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Modelling the Production and Propagation of Sound in Individual Human Vocal Tracts
KTH, School of Engineering Sciences (SCI), Mechanics, Biomechanics. KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
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 [en]
Biomechanics, Vocal Tract Acoustics, Numerical Flow Simulation, Fluid-Structure Interaction
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-240556ISBN: 978-91-7873-064-3 (print)OAI: oai:DiVA.org:kth-240556DiVA, id: diva2:1272685
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
List of papers
1. Influence of changes of the glottal waveform on vowel production
Open this publication in new window or tab >>Influence of changes of the glottal waveform on vowel production
2017 (English)In: Proceedings of Meetings on Acoustics (POMA), Acoustical Society of America, Paper ICA2016-586, 2017, 2017Conference paper, Published paper (Refereed)
Abstract [en]

Conditions of the vocal folds and upper airways can directly influence the fundamental frequency of the periodic movement of the glottis as well as the waveform of the source signal. This could further impair a patient's ability to excite resonances of the vocal tract and generate vowels. In this study, the Rosenberg model for the glottal pulse is applied to numerically investigate the propagation of the voice source signal from the glottis through a static vocal tract model. The geometries of the vocal tract are based on magnetic resonance imaging data for the different vowel pronunciations of a healthy male subject. For the computation of the pressure fluctuations and the associated distribution of frequency peaks as a result of the modulation through the vocal tract, direct compressible flow simulations are carried out by using a finite volume solver. The results are compared with the solution of a wave reflection analogue based on the area functions extracted from the same geometries and good agreement is reached. The effect of variations of glottal closure and fundamental frequency of the standard Rosenberg waveform on the computed acoustic signal is investigated. Thus, an estimation of the impact of glottal diseases on the ability of vowel production is attempted.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240550 (URN)
Conference
International Congress on Acoustics, Acoustical Society of America
Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19
2. On direct aeroacoustics calculations of the vocal tract
Open this publication in new window or tab >>On direct aeroacoustics calculations of the vocal tract
2019 (English)In: Direct and Large-Eddy Simulation XI, ERCOFTAC Series, Vol. 25 (ISBN: 978-3-030-04914-0), Springer, 2019, 2019Conference 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.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240551 (URN)
Conference
International Workshop on Direct and Large-Eddy Simulation XI
Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19
3. Compressible flow simulations of phonation using realistic vocal tract geometries
Open this publication in new window or tab >>Compressible flow simulations of phonation using realistic vocal tract geometries
(English)In: Journal of the Acoustical Society of America, ISSN 0001-4966, E-ISSN 1520-8524Article in journal (Refereed) Submitted
Abstract [en]

Voiced speech consists mainly of the source signal that is frequency-weighted by the acoustic filtering of the upper airways and vortex-induced sound through perturbation in the flow field. This study investigates the flow instabilities leading to vortex shedding and the importance of coherent structures in the supraglottal region downstream of the vocal folds for the far-field sound signal. Large eddy simulations of the compressible airflow through the glottal contriction are performed in realistic geometries obtained from three-dimensional magnetic resonance imaging data. Intermittent flow separation through the glottis is shown to introduce unsteady surface pressure through impingement of vortices. Additionally, dominant flow instabilities develop in the shear layer associated with the glottal jet. The aerodynamic perturbations in the near field and the acoustic signal in the far field is examined by means of spatial and temporal Fourier analysis. Furthermore, the acoustic sources due to the unsteady supraglottal flow are identified with the aid of surface spectra and critical regions of amplification of the dominant frequencies of the investigated vowel geometries are identified.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240552 (URN)
Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19
4. Analysis of the aerodynamic sound of speech through static vocal tract models of various glottal shapes
Open this publication in new window or tab >>Analysis of the aerodynamic sound of speech through static vocal tract models of various glottal shapes
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The acoustic spectrum of our voice can be generally divided into a harmonic and an inharmonic sound component. While the harmonic component based on the periodically modulated subglottal pressure is well described by reduced-order speech models, the accurate computation of the inharmonic component requires high-order flow simulations, which predict the vortex shedding and turbulent structures present in the shear layers of the glottal jet. This study characterizes the dominant frequencies in the unsteady flow of the intra- and supraglottal region. Both time-averaged and fluctuating values of the flow variables are computed and their distribution at various glottal shapes is compared. A realistic vocal tract geometry obtained through magnetic resonance imaging (MRI) is applied for the numerical domain, which is locally modified to account for different convergent and divergent glottal angles. The impact of the registered modes in the flow on the acoustic far field is computed through direct compressible flow simulations and cross-correlations. Furthermore, acoustic analogies are applied to localize the sources of the inharmonic broadband sound.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-240553 (URN)
Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19
5. Aeroacoustics of an elastic element in unsteady flow of low Reynolds numbers
Open this publication in new window or tab >>Aeroacoustics of an elastic element in unsteady flow of low Reynolds numbers
2016 (English)In: AIAA Technical Paper 2016-2700, 22nd AIAA/CEAS Aeroacoustics Conference, American Institute of Aeronautics and Astronautics, 2016, 2016Conference paper, Published paper (Refereed)
Abstract [en]

Vibrations of elastic structures are a common occurrence in numerous fields of engineering such as aeronautics, aerodynamics, civil engineering, and biomechanics. Particular e ort is dedicated to aeroacoustics of elements that are excited to oscillatory behaviour due to fluid instabilities. The current study is concerned with the numerical investigation of the flow-induced vibrations of a flexible, beam-like element in crossflow at low Reynolds numbers of Re = 100 − 1000 by means of fluid-structure interaction simulations. The aeroa-coustics in the near field are assessed with direct computation of the compressible airflow. Additionally, an acoustic analogy is applied, characterising the acoustic sources and the corresponding sound propagation. At low Reynolds numbers and high elastic moduli the dipole source produces the highest pressure perturbation in the near field. At higher Reynolds numbers and low elastic moduli, however, the monopole source due to structural vibrations becomes the important sound generating mechanism. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-194604 (URN)2-s2.0-84982943380 (Scopus ID)9781624103865 (ISBN)
Conference
22nd AIAA/CEAS Aeroacoustics Conference
Note

Conference Paper. QC 20161102

Available from: 2016-11-02 Created: 2016-10-31 Last updated: 2018-12-19

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