coherent and incoherent frequency modulation, indicating that it is mediated by the movement of each co mponent independently, rather than by grouping of coherently modulated components. This result is consistent in some respects with judgments of the prominence of competing vowels [S. E. McAdams, J. Acoust. Soc. Am. 85, 2148-2159 (1989)], which show that modulated vowels are more prominent than unmodulated vowels regardless of the type of modulation applied to the competing vowels. Experiment 2 used a paradigm similar to that developed by McAdams, in order to compare more directly the e ffect of FM on vowel identification and vowel prominence. On each tria l, three vowels were presented concurrently. Either none, one, or two of the vowels were modulated throughout, while modulation was applied to another vowel (the target) halfway through the stimulus. The vowels were either harmonic (with different fundamental frequencies) and coh erently modulated or inharmonic and incoherently modulated. Accuracy o f identification of the target vowel was not significantly different i n the harmonic/coherent and inharmonic/incoherent conditions and decli ned, in each case, as the number of modulated background vowels increa sed. Overall, the results of experiments 1 and 2, and of McAdams' prom inence judgment data, suggest that there is an auditory mechanism for detecting frequency modulation which can alert the listener to the pre sence of frequency modulated sounds, but which is insensitive to acros s-frequency differences in the pattern of that modulation. (C) 1995 Ac oustical Society of America.