((1)) This study was realized in partial fulfillment of the requirements for C.M.H. Marin's DEA diploma
at the Université Paris III (Marin, 1987). The original thesis in French may be obtained by writing to
C.M.H. Marin at IRCAM.
((2)) Requests for reprints should be addressed to S. McAdams at the Laboratoire de Psychologie
((3)) The research presented here concerns the simultaneous grouping of the components of a vocal sound
source. McAdams [J. Acoust. Soc. Am. 86, 2148-2159 (1989)] found that when three simultaneous
vowels at different pitches were presented with sub-audio frequency modulation, subjects judged
them as being more prominent than when no vibrato was present. In a normal voice, when the
harmonics of a vowel undergo frequency modulation they also undergo an amplitude modulation
which traces the spectral envelope. Our hypothesis was that this spectral tracing could be one of the
criteria used by the ear to group components of each vowel, which may help explain the lack of effect
of frequency modulation coherence among different vowels in the previous study. In this experiment,
two types of vowel synthesis were used in which the component amplitudes of each vowel either
remained constant with frequency modulation or traced the spectral envelope. The stimuli for the
experiment were chords of three different vowels at pitch intervals of five semitones (ratio 1.33). All
the vowels of a given stimulus were produced by the same synthesis method. The subjects' task
involved rating the prominence of each vowel in the stimulus. It was assumed that subjects would
judge this prominence to be lower when they were not able to distinguish the vowel from the
background sound. Also included as stimulus parameters were the different permutations of the three
vowels at three pitches and a number of modulation conditions in which vowels were unmodulated,
modulated alone, and modulated either coherently with, or independently of, the other vowels.
Spectral tracing did not result in increased ratings of vowel prominence compared to stimuli where no
spectral tracing was present. It would therefore seem that it has no effect on grouping components of
sound sources. Modulated vowels received higher prominence ratings than unmodulated vowels.
Vowels modulated alone were judged to be more prominent than vowels modulated with other
vowels. There was, however, no significant difference between coherent and independent modulation
of the three vowels. Differences among modulation conditions were more marked when the
modulation width was 6% than when it was 3%.