**5. Conclusion**

144 Otolaryngology

that maintains the anterior part of the vocal tract closed; therefore, the anterior part of the vocal tract opens and is "sucked out" by the speed of the airflow (McGowan, 1992), closing the vibration cycle, as occurs in the vocal fold vibration model. Therefore, supraglottic

The theory of source-filter interaction (Titze, 2008; Titze *et al.*, 2008) states that the acoustic pressure in the vocal tract changes the phonation threshold pressure and interferes with vocal fold vibration. The theory of source-filter interaction states that, when the vibratory motion of the point of articulation makes the acoustic pressure in the vocal tract oscillate, the phonation threshold pressure also oscillates, which might explain why the standard deviation of the closed quotient was higher during trill exercises in the present study.

According to Titze (2006), the objective of voice training is to promote the interaction between the source and the filter and therefore increase vocal intensity, efficiency, and economy (Titze, 2006). According to the author, lip trills and tongue trills are among the semi-occluded vocal tract exercises. Because semi-occluded vocal tract exercises promote a mechanical interaction between the source and the filter (Titze, 2008; Titze *et al.*, 2008) they change vocal fold impedance and therefore inhibit vocal fold vibration (Story *et al.*, 2000).

Vocal tract pressure during voiced fricatives must be constant, which distinguishes voiced fricatives from the exercises analyzed in the present study. New clinical studies comparing

In our studies, the mean closed quotient was highest during lip trills at high intensity, which distinguished lip trills from tongue trills and phonation of the sustained vowel /ε/. According to the literature (Story *et al.*, 2000; Titze, 2006), a more anterior obstruction translates to greater vocal tract impedance. Although lip trills are slightly more anterior than are tongue trills, the influence of supraglottic and subglottic pressure on vocal fold vibration is not linear (Zhang, 2009; Titze, 2008; Hatzikirou *et al.*, 2006, Titze, 2008), meaning that the airflow changes caused by increased intensity can lead to different vocal fold vibration proportions and differentiate between high- and low-intensity vibrations (Tao *et al.*, 2007;

Gaskill and Erikson (2008) found systematic differences between the closed quotient of lip trills and that of phonation of the sustained vowel /ε/; as we did in the present study, the authors argued that those differences might be due to the interaction between the source and the filter. In addition, the authors found differences between trained and untrained individuals in terms of the results obtained; the differences were attributed to the fact that

Some authors have reported that the sound produced by the larynx is not linear (Jing *et all,* 2001) and depends on numerous factors. Therefore, any difference in biomechanics, structure (such as tissue geometry, density, and viscosity), airflow control, or vocal tract control can cause differences in vocal fold vibration. Trained individuals have more control over those factors, and this can actually result in differences between trained and untrained individuals, as well as between trained individuals and patients with morphological changes in the vocal folds, in terms of vocal fold vibration during a given

pressure and, consequently, vocal tract impedance oscillate.

the two types of exercises should be conducted.

trained individuals have better control over glottic closure.

Becker *et al.*, 2009).

phonation task.

On the basis of our results, we can conclude that, in professional singers, the maximum amplitude of vocal fold vibration is greater during lip and tongue trills than during natural phonation. In addition, we observed considerable variation in the closed quotient during the exercises employed. Lip trills differed from tongue trills only at higher intensities and in terms of the closed quotient.
