Effect of frequency-modulation coherence for inharmonic stimuli: Frequency-modulation phase discrimination and identification of artificial double vowels

Abstract

The ability to compare patterns of frequency modulation (FM) in separate frequency regions was explored. In experiment 1, listeners had to distinguish whether the FM applied to two nonharmonically related sinusoidal carriers was in phase or out of phase. The FM rate was the same for each carrier. The starting phase of the modulation was randomized for each stimulus in a three alternative, forced-choice (3AFC) trial. Subjects were sensitive to relative FM phase for modulation rates of 2 and 4 Hz, but not for higher rates. In experiment 2, vowel identification was compared for artificial single and double vowels. The vowels were constructed from complex tones with components spaced at 2-ERB(N) (equivalent rectangular bandwidth) intervals, by increasing the levels of three components by 15 dB, to create three "formants." In the double vowels, the components of the two vowels were interleaved, to give 1-ERB(N) spacing. The three "formant" components were frequency modulated at 2, 4, or 8 Hz, with either the same or different rates for the two vowels. The identification of double vowels was not improved by a difference in FM rate across vowels, suggesting that differences in FM rate do not support perceptual segregation of inharmonic stimuli.