The physiology of oral whistling: a combined radiographic and MRI analysis

Abstract

The fluid mechanics of whistling involve the instability of an air jet, resultant vortex rings, and the interaction of these rings with rigid boundaries (see http://www.canal-u.tv/video/cerimes/etude_radiocinematographique_d_un_siffleur_turc_de_kuskoy.13056 and Meyer J. Whistled Languages. Berlin, Germany: Springer, 2015, p. 74-774). Experimental models support the hypothesis that the sound in human whistling is generated by a Helmholtz resonator, suggesting that the oral cavity acts as a resonant chamber bounded by two orifices, posteriorly by raising the tongue to the hard palate, and anteriorly by pursed lips (Henrywood RH, Agarwal A. Phys Fluids 25: 107101, 2013). However, the detailed anatomical changes in the vocal tract and their relation to the frequencies generated have not been described in the literature. In this study, videofluoroscopic and simultaneous audio recordings were made of subjects whistling with the bilabial (i.e., "puckered lip") technique. One whistling subject was also recorded, using magnetic resonance imaging. As predicted by theory, the frequency of sound generated decreased as the size of the resonant cavity increased; this relationship was preserved throughout various whistling tasks and was consistent across subjects. Changes in the size of the resonant cavity were primarily modulated by tongue position rather than jaw opening and closing. Additionally, when high-frequency notes were produced, lateral chambers formed in the buccal space. These results provide the first dynamic anatomical evidence concerning the acoustic production of human whistling. NEW & NOTEWORTHY We establish a new and much firmer quantitative and physiological footing to current theoretical models on human whistling. We also document a novel lateral airflow mechanism used by both of our participants to produce high-frequency notes.

Keywords: acoustics; aerodynamic whistle; magnetic resonance imaging; radiography; whistling.

Fig. 1.

Cartesian coordinates of anatomical landmarks obtained for analysis. A: upper incisor. B: second molar. C: anterior edge of tongue. D: lower incisor, and the hyoid as labeled. The x-axis was used as reference to correct for head movements.

Fig. 2.

Samples of audio spectrogram and pitch analysis output from Praat software for each of the three whistling task trials. The white dotted lines represent fundamental frequencies. The black star marks a period of inhalation.

Fig. 3.

Lateral air passages are used to generate high-frequency whistles. Bottom: the buccal space chambers are outlined, while the central oral resonance chamber is identified in top.

Fig. 4.

A: conformation of a low-frequency, approximately 500-Hz tone. B: still radiographic image of subject 1 whistling a high-frequency, ~2,500-Hz tone.

Fig. 5.

Relationship between tongue-incisor distance and whistle frequency for subject 1 during whistling trial 2. The variable a is a constant that is proportional to speed of sound, and b is the exponent that would convert a length to the volume in a Helmholtz resonator. The fitted values for these variables are a = 1113 and b = 1.45.

Fig. 6.

MRI images of the oral resonance chamber for three different whistle frequencies. For each frequency (see row labels), the left panel shows a midsagittal view, with the oral cavity outlined, the middle panel shows the three-dimensional reconstruction of the resonance chamber shown in situ, and the right panel shows the oral resonance chamber volume extracted. In all images, the air-filled oral resonance chamber is indicated by a marbled dark-gray shading and body tissues in shades of brown.

Fig. 7.

Volume (A) and area (B) data with fit to Helmholtz model. A: empirical fit (dashed line) to our three MRI-derived volumes and the corresponding whistled frequencies (approximate values). B: two empirical fits, using the two-parameter version of a Helmholtz model, shown in inset. The solid curve represents the fit to all data points, while the dashed line shows a fit to only whistles produced by the low-frequency mode, which is clearly different, indicating that fitted parameters (orifice area and/or tube length) vary between the two modes.