Abstract
The acoustical properties of the vocal tract, the air-filled cavity between the vocal folds and the mouth opening, are determined by its individual geometry, the physical properties of the air and of its boundaries. In this article, we address the necessity of complex impedance boundary conditions at the mouth opening and at the border of the acoustical domain inside the human vocal tract. Using finite element models based on MRI data for spoken and sung vowels /a/, /i/ and // and comparison of the transfer characteristics by analysis of acoustical data using an inverse filtering method, the global wall impedance showed a frequency-dependent behaviour and depends on the produced vowel and therefore on the individual vocal tract geometry. The values of the normalised inertial component (represented by the imaginary part of the impedance) ranged from 250,hbox {g}/hbox {m}^{2} at frequencies higher than about 3 kHz up to about 2.5times 10^{5},hbox {g}/hbox {m}^{2} in the mid-frequency range around 1.5–3 kHz. In contrast, the normalised dissipation (represented by the real part of the impedance) ranged from 65 to 4.5times 10^{5},hbox {Ns}/hbox {m}^{3}. These results indicate that structures enclosing the vocal tract (e.g. oral and pharyngeal mucosa and muscle tissues), especially their mechanical properties, influence the transfer of the acoustical energy and the position and bandwidth of the formant frequencies. It implies that the timbre characteristics of vowel sounds are likely to be tuned by specific control of relaxation and strain of the surrounding structures of the vocal tract.
Highlights
The human vocal tract (VT), the aeroacoustic cavity between the vocal folds and the open surface at the position of the lips, acts as a resonator of the pressure excitation due to the selfexcited vocal folds motion and airflow modulation caused by and based on the lung pressure (Fant 1960)
The differences in the acoustical characteristics are greater for the fourth and fifth formant (>150 Hz) which is caused by the higher sensitivity to small geometric variations and/or physical properties of the VT by diminishing the wavelength
We present a strategy to enhance MRI-based FEM models of the VT in order to match the formant frequencies and bandwidths as determined by using inverse filtering
Summary
The human vocal tract (VT), the aeroacoustic cavity between the vocal folds and the open surface at the position of the lips, acts as a resonator of the pressure excitation due to the selfexcited vocal folds motion and airflow modulation caused by and based on the lung pressure (Fant 1960). The topology of the frequency-dependent VT transfer function (TF) and its characteristic resonance frequencies, known as formant frequencies, is strongly determined by its geometry, especially the length and the area functions (Fant 1960; Story et al 1998) and the junction impedance to the surrounding air, i.e. the near- and far-field acoustics (Vampola et al 2011), and its interaction with the tissues inside the upper respiratory system (Sondhi 1974). The position of the formant frequencies and the associated bandwidths determine the articulated vowel and its quality. The formant frequencies and the bandwidths depend on each other (Hawks and Miller 1995)
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