What’s so difficult with singers’ difficult vowels?

Objective: Many pedagogues/teachers of singing claim that there are vowels that may be more beneficial in the training of classical singers. Several reasons may be involved the most researched one being formant tuning. A previous investigation showed that there are substantial differences in voice source parameters between vowels; these seem not to be related to the frequency distance between the first resonance of the vocal tract and its closest partial. Here, we investigate expert perceptions of difficult vowel production within a sequence and their possible voice source related features.

Method: Nine male professional and classically trained singers sang sequences of the vowels /i, e, a, o, u/ at different pitches. Eight teachers of classical singing identified vowels that sounded difficult to produce, henceforth Difficult. Each vowel within the sequence was analyzed with respect to tuning, vibrato and, by means of inverse filtering, voice source parameters. Also, for each sequence, the mean was determined for each of the respective associated parameters. For comparisons between parameters, all values were converted to z-scores. For each parameter the vowels showing the greatest deviations from the sequence mean, henceforth Extreme Cases, were identified.

Results: Preliminary results suggest that among the vowels, /u/ was most often identified as Difficult, followed by /i/. In almost all cases, a vowel classified as Difficult by most expert listeners was an Extreme Case in one or several voice source parameters, particularly often for MFDR but also for tuning and vibrato.

Discussion. Understanding the underlying variability of voice source parameters is crucial to the choice of vowel in voice training. Unexpectedly, the closed vowels /u/ and /i/ were more often perceived as Difficult than the remaining vowels and showed the greatest deviations from the mean across the sequence. Closed vowels should offer a more resistive vocal tract and thus could be expected to offer greater phonatory stability than open vowels. Additionally, prominent pedagogues have highlighted the /a/ vowel as being the most challenging. Our findings suggest that these ideas need to be contextualized to the level of voice proficiency.

Conclusions: The Difficult vowel ratings were most often found to be associated with Extreme cases regarding deviations from the sequence mean for different voice parameters. Of these, deviating MFDR was most often perceived as Difficult.