Pitches of concurrent vowels

When two vowels are presented simultaneously, listeners can report their phonemic identities more accurately if their fundamental frequencies (F0's) are different rather than the same. If the F0 difference (delta F0) is large, listeners hear two vowels on different pitches; if the delta F0 is small the vowels are identified less accurately and they do not evoke different pitches. The present study used a matching task to obtain judgments of the pitches evoked by "double vowels" created from pairwise combinations of steady-state synthetic vowels /i/, /a/, /u/, /ae/, and /[symbol: see text]/. One F0 was always 100 Hz; the other F0 was either 0, 0.25, 0.5, 1, 2, or 4 semitones higher. Experienced listeners adjusted the F0 of a tone complex to assign pitch matches to 50-ms or 200-ms double vowels. For delta F0's up to two semitones, listeners' matches formed a single cluster in the frequency region spanned by the two F0's. When the delta F0 was 4 semitones, the matches generally formed two clusters close to the F0 of each vowel, suggesting that listeners perceive two distinct pitches when the delta F0 is 4 semitones but only one clear pitch (possibly accompanied by one or more weaker pitches) with smaller delta F0's. When the duration was reduced from 200 ms to 50 ms, only a subset of the vowel pairs with a delta F0 of 4 semitones produced a bimodal distribution of matches. In general, 50-ms stimuli were matched less consistently than their 200-ms counterparts, indicating that the pitches of concurrent vowels emerge less clearly when the stimuli are brief. Comparisons of pitch and vowel identification data revealed a moderate correlation between match intervals (defined as the absolute frequency difference between first and second pitch matches) and identification accuracy for the 200-ms stimuli with the largest delta F0 of 4 semitones. The link between match intervals and vowel identification was weak or absent in conditions where the stimuli evoked only one pitch.