Dissertations / Theses on the topic 'Sound (acoustics)'

The original work presented in this thesis pertains to the design and characterisation of resonant-cavity-based acoustic metamaterials, with a focus on airborne sound. There are five separate experimental chapters, each with a unique approach to the design of periodic structures that can support and manipulate air-bound acoustic surface waves via diffractive coupling between resonant-cavities. The first two chapters concern measurement of the acoustic transmission though various kinds of periodic slit-arrays, whilst the latter three chapters utilise a near-field imaging technique to directly record and characterise the dispersion of trapped acoustic surface waves. The first experimental chapter investigates the effect that thermodynamic boundary layers have on the Fabry-Perot-like cavity resonances that are so often utilised in acoustic metamaterial design. At audio frequencies, these boundary layers have a decay length that is typically more than two orders of magnitude smaller than the width of the resonating slit-cavities, hence it may naively be assumed that their effect can be ignored. However, by studying in detail the effect that reducing slit-cavity width has on the frequency of the measured cavity-resonance, for both a single slit cavity and a slit-cavity array, it is found that these boundary layer effects become significant on a far larger scale than their characteristic thickness. This is manifested in the form of a reduction in the resonant frequency as the slit-width is narrowed. Significant attenuation of the resonance and a 5% reduction in the effective speed of sound through the cavity is measured when the boundary layers form only 5% of the total width of each slit. Hence, it is both shown that the prevalent loss free treatment of acoustic slit-cavities is unrealistic, and that one may control the effective speed of sound through the slit-cavities with a simple change in slit-width. The second chapter explores the effect of ‘compound’ grating structure on trapped acoustic surface waves, a compound grating having a basis comprised of more than one resonating element. The angle dependent acoustic transmission spectra of four types of aluminium slit-array are recorded, and for the compound gratings, it is found that sharp dips appear in the spectra that result from the excitation of a ‘phase-resonance’. This occurs as new degrees-of-freedom available to the acoustic near-field allow the fields of adjacent cavities within a unit-cell to be both out-of-phase and strongly enhanced. By mapping the transmission spectra as a function of in-plane wavevector, the dispersions of the modes supported by each sample are determined. Hence, the origin of the phase-resonant features may be described as acoustic surface waves that have been band-folded back into the radiative regime via diffraction from higher in-plane wavevectors than possible on a simple grating. One of the samples is then optimised via numerical methods that account for thermodynamic boundary layer attenuation, resulting in the excitation of a sharp, deep transmission minimum in a broad maximum that may be useful in the design of an acoustic filter. The third chapter introduces the near-field imaging technique that can be utilised to directly characterise acoustic surface waves, via spatial fast Fourier transform algorithms of high-resolution pressure field maps. The acoustic response of a square-lattice open-ended hole array is thus characterised. It is found that over a narrow frequency band, the lattice symmetry causes the acoustic surface power flow to be channelled into specific, predictable directions, forming ‘beams’ with a well defined width. In chapter four, the existence of the ‘acoustic line mode’ is demonstrated, a type of acoustic surface wave that may be supported by a simple line of open-ended hole cavities. The near-field imagine technique is again used to extract the mode dispersion. This acoustic line mode may be readily manipulated, demonstrated by arrangement of the line of holes into the shape of a ring. The existence of this type of mode offers a great deal of potential for the control of acoustic energy. Chapter five explores the effect of ‘glide-symmetry’ on a pair of acoustic line modes arranged side-by-side. A control sample not possessing glide- symmetry is first characterised, where measurement of the acoustic near- fields show that this sample supports two separate modes at different frequencies, with their phase either symmetric or anti-symmetric about the mirror plane between the lines of holes. One of these lines is then shifted along its periodicity by half of a grating pitch, thus creating glide-symmetry. The resulting sample is found to support a single hybrid mode, capable of reaching a much larger in-plane wavevector than possible on a simple grating with no gaps in its band-structure, and displaying a region of negative dispersion. The third sample demonstrates how one may increase the coupling strength between the two lines of holes via manipulation of the cavity shape, thus enhancing the glide-symmetry effect. The thesis concludes with preliminary investigations into other possible ways of manipulating acoustic surface waves, such as with the use of ‘screw-symmetry’.

A review of past research into sound propagation in woodland is presented. The attenuation of sound in woodland is small between about 800 and 2000Hz and greater at low and high frequencies. Attenuation measurements made in three contrasting woodlands are presented and compared with theoretical models. Propagation models using simple one- and two-parameter impedance models are used to calculate appropriate ground parameters for the prediction of impedance of the woodland soils. The ground parameters varied on different days in a single stand due to differences in moisture content and compaction. The overall differences between the stands are not significant. The woodland soil has a considerably lower impedance than other outdoor ground surfaces such as grassland or sand. A theoretical model for the attenuation of sound by thermoviscous absorption and scattering within an array of cylinders is assessed by means of a model experiment with wooden rods in an anechoic chamber. An input density 60% lower than the actual density gives a good agreement with measured attenuation. This modified model also predicts the attenuation by the cylinders in the presence of a ground surface. The scattering model is compared with the high frequency attenuation measured in the , woodland, using sampled trunk densities and radius, this underpredicts the observed attenuation, particularly in the stands with a dense branch and foliage structure. Addition of a second. dense, array of non rigid scatterers gives a good agreement with the measured data, thus modelling the scattering and absorbing effects of trunks, branches and leaves, in the high frequencies. Finally, a combined model is presented in which the attenuation caused by ground interference effects. at low frequencies. is added to a prediction of attenuation by the scattering model. across the whole frequency range. This model reproduces the frequency dependence of the attenuation of sound in woodland.

The search for a convenient connection between vibration patterns on a structure and the sound radiated from that structure is ongoing in structural acoustics literature. Common techniques are wavenumber domain methods, or representation of the vibration in terms of some basis, such as structural modes or elementary radiators, and calculating the sound radiation in terms of the basis. Most choices for a basis in this situation exhibit strong coupling between the basis functions, but there is one choice which does not: Acoustic radiation modes are by definition the basis that orthogonalizes the radiation operator, meaning the radiation modes do not exhibit any coupling in radiation of sound.Acoustic radiation modes are coming up on their 30th anniversary in the literature, but still have not found wide use. This is largely due to the fact that most radiation modes must be calculated through the computationally intensive boundary element method or boundary integral equations. Analytical expressions for radiation modes, or for the radiation resistance matrix from which they are derived, are only available for a few geometries. This thesis meets this problem head on, to develop additional analytical expressions for radiation resistance matrices of cylindrically curved structures.Radiation modes are developed in the context of their use to calculate sound power. Experimental and computational sound power calculations are presented in order to validate the use of the modes developed here. In addition, the properties and trends of the developed modes are explored.

Thesis (M.S.)--University of Alabama at Birmingham, 2009.
Title from PDF t.p. (viewed June 30, 2010). Additional advisors: Stephen A. Watts, Paul A. Richardson, John T. Tarvin. Includes bibliographical references (p. 36-38).

Product sound design has received much attention in recent years. This has created a need to develop and validate tools for developing product sound specifications. Elicitation of verbal attributes, identification of salient perceptual dimensions, modelling of perceptual dimensions as functions of psychoacoustic metrics and reliable auralisations are tools described in this thesis. Psychoacoustic metrics like loudness, sharpness and roughness, and combinations of such metrics into more sophisticated models like annoyance, pleasantness and powerfulness are commonly used for analysis and prediction of product sound quality. However, problems arise when sounds from several sources are analysed. The reason for this complication is assumed to be the human ability to separate sounds from different sources and consciously or unconsciously focus on some of them. The objective of this thesis was to develop and validate methods for product sound design applicable for sounds composed of several sources. The thesis is based on five papers. First, two case studies where psychoacoustic models were used to specify sound quality of saxophones and power windows in motor cars. Similar procedures were applied in these two studies which consisted of elicitation of verbal attributes, identification of most salient perceptual dimensions and modelling of perceptual dimensions as functions of psychoacoustic metrics. In the saxophone experiment, psychoacoustic models for prediction of prominent perceptual qualities were developed and validated. The power window experiment showed that subjects may judge only parts of the sound. Power window sound consists of the motor sound and the scratching of a window sliding over the seal. The motor sound was filtered out and models developed using motor sound alone showed good agreement with listening tests. This demonstrated the human ability to separate sound from different sources and pointed out the importance of handling auditory stream segregation in the product sound design process. In Paper III sound sketches (simple auralisations) was evaluated as a way to assess sounds composed of several sources. Auralisation allows control of the contributions of different sources to a sound at the listening position. This way, psychoacoustic analysis and listening tests may be carried out on the contributions from sources separately and as an ensemble. Sound sketches may also serve to specify a target sound for a product. In Papers IV and V, the precision of auralisations related to intended use was investigated. Auralisations were made by filtering engine sounds through binaural transfer functions from source locations to the listening position in a truck cabin. In Paper IV simplifications of auralisations of one source were compared to artificial head recordings. For idling sounds auralisations through binaural transfer functions with a resolution of 4 Hz or better, or smoothed with maximum 1/96 octave moving average filters were found to preserve perceived similarity to artificial head recordings. In Paper V the effect of simplifications of transfer functions on preference ratings of auralisations was examined. This is of interest in applications where audible differences may be acceptable as long as preference ratings are unaltered, e.g. when auralisations are used as rough sound sketches. At 500 rpm idle speed, a resolution of 32 Hz or better, or smoothing with maximum 1/24 octave moving average filters showed no significant alteration of subject preference ratings. These figures may serve as guide for required accuracy in auralisations used for evaluation of idling sounds in truck cabins. To conclude, psychoacoustic analysis of total sound may be used for prediction of perceived sound quality as long as the sound is generated by one source. When several sources generate sound, auditory stream segregation effects in combination with cognitive effects may deteriorate the results. Auralisation is a useful tool in such cases, since it makes it possible to analyse the effects of contributions from each source. It can also be used for making sound sketches which can serve as support in the design process.
Godkänd; 2008; 20080925 (ysko)

Thesis (M.Sc.Eng.) PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
When a circular piston vibrates within an aperture of a baffle, the flow within the annular gap between the piston and the baffle is opposite in phase, which causes a reduction in the radiated sound. That is, the acoustic power is always less than in the absence of a leaking edge flow. However, when the piston is backed by a cavity, the overall acoustic power can be increased. This thesis focuses on approximating the effect of the ‘leakage’ for a piston of radius a in a cylindrical cavity of radius b within a rigid baffle. First we consider the piston within a baffle without a cavity, which leads to a reduced acoustic power, then we examine the two cases of a closed cavity and an open cavity. The acoustic power is found to increase when the piston vibrates close to a resonant frequency of the closed or open cavity. The smallest resonant frequency fmin depends on the cavity depth and end-correction. The maximum ‘gain’ in acoustic power ~ 10 dB which depends on the nonlinear edge flow and also depends on the ratio a/b, the aspect ratio b/L and ζ0/b, where L is the cavity depth and ζ0 is the amplitude of piston displacement.
2031-01-01

This thesis aims at developing conceptual and methodological tools in order toadapt sound within the textile design area. Occupational groups working with soundare to a large extent problem driven. Accordingly, textile designers working withsound- affecting properties of textiles concentrate on their dampening qualities. Theambition with this research project is to make suggestions how textile designers canwork practically with textile sound design, in a more nuanced way.The overall aim of the thesis is to develop a vocabulary to make textile designersable to express the sound affecting qualities of textiles in a language full of nuances.As a starting-point the thesis briefl y describes commonly used methods andprocesses used to describe the expressiveness of a design, followed by a morethoroughly analyze of the textile design process illustrated by a practical example.These studies constitute a foundation to make it possible to see in what way thesemethods and processes will be affected when sound is added as new design tool.By studies of two sound design models, the fi rst attempts to develop a vocabularyconcerning how to describe sound affecting qualities of a textile are developed.Research focusing on language issues, especially on the development of conceptualtools done at the research institute CRESSON, provides descriptive soundconcepts, “sound effects”, embracing the interaction between human and his soundenvironment. These concepts are followed by a model of how to describe just asound or “sound object” in “itself” (not in relation to anything else), developed byPierre Schaeffer. These theoretical models have been complemented with empiricalstudies in form of a survey, named LISTEN. Interviews were performed from aphenomenological perspective. A number of informants were asked to tell aboutthe sound environment and single sounds occurring at their working places. Theinterviews were interpreted from a phenomenographic perspective. A number ofdesign projects are fi nally presented as practical examples of different ways to workwith textiles and sound.The theoretical models provided by Schaeffer have been used to make the fi rstsystematic attempts to describe sound environments; sounds and textiles soundaffecting properties.Since the model presented by Schaeffer is developed to be used within musicalcomposition the concepts have to be additionally modifi ed to be a useful tool withinthe textile design area. The thesis presents just the fi rst attempts to use this model.The next step to take in the research project is to adjust the theoretical systems ofCRESSON and Schaeffer to suit the special area of textile design. The interactiveideas of a sound-affecting textile will also be a subject of further development.

The sound radiated during a tennis impact has been shown to greatly influence the perception of 'feel' but has also been suggested to contribute to the perception of equipment quality, as is the case with many other consumer products. In an industry that is governed by design limitations, tennis racket manufactures are continually aiming to differentiate their products from their competitors' and the sound character of a racket is seen as one such method to do so. In order to control the sound radiated from a tennis racket it was first necessary to identify how a tennis racket radiates sound. Through a number of controlled player tests, involving groundstrokes and serves, the sound was recorded along with measurements of the physical vibrations excited in the frame. Analysis of the data revealed typical characteristics of the sound in the time and frequency domain. The sound was split into two sections to aid the analysis; an initial impulsive component that decayed very quickly and a ringing component that was of much lower amplitude but decayed at a much slower rate. The evolution of the frequency content over time was also investigated, however, the data provided much more information as to where each frequency component originated by analysing the data together with the experimental modal analysis data. The experimental modal analysis of a tennis racket was a vital stage in understanding which components of the racket were responsible for radiating the identified frequency component in the sound spectrum. The investigations identified frame bending modes, out-of-plane and inplane, as well as torsional modes, hoop modes and stringbed modes. To enable direct comparison between the natural frequencies excited during a tennis shot and the frequencies recorded from a freely suspended racket, the effect of the hand on the modal behaviour of the racket was analysed; experimental modal analysis data from a hand-gripped racket was compared with data generated by adding simulated mass to the modal model of the freely suspended racket. The first stringbed mode was identified as a key contributor to the sound of the racket, especially in the ringing component of the sound. Analysis of the physical vibrations in the frame following a tennis shot revealed that the first stringbed mode excited the frame of the racket, which is iv thought to be the reason why stringbed modes contribute to the sound more so than their surface area would suggest that they are capable of. Analysis of the relationship between subjective perceptions of players and calculated sound metrics suggest a negative correlation exists between the duration and loudness of a tennis shot and the appeal of the sound. Further investigations involving a jury to evaluate the appeal of a tennis impact sound, modified in terms of duration, revealed further evidence to support the theory that tennis impact sounds of greater duration are less appealing than those that decay quickly. This is the first study to investigate the sound radiated from a tennis racket and has identified how the sound is radiated from the racket and also suggests which parameters of the sound are considered appealing to tennis players. With this information it is possible to suggest design modifications that would influence the sound of a tennis racket in a predictable manner.

This dissertation is primarily concerned with the development of an effective methodology for reducing structure-borne sound radiation from an arbitrarily shaped vibrating structure. There are three major aspects that separate the present methodology from all the previous ones. Firstly, it is a non-contact and non-invasive approach, which is applicable to a class of vibrating structures encountered in engineering applications. Secondly, the input data consists of a combined normal surface velocity distribution on a portion of a vibrating surface and the radiated acoustic pressure at a few field points. The normal surface velocities are measured by using a laser vibrometer over a portion of the structural surface accessible to a laser beam, while the field acoustic pressures are measured by a small array of microphones. The normal surface velocities over the rest surface of the vibrating structure are reconstructed by using the Helmholtz Equation Least Squares (HELS) method. Finally, the acoustic pressures are correlated to structural vibration by decomposing the normal surface velocity into the forced-vibro-acoustic components (F-VAC). These F-VACs are mutually orthogonal basis functions that can uniquely describe the normal surface velocity. The weightings of these F-VACs represent the relative contributions of structural vibrations into the sound radiation. This makes it possible to suppress structure-borne acoustic radiation in the most cost-effective manner simply by controlling the key F-VACs of a vibrating structure. The effectiveness of the proposed methodology for reducing structure-borne acoustic radiation is examined numerically and experimentally, and compared with those via traditional experimental modal analyses. Results have demonstrated that the proposed methodology enables one to reduce much more acoustic radiation at any selected target frequencies than the traditional approach.

The search for quieter internal combustion engines drives the quest for a better understanding of the acoustic properties of engine duct components. Simulations are an important tool for enhanced understanding; they give insight into the flow-acoustic interaction in components where it is difficult to perform measurements. In this work the acoustics is obtained directly from a compressible Large Eddy Simulation (LES). With this method complex flow phenomena can be captured, as well as sound generation and acoustic scattering. The aim of the research is enhanced understanding of the acoustics of engine gas exchange components, such as the turbocharger compressor.In order to investigate methods appropriate for such studies, a simple constriction, in the form of an orifice plate, is considered. The flow through this geometry is expected to have several of the important characteristics that generate and scatter sound in more complex components, such as an unsteady shear layer, vortex generation, strong recirculation zones, pressure fluctuations at the plate, and at higher flow speeds shock waves. The sensitivity of the scattering to numerical parameters, and flow noise suppression methods, is investigated. The most efficient method for reducing noise in the result is averaging, both in time and space. Additionally, non-linear effects were found to appear when the amplitude of the acoustic velocity fluctuations became larger than around 1~\% of the mean velocity, in the orifice. The main goal of the thesis has been to enhance the understanding of the flow and acoustics of a thick orifice plate, with a jet Mach number of 0.4 to 1.2. Additionally, we evaluate different methods for analysis of the data, whereby better insight into the problem is gained. The scattering of incoming waves is compared to measurements with in general good agreement. Dynamic Mode Decomposition (DMD) is used in order to find significant frequencies in the flow and their corresponding flow structures, showing strong axisymmetric flow structures at frequencies where a tonal sound is generated and incoming waves are amplified.The main mechanisms for generating plane wave sound are identified as a fluctuating mass flow at the orifice openings and a fluctuating force at the plate sides, for subsonic jets. This study is to the author's knowledge the first numerical investigation concerning both sound generation and scattering, as well as coupling sound to a detailed study of the flow.With decomposition techniques a deeper insight into the flow is reached. It is shown that a feedback mechanism inside the orifice leads to the generation of strong coherent axisymmetric fluctuations, which in turn generate a tonal sound.

QC 20121113

This thesis resumes the work done to study the acoustical performance of several building construction types proper from the south of Europe. It was made special attention to the flanking contribution. Initially, it was performed an exhaustive study of the existent bibliography with the aim of identifying and valuing the several acoustic experimental measurement methods related to the estimation of flanking contribution. Therefore, three possible applying methods were found: (i) Sound pressure method used in combination with elements shielding; (ii) Sound intensity method capable of obtaining the most reliable results without using the shielding process; (iii) Vibration velocity measurement procedure whose foundation lies on the relation between the vibration velocity of each element and the different flanking paths. This method is based on the velocity measures obtained from the transmitted vibrations by these elements, which are converted on acoustic estimation. From these three measurement methods the sound intensity is the only standardized and allows quantifying the contribution of each flanking path separately. For that reason, the intensity method was used as reference. It was also compared with the sound pressure method, respecting the estimation of airborne sound insulation without including the flanking contribution, and with the vibration velocity method when considering the flanking transmission. There were performed several experimental measurements to compare the results in laboratory and field conditions, for both structure and airborne sound transmission, through the three measurement methods. Taking the results obtained in consideration it is intended to establish a vibration velocity method as a measurement procedure on the estimation of airborne sound insulation with flanking contribution evaluation. At the same time it was calculated the sound insulation prediction by means of the methods referred on the EN 12354-1 (2000). Gathering all this information and comparing all the results, it is possible to conclude if the prediction methods are suitable for the south European countries, especially to the Spanish buildings. It must be referred that the vibration velocity method is only capable of producing results through the application of mathematical models which simulate the radiation efficiency estimation. The authors of these models were selected, as the most recent and known, among the bibliography and their models were applied to the specific conditions found for this thesis. These results were expressed by means of graphics representing vibrational velocity levels spatially distributed. The experimental work developed for this thesis implies a profound knowledge of all the measurement methods, and allowing obtaining data capable of concluding on the advantages and disadvantages of each method. Finally, conclusions and specific proposals are presented in order to stimulate the development of new studies. Esta tesis resume el esfuerzo que se ha realizado al estudiar las prestaciones acústicas de diversas topologías constructivas propias del sur de Europa, haciendo especial hincapié en el efecto de las transmisiones indirectas. Inicialmente, se ha estudiado de una forma exhaustiva la bibliografía existente con el objeto de identificar y valorar los diversos métodos de medida experimentales relacionados con la estimación de las transmisiones indirectas. Asimismo, se han encontrado tres posibles métodos para aplicación: (i) El método de presión, utilizado en combinación con el aislamiento de tabiques o apantallamiento; (ii) El método de intensidad acústica, capaz de obtener resultados muy fiables sin recurrir al apantallamiento; (iii) El procedimiento de medida de vibraciones, cuyo fundamento radica en la relación entre la velocidad de vibración de cada uno de los elementos implicados y de los distintos caminos de flanco. Este tiene como base medidas de velocidad, midiendo las vibraciones transmitidas por estos elementos que después se convierten en estimación acústica. De estos tres métodos de medida, el único estandardizado, que proporciona la contribución de cada camino de flanco por separado, es el método de intensidad. Por ello, este método ha sido utilizado como “método de referencia”. Se comparó con el método de presión, en lo que respecta a medida de aislamiento a ruido aéreo sin incluir transmisiones por flanco. Se comparó también con el método de vibraciones por velocidad, cuando se han tenido en cuenta las transmisiones por flanco. Se han realizado diversas tandas de medidas experimentales para comparar los resultados de acuerdo a los tres métodos de medida, tanto en condiciones de laboratorio como en condiciones reales (in situ), así como para medidas de impacto y aéreas. En base a los resultados obtenidos, se plantea establecer el método de vibración como posible procedimiento de medida en la estimación del aislamiento sonoro de edificaciones con la estimación de las transmisiones de flanco. vii En paralelo con estos experimentos, se calculó la predicción de la transmisión acústica incluida en la normativa EN 12354-1 (2000). Con esta información se añade otro conjunto de datos, comparables con los resultados experimentales, permitiendo concluir sobre la aplicabilidad, o no, de este método de predicción en los edificios del sur de Europa, más en concreto en los españoles. Con el método de vibraciones solamente es posible obtener resultados mediante la aplicación de modelos matemáticos de estimación de la eficiencia de radiación acústica. Los autores de estos modelos se han escogido entre la bibliografía como los más conocidos y más novedosos, aplicados a las específicas condiciones de entorno elegidas y posibles para esta tesis. Los resultados de estos datos se expresan mediante gráficas de distribución espacial de niveles de velocidad de vibración. El trabajo desarrollado experimentalmente implica un conocimiento profundo de todos los métodos de medición acústica referidos en esta tesis, permitiendo obtener datos que indiquen las ventajas y inconvenientes de cada uno de los métodos, así como los resultados comparados de las estimaciones obtenidas por cada uno de ellos. Al final, se presentan las conclusiones y propuestas específicas, para el desarrollo de nuevos estudios. Esta tese resume o esforço que foi feito ao estudar as prestações acústicas de diversas topologias construtivas próprias do sul da Europa, com especial atenção no efeito das transmissões marginais de flanco. Inicialmente, foi estudada de uma forma exaustiva a bibliografia existente com o objectivo de identificar e valorizar os diversos métodos de medida experimentais relacionados com a estimação das transmissões marginais, em concreto as de flanco. Com isto, foram encontrados três possíveis métodos para utilizar: (i) O método de pressão, utilizado em combinação com o isolamento independente de paredes; (ii) O método de intensidade acústica, capaz de obter resultados muito fiáveis e sem recorrer ao isolamento de paredes; (iii) O procedimento de medida de vibrações, cujo fundamento reside na relação entre a velocidade de vibração de cada uma das paredes implicadas e dos diferentes caminhos de flanco. Este procedimento tem como base medidas de velocidade, onde são medidas as vibrações transmitidas por estes elementos que posteriormente são convertidas em estimação acústica. De entre estes três métodos de medida, o único normalizado, que proporciona a contribuição de cada caminho de flanco de forma separada, é o método de intensidade. Por isto, este método foi utilizado como “método de referência”. Este método foi comparado com o método de pressão, no que diz respeito a medida de isolamento a ruído aéreo sem incluir as transmissões de flanco. Foram realizadas diversas séries de medidas experimentais para comparar os resultados para os três métodos de medida, quer em condições laboratoriais como em condições reais ou in situ, assim como para medições com excitação por percussão e aérea. Com base nos resultados obtidos pretende-se estabelecer um método de vibrações como possível procedimento de medida na estimação do isolamento sonoro dos elementos de compartimentação dos edifícios com a contabilização das transmissões de flanco. xi Em paralelo com estes ensaios calculou-se a previsão da transmissão acústica através da norma EN 12354-1 (2000). Com esta informação é acrescentado um outro conjunto de dados, comparáveis com os resultados experimentais, permitindo concluir sobre a aplicabilidade, ou não, de este método de previsão aos edifícios do sul da Europa, mais concretamente aos espanhóis. Com o método de vibrações isoladamente é possível obter resultados mediante a aplicação de modelos matemáticos que estimam a eficiência de radiação acústica. Os autores de estes modelos foram escolhidos de entre a bibliografia, como os mais conhecidos e também mais recentes, foram aplicados às condições do meio eleitas e possíveis para esta tese. Os resultados foram expressos mediante gráficos de distribuição espacial de níveis de velocidade de vibração. O trabalho desenvolvido experimentalmente implica um conhecimento profundo de todos os métodos de medição acústica referidos neste tese, permitindo obter dados que indiquem as vantagens e os inconvenientes de cada um dos métodos, assim como os resultados comparados das estimações obtidas por cada um deles. No fim, apresentam-se conclusões e propostas específicas para o desenvolvimento de novos estudos.
PRODEP III

In the ocean, natural and artificial processes generate clouds of bubbles which scatter and attenuate sound. Measurements have shown that at the individual bubble resonance frequency, sound propagation in this medium is highly attenuated and dispersive. Theory to explain this behavior exists in the literature, and is adequate away from resonance. However, due to excessive attenuation near resonance, little experimental data exists for comparison. An impedance tube was developed specifically for exploring this regime. Using the instrument, unique phase speed and attenuation measurements were made for void fractions ranging from 6.2 × 10^−5 to 2.7 × 10^−3 and bubble sizes centered around 0.62 mm in radius. Improved measurement speed, accuracy and precision is possible with the new instrument, and both instantaneous and time-averaged measurements were obtained. Behavior at resonance was observed to be sensitive to the bubble population statistics and agreed with existing theory, within the uncertainty of the bubble population parameters. Scattering from acoustically compact bubble clouds can be predicted from classical scattering theory by using an effective medium description of the bubbly fluid interior. Experimental verification was previously obtained up to the lowest resonance frequency. A novel bubble production technique has been employed to obtain unique scattering measurements with a bubbly-liquid-filled latex tube in a large indoor tank. The effective scattering model described these measurements up to three times the lowest resonance frequency of the structure.
United States Navy Office of Naval Research Ocean Acoustics Program

It is difficult to predict the sound radiation from large factory roofs. The existing infinite panel theories of sound insulation are not sufficient when the sound radiates at grazing angles. It has been shown that the reason for the collapse of the theory is the well known result for the radiation efficiency. This research will present a simple analytic strip theory, which agrees reasonably well with numerical calculations for a rectangular panel. Simple analytic strip theory has lead to the conclusion that it is mainly the length of the panel in the direction of radiation, rather than its width that is important in determining its radiation efficiency. The findings of the current research also indicated that apart from the effect due to coincidence, a panel was non-directional compared to an opening.

The health, safety, comfort and productivity of a room’s occupants is greatly influenced by the sound field within it. An acoustical engineer is often consulted during the design of a room to prevent or alleviate unwanted acoustical problems. Prediction models are often used to find the most cost-effective solution to a given acoustical problem. The accuracy of sound-field prediction varies with the particular model, as do the parameters predicted. Most models only predict sound-pressure levels. Many only predict energetic quantities, ignoring wave phase and, therefore, interference and modal effects in rooms. A ray-tracing model, capable of predicting sound-pressure level, reverberation time and lateral energy fraction was translated into MATLAB code and modified to increase accuracy by including phase. Modifications included phase effects due to path length travelled and phase changes imparted by surface reflections as described by complex reflection coefficients. Further modifications included predicting steady-state and transient sound-intensity levels, providing information on the direction of sound-energy flow. The modifications were validated in comparison with free-field theory and theoretical predictions of sound fields in the presence of a single surface. The complex reflection coefficients of four common building materials were measured using two methods—an impedance tube and the spherical-decoupling method. Using these coefficients, the modified program was compared with experimental data measured in configurations involving one or more surfaces made of these materials, in an anechoic chamber, a scale-model room, and a full-scale office space. Prediction accuracy in the anechoic chamber, and in the presence of a single reflecting surface, greatly improved with the inclusion of phase. Further comparison with full-scale rooms is required before the accuracy of the model in such rooms can be evaluated definitively.

Thesis (M.S. in Engineering Acoustics)--Naval Postgraduate School, December 1990.
Thesis Advisor(s): Coppens, Alan B. ; Sanders, James V. "December 1990." Description based on title screen as viewed on March 31, 2010. DTIC Identifier(s): Sound Pressure, Sound Transmission, Transmission Loss, Parabolic Equation Models, Computerized Simulation, Underwater Acoustics, Acoustic Velocity, Ocean Bottom, Ocean Models, Theses. Author(s) subject terms: Image Method, Parabolic Equation Model, Wedge-shaped Ocean. Includes bibliographical references (p. 37). Also available in print.

Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2009
In acoustics, pulse echo methods are well known as a means of measuring time of Hight. Traditional techniques for generating acoustic waves in solid ferromagnetic waveguides include piezoelectric, capacitive and magnetostriction. Piezoelectric and capacitive techniques are preferred due to the inefficiency of magnetostriction caused by electro-mechanical coupling losses and the fact that most ferromagnetic materials show low levels of magnetostriction. The aim of this study was to optimise the magnetostrictive effects for sensing applications based on a ferromagnetic waveguide using the pulse echo method. The results obtained were implemented in the design of an acoustic thermometer. Two configurations for signal generation and recovery were examined, the use of a single wound copper coil acting as a transceiver coil, and the use of separate transmit and receive coils. Results obtained using the latter configuration indicated better signal to noise ratio's and provided the flexibility to manipulate the point of signal recovery. The pulse echo method was implemented and optimised. An acoustic thermometer based on an existing design was developed by inducing a partial reflection from a set position in the waveguide, defining a sensing probe. Awareness of the elastic properties of the waveguide material enabled the guaging of its temperature by measuring the acoustic pulse velocity in the probe. The accuracy of the instrument was increased through signal conditioning, examined together with cross correlation and an increased sampling frequency. Systematic errors were resolved through calibration, giving the instrument an overall accuracy of ±O.56"C for the range of temperatures between 2O"C and 400"C.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Rogers, P. H.; Committee Member: Ferri, A. A.; Committee Member: Ruzzene, M.; Committee Member: Smith, M. K.; Committee Member: Trivett, D.; Committee Member: Zinn, B. T. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The development of a novel type of resonant sound absorber is described, designed specifically for the absorption of sound at low frequencies and at high intensities. A review of previous work on resonant sound absorbers shows that existing theoretical models for describing the phenomena are incomplete and that there remains a need for further extensions to the relevant models to cater for non-linear effects which become particularly important at high intensities. It is also shown that there are limitations to the current methods for testing absorbers which make them less suitable at low frequency and when the signals are nonharmonic. In the present work a theoretical model of a Helmholtz type of absorption resonator working in the nonlinear regime is developed using nonlinear hydrodynamic equations for viscous incompressible fluids. The model is able to predict the input impedance, the resonant frequency and the absorption coefficient of the device under nonlinear conditions. The model is also used to account for "difference frequency" generation, the reflection of signals of any shape from the surface, and the isolation characteristics of this type of absorber when a porous layer is fixed to the back of the resonant cavity. A new method for the development of acoustic characteristic~ of resonant sound absorbers is described, which was developed as part of the present work and which overcomes the limitations of existing methods. Measurements carried out using this and other techniques show that the acoustic characteristics of the absorber described are very close to those predicted from the model. A new type of packless absorber (one without fibrous material) based on the above theoretical model is described. Such an absorber has been constructed and tested in a reverberant sound field and is shown to provide effective sound absorbtion under conditions typical of a working industrial environment.

Detta är en interventionsstudie av hur rumsakustiken i ett klassrum påverkar en lektion beträffande tidseffektivisering samt taluppfattbarhet. Studien jämför resultat av olika mätningar och tester före och efter en rumsakustisk åtgärd i form av ett nedpendlat akustiktak. Åtgärden utförs i ett klassrum på S:t Hansskolan i Visby, i syfte att undersöka huruvida en förbättring av ljudmiljön i klassrummet kan ge positiva samhällsekonomiska effekter. De mätningar och tester som genomförts ger tillsammans en bild av klassrummets akustiska egenskaper före och efter åtgärden. Mätningarna innefattar tidtagning av uppstartstid av en lektion samt olika rumsakustiska mätningar och tester beträffande taluppfattbarhet. Mätningar visar att klassrummets ljudmiljö efter den rumsakustiska åtgärden har förbättrats med kortare efterklangstider, förbättrade förhållanden med rummets reflexer samt ett förbättrat talöverföringsindex. Även ett lyssningstest har genomförts och resultaten visar att taluppfattbarheten har ökats. Lärarens tidtagning visar att uppstartstiden förkortats med åtminstone 15 minuter per vecka. Sammantaget visar studien att den rumsakustiska åtgärden inneburit en ökning i nyttjad lektionstid som dessutom blivit mer effektiv genom en uppmätt ökad taluppfattbarhet. Observerade nyttoeffekter anses väga tyngre än investeringskostnaden till den grad att installationen rekommenderas till befintliga skolmiljöer och inte bara vid nyproduktion.
This is an interventional study of how classroom acoustics affects a class in regards to time effectiveness and speech intelligibility. The study compares results of measurements and tests before and after acoustical treatments in the form of a new acoustic ceiling. The treatments are made in a classroom in S:t Hansskolan elementary school, with the aim to see whether an improvement in room acoustics can lead to socio economic gains through an increase in the effective time for a lesson. The combined measurements and tests show the state of the acoustical environment before and after the treatments. Measurements includes the time it takes to get a lesson going, as well as various acoustical measurements and tests regarding speech intelligibility. Results shows that the room acoustics have improved with reduced reverberation times, an increase in early reflexes compared to late, as well as improved speech transmission index values. The class also scores higher on hearing in noise tests, which implies an increase in speech intelligibility. The teacher’s timekeeping shows that the time it takes to start classes has shortened by at least 15 minutes per week. On a whole the study shows that the acoustic treatments has led to an increase in use of planned time for each lesson, which through increased speech intelligibility also has become more effective. Observed socio economic effects outweigh the cost of the installation to the point that it is recommended not only to new classrooms but also to existing school environments.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2005.
Berthelot, Yves, Committee Member ; Cunefare, Kenneth A, Committee Chair ; Lynch, Christopher, Committee Member. Includes bibliographical references.

This article focuses on computer noise analysis and noise reduction processing. With the popularity of computers, people are increasingly demanding the comfort of using computers. Solving the noise problem of the computer case can make the working environment more comfortable. People working in a noisy environment for a long time can cause anxiety and the quality of work is not high. The main purpose of this paper is to analyse the characteristics of computer noise and to reduce the noise of the chassis through the secondary sound source. Through the comparison of the experimental and simulation results, the noise reduction effect of the secondary sound source on the computer case is obtained. This paper can provide a scientific reference for the manufacture of computer chassis and improvement of noise.

Audio signal processing systems for converting two-channel (stereo) recordings to four or five channels are increasingly relevant. These audio upmixers can be used with conventional stereo sound recordings and reproduced with multichannel home theatre or automotive loudspeaker audio systems to create a more engaging and natural-sounding listening experience. This dissertation discusses existing approaches to audio upmixing for recordings of musical performances and presents specific design criteria for a system to enhance spatial sound quality. A new upmixing system is proposed and evaluated according to these criteria and a theoretical model for its behavior is validated using empirical measurements.
The new system removes short-term correlated components from two electronic audio signals using a pair of adaptive filters, updated according to a frequency domain implementation of the normalized-least-means-square algorithm. The major difference of the new system with all extant audio upmixers is that unsupervised time-alignment of the input signals (typically, by up to +/-10 ms) as a function of frequency (typically, using a 1024-band equalizer) is accomplished due to the non-minimum phase adaptive filter. Two new signals are created from the weighted difference of the inputs, and are then radiated with two loudspeakers behind the listener. According to the consensus in the literature on the effect of interaural correlation on auditory image formation, the self-orthogonalizing properties of the algorithm ensure minimal distortion of the frontal source imagery and natural-sounding, enveloping reverberance (ambiance) imagery.
Performance evaluation of the new upmix system was accomplished in two ways: Firstly, using empirical electroacoustic measurements which validate a theoretical model of the system; and secondly, with formal listening tests which investigated auditory spatial imagery with a graphical mapping tool and a preference experiment. Both electroacoustic and subjective methods investigated system performance with a variety of test stimuli for solo musical performances reproduced using a loudspeaker in an orchestral concert-hall and recorded using different microphone techniques.
The objective and subjective evaluations combined with a comparative study with two commercial systems demonstrate that the proposed system provides a new, computationally practical, high sound quality solution to upmixing.