The effect of phonation into a straw on the vocal tract adjustments and formant frequencies. A preliminary MRI study on a single subject completed with acoustic results

Abstract

Phonation into a straw is used in voice training and therapy. This study investigated its’ effects on the vocal tract adjustments.

Magnetic resonance images were taken from one female phonating first on [a:], then into a plastic drinking straw and then again on [a:]. Acoustic samples were recorded from her and 10 other females. The samples were analyzed for fundamental frequency, sound pressure level (SPL), five lowest formant frequencies (F₁–F₅) and SPL of the speaker's formant cluster range (2.5–4.5 kHz).

During and after phonation into a straw, the midsagittal area of the vocal tract increased and the velar closure improved. The ratio of the transversal area of the lower pharynx over that of the epilarynx increased both during (27%) and after the straw (20%). The overall SPL and the SPL of the speaker's formant cluster increased. The frequencies of F₂, F₄ and F₅ decreased, while F₃ increased. The distances between F₄ and F₃ and F₅ and F₄ decreased by 49%. For the 10 subjects, F₃ increased, F₄ decreased and the distance between F₃ and F₄ decreased significantly.

The results suggest that exercising with a straw helps establishing a speaker's formant cluster, which increases loudness and thus improves vocal economy.

Introduction

Voiced fricatives, nasals and phonation into tubes are widely used exercises in voice training and therapy [1], [2], [3], [4], [5], [6], [7], the aim being to reach the ‘phonation balance’ (i.e. voice that is neither breathy nor pressed) and maximize the sonority of the voice. Typically, the voice is immediately perceived as being louder and produced with less effort [6]. The effect of these exercises has been interpreted to be due to the fact that lengthening and/or narrowing of the vocal tract lowers the first formant frequency and increases the acoustic input impedance (i.e. ratio of the acoustic input air pressure to the input airflow velocity) of the vocal tract [6], [8]. This increases the vibratory sensations in the vocal tract during phonation. Especially important is an increase of the reactance (i.e. the imaginary part of the vocal tract input impedance) in the fundamental frequency range of speech [8], [9]. Increased reactance has been reported to mechanically assist vocal fold vibration and also modify the glottal volume velocity waveform so that the sound pressure level (SPL) of the voice source increases and the spectral tilt decreases (i.e. the overtones get stronger [10], [11]. This makes the sound louder and brighter as such if the increased reactance is caused by a narrowing in the lower part of the vocal tract, near the vocal folds, and not in the mouth cavity, which in turn would reduce the radiated SPL (as in the case of closed vowels compared to open ones). Modelling studies and preliminary experiments with a single subject setup suggest that exercises implying a narrowing or lengthening of the front part of the vocal tract may guide in optimizing the glottal or lower vocal tract setting or both [9], [12], [13], in order to maximize the vibratory sensations in the vocal tract during the exercise. This can also explain why the effect could remain after the exercise. Optimal glottal setting implies ‘phonation balance’, i.e. an optimal adduction in relation to subglottic air pressure and may also include sufficiently high thyroarytenoid activity in relation to cricothyroid activity to maximize the mobility of the vocal folds [13]. An optimal vocal tract setting is supposed to be characterized by a narrowing of the epilaryngeal region (space from the vocal folds up to the epiglottis) in relation to the low pharynx [10]. This would not only increase the reactance of the vocal tract over a wide frequency range, but also result in a cluster of the upper formants F₃–F₅ (a singer's formant cluster), typical of Western classical singing, located in the range of 2–3 kHz [14]. In a good speaking voice a speaker's formant cluster has been found between 3 and 4 kHz [15], [16], formed by either a cluster of F₄ and F₅ [15] or F₃ and F₄ [16]. It is plausible to suggest that the origin of the speaker's formant cluster also lies in the adjustments of the low pharynx and the epilarynx. A formant cluster in the range of 2–4 kHz increases the loudness of the sound even when the voice source remains constant, thus improving vocal economy.

This pilot study aimed at investigating the possible shape and size changes in the vocal tract during and immediately after a vocal exercise consisting of phonation into a straw. Magnetic resonance imaging (MRI) technique was applied to a single subject. A detailed acoustic analysis was carried out for vowel phonation before and after phonation into a straw for 11 subjects. The hypotheses were: (1) a narrowing of the lower epilaryngeal region (and/or widening of the lower pharyngeal region) occurs during and after phonation into a straw, and (2) a cluster of two or three formants in the range of F₃, F₄ and F₅ is formed after phonation into a straw.