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 interfering vowels. In experiment 1, identification thresholds were measured for each of five target vowels, masked, in each trial, by one of ten masking vowels. Both target and masking vowels were synthesized using harmonically spaced frequency components. Inharmonic spacing was used in order to prevent powerful grouping processes which exploit fundamental frequency from dominating the results. The target vowels were synthesized with sinusoidal frequency modulation on each frequency component which was either coherent (same phase) or incoherent (random phases). The masking vowels were synthesized with components which were either modulated in the same way as the target vowel or were unmodulated. Identification thresholds were lower when the masking vowel had no modulation. The effect occurred for both coherent and incoherent frequency modulation, indicating that it is mediated by the movement of each component 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 effect of FM on vowel identification and vowel prominence. On each trial, 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 coherently modulated or inharmonic and incoherently modulated. Accuracy of identification of the target vowel was not significantly different in the harmonic/coherent and inharmonic/incoherent conditions and declined, in each case, as the number of modulated background vowels increased. Overall, the results of experiments 1 and 2, and of McAdams' prominence judgment data, suggest that there is an auditory mechanism for detecting frequency modulation which can alert the listener to the presence of frequency modulated sounds, but which is insensitive to across-frequency differences in the pattern of that modulation.