Zwicker, U.T. (1984). Auditory recognition of diotic and dichotic vowel pairs. Speech Commun. 3, 265-277
Ease of recognition of two simultaneous vowels was investigated for vowels (1) presented either diotically or dichotically, and (2) with identical or different fundamental frequencies. Recognition rates of pairs of vowels with identical fundamental frequencies were typically slightly above 60%, and rose to nearly 80% when the fundamentals of the vowels were different. The results of Scheffers for the diotic condition were confirmed. Usually the observers reportedone vowel in a pair to be dominant and easier to recognize. Thus, the simultaneous recognition of two diotic vowels may involve the comparison of their combined spectral pitch pattern with spectral patterns of vowels stored in memory. As part of this process the pattern corresponding to the dominant vowel of the pair may somehow be substracted from the combined sound in order to aid recognition of the other vowel. Recognition rates in the dichotic condition were generally higher than in the diotic condition, but the size of this difference varied markedly from observer to observer. For some observers the difference was insignificant. Additional experiments incorporated the simultaneous presentation of masking noise with the vowels, under three conditions of interaural phase correlation. Vowel recognition was better when subjects were able to laterally (spatially) separate the noise from the vowels, and best when the two vowels and the noise could be lateralized to three different spatial positions. Ease of lateralization depended in turn on the kind of interaural correlation of the noise, and whether the vowels were presented dichotically or diotically.