Vocal vibrato has a number of manifestations. Typically vibrato is first thought of as a variation in fundamental frequency (f_o), and secondarily as a variation in intensity and sound quality. Other variations that accompany these three types of vibrato include movement of throat structures (changes in larynx height, alterations in arytenoid adduction, pharyngeal wall movement, tongue movement, etc.), and especially muscle contraction variations that accompany all mentioned vibrato types.

Another vibrato manifestation is airflow variation. Airflow vibrato has received little attention in the past.  The reason may be because it apparently has no auditory perceptual impact. However, for a complete understanding of the production of vibrato, or any identifiable voicing gesture, one must study the aerodynamics of the phenomenon as well as the neuromuscular, physiological, acoustic, and perceptual.

Airflow vibrato is defined as the variation of the average airflow during vibrato singing. Those in the voice laboratory at Bowling Green State University have been interested in the study of airflow vibrato by asking questions such as (1) Is there airflow vibrato in singing? (2) What does airflow vibrato look like? (3) Does it occur at exactly the same time as fundamental frequency vibrato? (4) Is airflow vibrato rate the same as the f_o vibrato rate during the same utterance? (5) Are there more frequencies within an airflow vibrato cycle than in the f_o vibrato cycle? (6) What causes airflow vibrato? (7) How might knowing airflow vibrato help in training singing? (8) Is there an airflow vibrato expectation for experienced singing?

Our studies have been performed by students and colleagues Sri Nandamudi and Brooke Howell. Our findings so far include the consistent finding that (1) airflow vibrato rate is essentially the same as for f_o vibrato (see Figure 1), (2) airflow vibrato typically (but not always!) leads f_o vibrato but not by a specific phase amount (in Figure 1 it is a lead of 146°), and (3) airflow vibrato can vary nearly sinusoidally like f_o vibrato (in Figure 1 the signal is quite smooth, but also other frequencies of the airflow signal can be seen as the inconsistent “wriggles”), or have double or triple oscillations within one vibrato cycle (see Figure 2).

What is confusing is the lack of certainty relative to what creates airflow vibrato and why it typically leads f_o vibrato. We naturally would speculate that the variation of airflow during airflow vibrato depends on variations of the airflow resistance at the glottal level. We do not find vocal fold contact quotient per se to be different at the high points compared to the low points of the airflow vibrato cycle (Nandamudi, 2017), suggesting that variations in the anterior glottis may not be much involved. Because bleat creates glottal adduction variations at 2 to 3 times f_o vibrato rates, the presence of airflow vibrato variations of 2 to 3 times f_o vibrato rates may be related to posterior glottal adduction variation similar to that present for bleat (Nandamudi & Scherer, 2019). That is our current hypothesis requiring further experimentation.