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On direct aeroacoustics calculations of the vocal tract
KTH, School of Engineering Sciences (SCI), Mechanics.
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-2906-9306
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0001-7330-6965
2019 (English)In: Direct and Large-Eddy Simulation XI, ERCOFTAC Series, 2019, Vol. 25Conference paper, Published paper (Refereed)
Abstract [en]

Voice production and the verbal expression through speech are crucial components of human communication. The human voice is not just conveying information directly through words, but also indirectly as paralinguistic information such as the speaker's emotional state through tonality.

As such, voice is generated through a two-part process: First, a source signal is produced by the vocal folds that are pulsating the lung pressure and volumetric flow rate in a particular frequency through periodic opening and closing. Second, the vocal tract causes an attenuation or amplification of this source signal at certain frequencies depending on its specific shape. The voice generation process can therefore be described by a source-filter model with the vocal folds acting as the source and the vocal tract as an acoustic filter. Thus, we are able to produce different vowels and sounds as we manipulate the vocal tract during phonation.

However, the ability to speak can be compromised due to clinical conditions affecting the opening between the vocal folds (i.e. glottis) or the vocal tract. Certain voice disorders such as partial or total vocal fold paralysis and laryngeal cancer are known to affect the source signal and its waveform considerably.

Nevertheless, the actual cause-effect relations between physiological changes in the vocal tract and the acoustic pressure in the far field are unclear. In acoustics, the far field is defined as the region away from the source, where sound pressure levels follow the inverse square law and show a decrease of approximately 6 dB for each doubling of the distance from the source.

An additional factor in voice production is the shedding of intraglottal vortical structures. The sound output generated by vortices becomes important in cases of incomplete glottal closure or paralysed vocal folds. In this study, the acoustic signal generated through speech is computed directly as pressure fluctuations resulting from unsteady large eddy simulations, applied to magnetic resonance imaging (MRI) data. Thus, a time-resolved solution for the acoustic pressure in the upper airways is achieved, contributing to the knowledge of cause-effect relations in phonation and opening up new therapeutic options for vocal tract and airway disorders by the use of computational fluid dynamics.

Place, publisher, year, edition, pages
2019. Vol. 25
Series
ERCOFTAC Series ; 25
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-240551ISBN: 978-3-030-04914-0 (print)OAI: oai:DiVA.org:kth-240551DiVA, id: diva2:1272621
Conference
the 11th workshop on Direct and Large Eddy Simulation (DLES),Pisa, Italy in May 2017
Note

QC 20190215

Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2019-02-15Bibliographically approved
In thesis
1. Modelling the Production and Propagation of Sound in Individual Human Vocal Tracts
Open this publication in new window or tab >>Modelling the Production and Propagation of Sound in Individual Human Vocal Tracts
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Voice generation and the expression through speech are of vital importance for communication. The human upper airways are the origin of the process of speech production, which involves a modulation of the periodically pulsed pressure from the lungs by the vocal tract volume. In this work, phonation and voiced speech are investigated through both low- and high-order models, which are applied to vocal tract geometries of increasing complexity. Initially, the effect of variations of vocal fold closure, fundamental frequency, and vocal tract length on the computed acoustic signal is examined through parameter studies based on one-dimensional wave reflection analogues. Eventually, unsteady large eddy simulations based on the compressible Navier-Stokes equations are carried out to compute the pressure fluctuations and the associated distribution of resonance modes as a result of the interaction with the static vocal tract. Thus it is possible to calculate tonalities from the entire audible range of frequencies from 20 to 20000 Hz. In particular the inharmonic broadband sound component produced predominantly by coherent structures in the upper airways and at frequencies above 2 kHz is resolved in the current study, which is not captured by low-order models based on wave equations. Furthermore, three-dimensional numerical meshes based on surface representations of the human upper airways under voiced speech from magnetic resonance imaging (MRI) data of a healthy male subject are applied. These are necessary to resolve high-order acoustic modes that would not be represented by simplified geometries. Validation and verification of the chosen methods are achieved through comparison with experimentally obtained speech data, as well as Helmholtz eigenfrequencies of the considered vowel pronunciations. The main scope of this work is the assessment of acoustic sources and the conditions for aerodynamic sound being produced and propagated in the upper airways during phonation. The distribution of acoustic sources involved in the generation of the dominant frequencies are identified by application of acoustic analogies as well as surface Fourier transformation of the acoustic pressure fluctuations. However, the human upper airways do not only embrace the source of phonation and affect the modulation of the voice. Moreover, unwanted sounds may be generated in the upper airways due to elastic, collapsible parts that are susceptible to flow-induced vibration and resonance. The sound resulting from fluid-structure interaction in the upper respiratory tract, commonly known as snoring, can be an important indicator for underlying breathing disorders, such as obstructive sleep apnea (OSA). In a smaller part of this project, the flow structures and acoustic sources as a result of the interaction of shear flow of various Reynolds numbers with an elastic element are computed. The geometric dimensions are chosen to be representative of average physical values of the upper respiratory tract. Onset of tissue vibrations and resonance effects are investigated for a range of parameters of both solid and fluid. The obtained results of this work are aimed to contribute also to the development of a computational tool that assists physicians in the assessment of the airway function and the effectiveness of treatment plans prior to their application.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019
Series
TRITA-MEK, ISSN 0348-467X ; 2019:02
Keywords
Biomechanics, Vocal Tract Acoustics, Numerical Flow Simulation, Fluid-Structure Interaction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240556 (URN)978-91-7873-064-3 (ISBN)
Public defence
2019-01-31, F3, Lindstedtsvägen 26, Plan 2, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20181220

Available from: 2018-12-21 Created: 2018-12-19 Last updated: 2018-12-21Bibliographically approved

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Dahlkild, AndersMihaescu, Mihai

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