Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. / Includes bibliographical references. / M.S.

Ces travaux s’inscrivent dans le cadre des recherches destinées à réduire ou à éliminer le bruit de crissement émis par les véhicules ferroviaires en courbe ou au freinage. Ces bruits peuvent en effet atteindre des niveaux sonores très élevés, jusqu’à 105 dB(A) à proximité des rames en cas de freinage par exemple, et constituent une gêne importante pour les usagers et les riverains. Afin d’apporter des solutions de réduction de ce bruit de crissement, on se concentre sur la modélisation des vibrations auto-entretenues de structures avec contact frottant ainsi que sur la compréhension des mécanismes à l’origine du crissement résultant, en particulier dans le cas d’un frein à disque ferroviaire. On fait l’hypothèse que les vibrations ont pour origine un couplage de modes de la structure dans les directions tangentielles et normale à l’interface en contact frottant. Ce couplage entraîne une instabilité de l’équilibre quasi-statique glissant du système et l’apparition de vibrations auto-entretenues. On cherche à déterminer ces vibrations par le calcul ce qui s’effectue en deux étapes. Dans un premier temps, l’analyse de stabilité de l’équilibre quasi-statique du système permet de déterminer l’occurrence des vibrations. Dans cette thèse, deux hypothèses de modélisation du frottement – plan ou rectiligne – sont comparées dans le cas d’un modèle académique minimal. Dans le cas non-amorti, on montre que l’hypothèse simplificatrice du frottement rectiligne tend à stabiliser le système. Dans le cas amorti, on constate que les résultats établis dans la littérature pour un frottement rectiligne, en particulier le paradoxe de déstabilisation, ne sont pas facilement généralisables au cas du frottement plan bien que des similitudes existent. Dans un deuxième temps, une analyse non-linéaire est nécessaire pour déterminer les amplitudes et le contenu fréquentiel précis des vibrations auto-entretenues. Cette étape est généralement effectuée à l’aide d’une intégration temporelle numérique à partir de conditions initiales données, proches de l’équilibre glissant. En examinant l’évolution temporelle des vibrations obtenues à l’aide de cette technique, on peut distinguer d’une part le régime transitoire au cours duquel les non-linéarités apparaissent et stabilisent progressivement la solution instable, et d’autre part le régime stationnaire (ou régime de fonctionnement) où les amplitudes vibratoires sont stabilisées. Dans le cas du modèle académique, une analyse rigoureuse des échanges énergétiques au cours de l’évolution temporelle permet de mettre en évidence l’origine de la stabilisation des vibrations, en particulier la baisse du taux de puissance injectée au contact résultant de l’accumulation des événements fortement non-linéaires : décollements, adhérences ou chocs. L’intégration temporelle directe n’est toutefois pas adaptée à des modèles numériques comportant de nombreux degrés de liberté et pour lesquels la détermination du régime stationnaire est généralement suffisante pour proposer des solutions de réduction. Le surcoût de calcul engendré par le régime transitoire est en effet très limitant. C’est pourquoi, une méthode est proposée pour approximer directement le régime stationnaire dans le cas d’un système mono-instable. Cette méthode combine une technique de tir à une initialisation basée sur les observations énergétiques ci-dessus. Elle est d’abord validée sur le modèle minimal puis adaptée au modèle éléments finis de frein ferroviaire moyennant une approximation originale de la technique de tir : la réduction de l’espace des phases des conditions initiales. Les approximations des régimes stationnaires sont comparées avec ceux obtenus par l’intégration temporelle directe. Les avantages et les limitations de la méthode sont discutés. / This work is part of research intended to mitigate or eliminate the squeal noise produced by railway vehicles in curves or during braking. For instance, brake noise can reach very high sound levels up to 105 dB(A) close to the trains and constitutes an important nuisance for passengers and residents in the station. In order to provide some mitigation measures, the study focuses on the modelling of self-sustained structural vibrations in presence of frictional contact and the understanding of the squeal generation mechanism, especially for of a railway disc brake. It is assumed that vibration is due to structural mode coupling in the normal and tangential directions at the frictional interface. This coupling leads to instability of the sliding quasistatic equilibrium of the system and the occurrence of self-sustained vibrations. Two steps are necessary to calculate these vibrations. Firstly, the occurrence of the vibrations is determined through the stability analysis of the quasi-static equilibrium. In this thesis, two modelling assumptions for the friction law – planar or rectilinear – are compared in the case of an academic model. Without damping, the simplified rectilinear friction law tends to stabilize the system. With damping, the results established in the literature in case of rectilinear friction, especially the destabilization paradox, cannot be applied generally in case of planar friction even if similarities may be observed. Secondly, a nonlinear analysis is necessary and performed to calculate the amplitudes and the detailed spectral content of the self-sustained vibrations. This step is generally performed by using numerical time integration from given initial conditions, close to the equilibrium. Taking a look at the vibrations time histories obtained by this technique, two different states may be distinguished. In the transient state, nonlinear forces appear and gradually stabilize the unstable solution whereas in the steady (stationary) state, vibrations are stabilized. In the case of the academic model, the cause of the stabilization is highlighted thanks to a rigorous analysis of energy exchanges, especially the decrease of the power rate injected via the contact interface resulting from the accumulation of strongly nonlinear events like loss of contact, stick or impacts. However, direct time integration is not very appropriate in the case of large numerical models with many degrees of freedom. Indeed, the additional computational cost due to the transient state is very high although the determination of the steady state is generally sufficient from a practical point of view. Consequently, a method is proposed in order to directly approximate the steady state in case of mono-instability. This method combines a shooting technique with an initialization based on the above energy considerations. It is first validated on the minimal model and then transposed to the finite element model of a railway disc brake through an original approximation of the shooting technique : the reduction of the initial conditions phase space. The estimations of the steady states are compared with those obtained by direct time integration. The advantages and the limitations of the method are discussed.

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Yang Chen. / "November 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (p. 131-141) / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

Wang, Ning. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references (leaves 106-115). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Introduction to Speech and Speaker Recognition --- p.1 / Chapter 1.2 --- Difficulties and Challenges of Speaker Authentication --- p.6 / Chapter 1.3 --- Objectives and Thesis Outline --- p.7 / Chapter 2 --- Speaker Recognition System --- p.10 / Chapter 2.1 --- Baseline Speaker Recognition System Overview --- p.10 / Chapter 2.1.1 --- Feature Extraction --- p.12 / Chapter 2.1.2 --- Pattern Generation and Classification --- p.24 / Chapter 2.2 --- Performance Evaluation Metric for Different Speaker Recognition Tasks --- p.30 / Chapter 2.3 --- Robustness of Speaker Recognition System --- p.30 / Chapter 2.3.1 --- Speech Corpus: CU2C --- p.30 / Chapter 2.3.2 --- Noise Database: NOISEX-92 --- p.34 / Chapter 2.3.3 --- Mismatched Training and Testing Conditions --- p.35 / Chapter 2.4 --- Summary --- p.37 / Chapter 3 --- Speaker Recognition System using both Vocal Tract and Vocal Source Features --- p.38 / Chapter 3.1 --- Speech Production Mechanism --- p.39 / Chapter 3.1.1 --- Speech Production: An Overview --- p.39 / Chapter 3.1.2 --- Acoustic Properties of Human Speech --- p.40 / Chapter 3.2 --- Source-filter Model and Linear Predictive Analysis --- p.44 / Chapter 3.2.1 --- Source-filter Speech Model --- p.44 / Chapter 3.2.2 --- Linear Predictive Analysis for Speech Signal --- p.46 / Chapter 3.3 --- Vocal Tract Features --- p.51 / Chapter 3.4 --- Vocal Source Features --- p.52 / Chapter 3.4.1 --- Source Related Features: An Overview --- p.52 / Chapter 3.4.2 --- Source Related Features: Technical Viewpoints --- p.54 / Chapter 3.5 --- Effects of Noises on Speech Properties --- p.55 / Chapter 3.6 --- Summary --- p.61 / Chapter 4 --- Estimation of Robust Acoustic Features for Speaker Discrimination --- p.62 / Chapter 4.1 --- Robust Speech Techniques --- p.63 / Chapter 4.1.1 --- Noise Resilience --- p.64 / Chapter 4.1.2 --- Speech Enhancement --- p.64 / Chapter 4.2 --- Spectral Subtractive-Type Preprocessing --- p.65 / Chapter 4.2.1 --- Noise Estimation --- p.66 / Chapter 4.2.2 --- Spectral Subtraction Algorithm --- p.66 / Chapter 4.3 --- LP Analysis of Noisy Speech --- p.67 / Chapter 4.3.1 --- LP Inverse Filtering: Whitening Process --- p.68 / Chapter 4.3.2 --- Magnitude Response of All-pole Filter in Noisy Condition --- p.70 / Chapter 4.3.3 --- Noise Spectral Reshaping --- p.72 / Chapter 4.4 --- Distinctive Vocal Tract and Vocal Source Feature Extraction . . --- p.73 / Chapter 4.4.1 --- Vocal Tract Feature Extraction --- p.73 / Chapter 4.4.2 --- Source Feature Generation Procedure --- p.75 / Chapter 4.4.3 --- Subband-specific Parameterization Method --- p.79 / Chapter 4.5 --- Summary --- p.87 / Chapter 5 --- Speaker Recognition Tasks & Performance Evaluation --- p.88 / Chapter 5.1 --- Speaker Recognition Experimental Setup --- p.89 / Chapter 5.1.1 --- Task Description --- p.89 / Chapter 5.1.2 --- Baseline Experiments --- p.90 / Chapter 5.1.3 --- Identification and Verification Results --- p.91 / Chapter 5.2 --- Speaker Recognition using Source-tract Features --- p.92 / Chapter 5.2.1 --- Source Feature Selection --- p.92 / Chapter 5.2.2 --- Source-tract Feature Fusion --- p.94 / Chapter 5.2.3 --- Identification and Verification Results --- p.95 / Chapter 5.3 --- Performance Analysis --- p.98 / Chapter 6 --- Conclusion --- p.102 / Chapter 6.1 --- Discussion and Conclusion --- p.102 / Chapter 6.2 --- Suggestion of Future Work --- p.104

Noise in the Intensive Care Unit (ICU) has been associated with patients experiencing psychological and physical disorders such as anxiety, sleep deprivation, and worsening of hypertension and diabetes. Researchers have suggested that the use of a noise reduction protocol can result in a decrease in noise in the ICU and a subsequent improvement in Hospital Consumer Assessment of Healthcare Providers and Systems (HCAHPS) scores. The research question for this project examined the effectiveness of a newly developed noise protocol in minimizing noise in the ICU, since the patients at the facility of study reported noise as being a nuisance that was hampering their sleep and healing; this nuisance has also been reflected in the hospital's low HCAHPS scores. The theoretical premise of the project was the theory of comfort, which suggests that engaging in health-seeking behaviors bring patients comfort. The sources of evidence that guided the project included a literature review using the keywords noise in ICU, sleep disruption, and hospital noise; HCAHPS scores over the past 5 years; and the analysis of data obtained from interviews of 48 nurses and 4 intensivists (critical care doctors) who responded to an open invitation to participate. The interviews were analyzed using codes; the emerging themes were that the protocol was useful, did not interfere with work flow, and allowed patients to rest uninterruptedly. The result from the project can be used by the hospital leadership team to advance the noise reduction protocol to areas of the hospital outside of ICU, and as a training tool to educate the hospital staff on the importance of maintaining a noise-friendly environment.

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In maritime vessels, it is desirable to minimise the structural and acoustic responses for several reasons, including passenger comfort, minimisation of crew fatigue, and in the case of military vessels, to avoid detection. The propeller-shafting system represents one of the most critical areas which must be addressed in order to reduce the low frequency acoustic signature. The propeller-shafting system is primarily excited by axial oscillations at the propeller. The force transmitted along the propeller-shafting system from these disturbances results in axial excitation of the hull and subsequent sound radiation. The aim of this thesis is to apply a combination of passive and active control techniques, in order to minimise the low frequency radiated noise signature of a pressure hull submerged in a fluid. Dynamic models of the propeller-shafting system, foundation and cylindrical hull including complicating factors such as fluid loading, bulkheads and onboard equipment are developed and described using the transmission matrix approach. This modular description enables greater flexibility for dynamic modelling of the propeller shafting system, and can be easily manipulated for future design modifications. The far-field radiated sound pressure from the submarine hull is evaluated and related to the force delivered to the hull by the propeller-shafting system. A passive optimisation scheme involving a genetic and general non-linear constrained algorithm is used to minimise fitness functions associated with the vibration of the propeller, vibration transmission to the hull and far-field radiated sound pressure over a low frequency range. This results in optimal resonance changer parameters for single and multiple resonance changers in a variety of configurations. A new quasi-adaptive resonance changer system is proposed and optimised to minimise the radiated sound pressure or propeller velocity. The optimal use of an adaptive resonance changer is investigated in both the frequency and time domains to reduce the hull velocity and subsequently the far-field radiated sound pressure. Fully active control is also evaluated by introducing a control force to the resonance changer with the aim of minimising either the propeller velocity or the radiated noise level. Finally, the concept of hybrid control is investigated by coupling passive, active and semi-active control techniques together to improve the overall performance.