The acoustic scattering properties of the human head and torso match well with those of simple geometric shapes. Consequently, analytical scattering models can be utilised to account for the sound localisation cues introduced by these features. The traditional use of such models assumes that the head surface is completely rigid in nature. This thesis is concerned with modelling and understanding the effect of terminal scalp hair (i.e., a non-rigid head surface) on the auditory localisation cues. The head is modelled as a sphere, and the acoustical characteristics of hair are modelled using a locally-reactive equivalent impedance parameter. This allows the scattering boundary to be defined on the inner rigid surface of the head. The boundary assumptions are validated experimentally, through impedance measurement at oblique incidence and analysis of the near-field scattering pattern of a uniformly covered sphere. The impedance properties of human hair are also discussed, including trends with variations in sample thickness, bulk density, and fibre diameter. A general solution for the scattering of sound by a sphere with an arbitrarily distributed, locally reactive surface impedance is then presented. From this, an analytical solution is derived for a surface boundary that is evenly divided into two uniformly distributed hemispheres. For this boundary condition, cross-coupling is shown to exist between incoming and scattered wave modes of equi-order when the degrees are non-equal and opposite in parity. The overall effect of impedance on the resultant scattering characteristics is discussed in detail, both for uniform and for hemispherically divided surface boundaries. Finally, the analytical formulation and the impedance characteristics of hair are collectively utilised to investigate the effect of hair on human auditory localisation cues. The hair is shown to produce asymmetric perturbations to both the monaural and binaural cues. These asymmetries may help to resolve localisation confusions between sound stimuli positioned in the front and rear hemi-fields. The cue changes in the azimuth plane are characterised by two predominant features and remain consistent regardless of the decomposition baseline (i.e., the inclusion of a pinna offset, neck, etc). Experimental comparisons using a synthetic hair material show a good agreement with simulated results.

Afin de réduire les bruits rayonnés en sortie de guide d’onde, des traitements acoustiques localement réactifs, comme les structures « Plaque perforée/Nid d’abeilles », peuvent être appliqués en liner. La conception de ces liners devient alors un challenge important avec l’apparition de nouvelles normes sur le bruit et impose de posséder une très bonne connaissance du comportement de ces traitements, en particulier leur impédance de surface. Néanmoins, la caractérisation de cette impédance n’est pas une chose facile et est généralement réalisée via des modèles semi-empiriques comme ceux de Guess, Elnady, Allam ou expérimentalement par des méthodes de mesures directes ou inverses. Ces approches inverses permettent, par la confrontation d’une modélisation du problème avec des observations expérimentales, de retourner, au travers d’une fonction coût, l’impédance du traitement. Ces méthodes ont l’avantage de réaliser une observation dans les conditions réelles d’utilisation du traitement. En effet, de nombreux paramètres influencent l’impédance de surface comme la présence d’un écoulement, l’incidence de l’onde… Dans cette thèse, une méthode d’identification de l’impédance est proposée. A partir de l’impédance de surface d’un traitement « Plaque perforée/Nid d’abeille » prédite par un modèle empirique via ses paramètres géométriques (épaisseur de la plaque, diamètre des perforations…), une méthode basée sur l’approche Bayésienne est implémentée afin de remonter à l’impédance réelle du traitement. Le problème étudié consiste en la propagation d’une onde dans un tronçon rectangulaire traité sur sa face supérieure et la mesure des pressions acoustiques est réalisée sur le banc d’essai de l’Université Technologique de Compiègne avec un écoulement rasant. Un modèle de propagation d’ondes dans le conduit est développé via la technique du raccordement modal, afin de prédire la pression aux positions des microphones pour n’importe quelle valeur d’impédance. A partir de la mesure et de la simulation, la règle de Bayes peut être appliquée afin de construire la densité de probabilité a posteriori. Cette densité de probabilité est alors échantillonnée au travers d’un algorithme Évolutionnaire de Monte Carlo par Chaîne de Markov (EMCMC). L’intérêt principale de cette méthode, est d’obtenir de nombreuses d’informations statistiques sur les paramètres caractérisant l’impédance de surface comme leur distribution et leur corrélation. / Locally reactive acoustic liners such as honeycomb structures with perforated panels can be modeled with a surface impedance in standard numerical models. However, the characterization of this impedance is not always straightforward. Empirical models or standing wave tube measurements are generally used to get the behavior of these acoustic treatments. Unfortunately, these methods provide only an evaluation of the impedance under specific conditions. Moreover, the conditions of use can change significantly the acoustic liners behavior as grazing flow conditions or oblique incident waves. A characterization of locally reactive acoustic liners is presented here. Starting from a set of parameters and represents a surface impedance using empirical model, an inverse method based on Bayesian approach is used to return the surface impedance taking in consideration the real conditions of use. A rectangular duct treated by a liner on its upper face is considered and these conditions are similar to the experiment present at the Université de Technologie de Compiègne. This inverse method requires a direct model to predict the pressure at some microphone positions with any surface impedance. The model used in the following is based on the Mode-Matching method. From the direct analytical model, the Bayes'rule is then used to get the posterior probability density function of the estimated impedance. An Evolutionary Monte Carlo by Markov chain (EMCMC) method is used to sample this posterior probability density. This method provides not only the best set of parameters but also some statistical information for each parameter.

This study was conducted to examine the effect of flow turbulence on sound waves propagating across a velocity field. The resulting information can be used to determine the potential for increasing the accuracy of an ultrasonic flowmeter, and understand the data scatter typically seen when using an ultrasonic flowmeter. A modification of the Ray Trace Method was employed which enabled the use of multiple rays in a very fine grid through a flow field. This technique allowed for the computation of the statistical variation of the propagation times for sound pulses traversing a flow field. The statistical variation was studied using two flow fields: 1) a uniform flow field with a superimposed vortex street and 2) an experimentally measured channel flow. The uniform flow field with a superimposed vortex street allowed for the examination of the effects of a large-scale flow structure on sound wave propagation, and for the verification of the analysis technique. Next by using the measured turbulent channel flow, as an example, the statistical variation of sound pulse propagation time was computed for flow likely to be encountered in actual flow measurement situations. Analysis was also conducted to determine the maximum allowable repetition rate of measurements with regard to the optimal time of flight measurements. Both the propagation time of a sound pulse moving across a uniform flow field with superimposed vortex street, and the resultant computed flow were observed to vary at the same frequency of the vortex street. Further, the magnitude of the variations was proportional with the strength of the individual vortices in the vortex street. A sound pulse propagating back and forth across a measured turbulent channel flow, afforded individual time difference variation from the mean propagation time of up to 5%. It was shown that a minimum variation occurred when the sound pulses were transmitted at a 75 degree angle to the flow axis. It was also determined that the average speed of sound in a flow field affected the final flow measurements by decreasing the measured delta time difference between the upstream and downstream propagating sound waves, and therefore the measured flow. The width of the sound path also contributed to decreasing the variation of the individual measurements by integrating over a larger sound path. These findings suggest that turbulence in a flow field affects ultrasonic flowmeter measurements by creating differences in the propagation times of individual sound pulses. Thus, turbulence and large-scale flow structures can result in variations in volumetric flow rate determination made by an ultrasonic flowmeter system.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Electrical Engineering and Computer Science. / Microfiche copy available in Archives and Engineering. / Vita. / Bibliography: leaves 138-140. / Ph.D.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Microfiche copy available in Archives and Engineering. / Vita. / Includes bibliographical references. / by Paul R. Donavan. / Sc.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Paola Crandall Fano. / M.S.

Thesis. 1978. M.S. cn--Massachusetts Institute of Technology. Dept. of Mechanical Engineering. / Includes bibliographical references. / by Ilene J. Busch-Vishniac. / M.S.cn

Thesis. 1978. M.C.P.--Massachusetts Institute of Technology. Dept. of Urban Studies and Planning. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ROTCH. / Includes bibliographical references. / M.C.P.

Thesis (M.S.)--University of Delaware, 2006. / Principal faculty advisor: Robert P. Gilbert, Dept. of Mathematical Sciences. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.