Nasal and vocal tract coupling: Variation of the velopharyngeal opening in 3-D casts
Channel: johan sundberg
Date: 2020-05-15

Miriam Havel MD, Dept. of Otorhinolaryngology, Head&Neck Surgery, Section Phoniatrics, Munich University Hospital (LMU), Munich, Germany 

Johan Sundberg PhD, 2Dept. of Speech, Music and Hearing, School of Computer Science and Communication, KTH (Royal Institute of Technology), Stockholm, Sweden

Abstract: Background: The acoustic contribution of the nasal cavity to the resonance properties of the vocal tract depends on the velopharyngeal opening (VPO). In this work, we investigate the acoustic effects of different sizes of the VPO by coupling 3 D casts of vocal tracts to a 3 D cast of a nasal tract.  
 
Method: 3D replicas of human vocal tracts for the vowels /a, ae, i/ were connected to a 3D replica of a human nasal tract via tubes obtained from standard syringes. The system was excited by sine tone sweeps from an earphone placed at the glottis. The response was picked up by a microphone at the lip opening. The acoustical responses obtained for different diameters and lengths of the coupling tube were recorded and the effect on the transfer function was analyzed.  
 
Results: The lowest resonance of the nasal tract alone was 535 Hz. For all three vowels, a marked dip appeared in the transfer functions near 600Hz, when a VPO coupling of at least 2.3 mm diameter and 14 mm length was introduced, i.e. when the nasal tract was acoustically included in the resonator system. With increasing size of the VPO the dip moved upward in frequency and became deeper. In addition, minor dips appeared above 2000Hz. With a specific narrow coupling tube, the level difference between the first and fourth formants was a few dB smaller than for the remaining coupling tubes tested.   
 
Conclusions: The transfer functions obtained in 3D replicas of coupled human nasal and vocal tracts showed systematic effects that depended on the diameter of the introduced velopharyngeal opening. The results support the assumption that the high frequency part of a spectrum can be slightly enhanced by an optimized size of the VPO.  
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