Falsetto Register and Vowels

THIS IS IN DIRECT RESPONSE to a question from Jeremy Silver, singer and singing teacher. He wrote the following question: Why can one only sing pure falsetto on the /i/ or /u/ vowel? Is this because of vocal tract tuning? I have been having a debate with other voice teachers about this phenomenon. I have been taught and found that even in approximated falsetto (where there is still not TA activity) you cannot produce any vowels but /i/ or /u/. If you bring in a bit of vocalis/TA, then you can start to sound /a/ or /e/ or /o/. Is this correct? Also, Dr. Titze, is it possible to super-approximate the vocal cords while in pure falsetto so that you can get enough of the harmonic envelope to produce all the vowels clearly? It has been proposed to me that this is possible by the use of lateral cricoarytenoid (LCA) and interarytenoid (IA) muscles while keeping the larynx low. I don't believe it because I do not think you can keep the larynx low if you do this, and I'm not sure that you can produce all the vowels clearly without some TA involvement. You raise some fascinating questions that are at the forefront of voice research at the moment. The interaction between vocal fold movement, glottal airflow, and specific vowels is what has been called the nonlinear source-filter theory of vowel production. Generally speaking, vibration of the vocal folds can be strengthened by acoustic pressures in the vocal tract when one or more of the lower harmonies is slightly below a formant frequency (to the left of the formant if you are looking at a frequency spectrum). In this region, the vocal tract provides what acousticians call to a frequency produced at the source. This inertive reactance assists the vocal folds in their vibration, producing greater amplitude, more collision, and the characteristics of modal (chest) register. The vowels /i/ and /u/ both have a low first formant (F1 = 250-300 Hz). That means that any pitch above C^sub 4^ (261 Hz) is likely to have its fundamental (first harmonic) above the first formant. It will not be reinforced by the vocal tract. Relative to other pitch-vowel combinations, the vibrational amplitude of the vocal folds will be reduced, resulting in less collision and likely a falsetto registration. For this reason, pure falsetto is easy to produce with /i/ and /u/. If we change the vowel to /a/ or /ae/, the first formant rises and (for the same pitch) the vocal folds get more reinforcement from the vocal tract because the first harmonic is below F1. The larger amplitude and greater collision produces a chest-like quality. For this reason, yodelers will use the /i/-/ae/ and /u/-/a/ vowel sequences with pitch jumps like C^sub 4^ to A^sub 4^ (261 Hz to 440 Hz). This accentuates the registration, which is the hallmark of good yodeling. Your questions about muscle use in these cases are a bit more difficult to answer. …