Dissertations / Theses on the topic 'High frequency sound'

This work sets forth a new method for predicting the spatialvariation of mean square pressure within two-dimensionalenclosures containing high-frequency broadband sound fieldsand light to moderate absorption. In the new method, theenclosure boundaries are replaced by a continuousdistribution of broadband uncorrelated sources, each ofwhich provides a constituent field expressed in terms ofmean square pressure and time average intensity variables.Superposition of these fields leads to the overall meansquare pressure and time average intensity as a function ofposition. Boundary conditions for radiating and absorbingsurfaces are recast in terms of energy and intensityvariables. The approach is implemented as a boundaryelement formulation for efficient evaluation of the pressureand intensity fields in enclosures. In contrast totraditional boundary element methods, the new method isindependent of frequency. A two-dimensional model problemenclosure is investigated to verify the new method. The exact analytical solution for the mean square pressuredistribution within the model problem enclosure is obtainedand compared to the results predicted by the new method.The comparisons indicate that the new method is asignificant improvement upon classical diffuse field theoryand computationally efficient relative to traditional boundary element methods and ray tracing techniques.

The Sensor Research Laboratory (SRL) at Naval Postgraduate School (NPS) has developed a micro-electromechanical system (MEMS) based directional sound sensors that mimics the aural system of the Ormia Ochracea Fly. The goal of this research is to characterize a set of directional sound sensors with varying configurations that operate in the high frequency range (15?20 kHz). The sensor consists of two identical wings coupled in the middle and the entire structure is connected to a substrate using two legs in the middle. In response to sound, the coupled wings oscillate with rocking and bending like motions at frequencies that depend on the mechanical characteristics of the structure. A simulation of sensor characteristics using COMSOL finite element software showed a resonant frequency of about 20 kHz for each device. The devices were fabricated by the MEMSCAP foundry service using silicon-on-insulator (SOI) substrate with a 25 ?m device layer. Using a laser vibrometer, response to incident sound pressure was measured at different frequencies and angles. All the devices showed that measured and simulated frequencies were in reasonably close agreement. The measurements showed good sensitivity to the direction of sound as predicted.

This thesis aims to investigate the acoustic properties of ultrasound contrast agents (UCAs) at high ultrasound frequencies. In recent years, there has been increasing development in the use of high frequency ultrasound in the fields of preclinical, intravascular, ophthalmology and superficial tissue imaging. Although research studying the acoustic response of UCAs at low diagnostic ultrasonic frequencies has been well documented, quantitative information on the acoustical properties of UCAs at high ultrasonic frequencies is limited. In this thesis, acoustical characterisation of three UCAs was performed using a preclinical ultrasound scanner (Vevo 770, VisualSonics Inc., Canada). Initially the acoustical characterisation of five high frequency transducers was measured using a membrane hydrophone with an active element of 0.2 mm in diameter to quantify the transmitting frequencies, pressures and spatial beam profiles of each of the transducers. Using these transducers and development of appropriate software, high frequency acoustical characterisation (speed and attenuation) of an agar-based tissue mimicking material (TMM) was performed using a broadband substitution technique. The results from this study showed that the acoustical attenuation of TMM varied nonlinearly with frequency and the speed of sound was approximately constant 1548m·s-1 in the frequency range 12-47MHz. The acoustical properties of three commercially available lipid encapsulated UCAs including two clinical UCAs Definity (Lantheus Medical Imaging, USA) and SonoVue (Bracco, Italy) and one preclinical UCAs MicroMarker (untargeted) (VisualSonics, Canada) were studied using the software and techniques developed for TMM characterisation. Attenuation, contrast-to-tissue ratio (CTR) and subharmonic to fundamental ratio were measured at low acoustic pressures. The results showed that large off-resonance and resonant MBs predominantly contributed to the fundamental response and MBs which resonated at half of the driven frequency predominantly contributed to subharmonic response. The effect of needle gauge, temperature and injection rate on the size distribution and acoustic properties of Definity and SonoVue was measured and was found to have significant impacts. Acoustic characterisations of both TMM and UCAs in this thesis extend our understanding from low frequency to high frequency ultrasound and will enable the further development of ultrasound imaging techniques and UCAs design specifically for high frequency ultrasound applications.

The first part of the study was to characterise the acoustic properties of an IEC agar-based tissue mimicking material (TMM) at ultrasound frequencies centred around 20 MHz. The TMM acoustic properties measured were the amplitude attenuation coefficient (dB cm-1MHz-1), the sound speed (ms-1) and the backscattered power spectral density characteristics of spectral slope (dB MHz-1), y-axis intercept (dB) and reflected power (dB). The acoustic properties were measured over a temperature range of 22 - 37oC. Both the attenuation coefficient and sound speed, both group and phase, showed good agreement with the expected values of 0.5 dB cm-1 MHz-1 and 1540 ms-1 respectively with average values of 0.49 dB cm-1MHz-1 (st.dev. ± 0.03) and 1541.9 ms-1 (st.dev. ± 8.5). Overall, this non-commercial agar-based TMM was shown to perform as expected at the higher frequency range of 17-23 MHz and was seen to retain its acoustic properties of attenuation and speed of sound over a three year period. For the second part of the study, composite sound speed was measured in carotid plaque embedded in TMM. The IEC TMM was adapted to a clear agar gel. The contour maps from the attenuation plots were used to match the composite sound speed data to the photographic mask of plaque outline and thus the histological data. By solution of sets of simultaneous equations using a matrix inversion, the individual speed values for five plaque components were derived; TMM, elastin, fibrous/collagen, calcification and lipid. The results for derived sound speed in the adapted TMM were consistently close to the expected value of soft tissue, 1540 ms-1. The fibrous tissue showed a mean value of 1584 ms-1 at body temperature, 37oC. The derived sound speeds for elastic and lipid exhibited large inter-quartile ranges. The calcification had a significantly higher sound speed than the other plaque components at 1760 - 2000 ms-1.

Interaural Timing Differences (ITDs) are a cue for sound localisation. In response to low-frequency sounds, sensitivity to ITDs can be conveyed by the fine-structure of the sound waveform. In response to high-frequency sounds, sensitivity to ITDs can only be conveyed by the amplitude modulated envelope of the sound waveform. Sensitivity to ITDs within high-frequency sounds has classically been described as poorer than in response to low-frequency sounds. However, using a "transposed" sound stimulus, it has been shown that human sensitivity to ITDs in high-frequency sounds can be equivalent to sensitivity to ITDs in low-frequency sounds. In the present study, sensitivity to ITDs was investigated in the responses of neurons from the Inferior Colliculus of the guinea pig using transposed, and conventional, stimuli. A neural correlate of the improvement in sensitivity to ITDs provided by transposed tones was found. ITD-tuning functions had greater depths of modulation in response to transposed tones as compared to conventional stimuli, and neural discrimination thresholds for ITDs in transposed tones were similar to those obtained in response to low-frequency tones. Neural coding of ITDs at low frequencies has been shown to depend on a neuron's frequency tuning. Therefore, the responses of neurons were examined for evidence of frequency-dependent tuning to ITDs in the envelope of high-frequency stimuli. The frequency-dependent ITD-tuning that was found contradicts a model of ITD coding proposed in 1948 by Jeffress. ITD-coding at high-frequencies, similarly to at low- frequencies, may use a population of neurons which are broadly tuned to ITDs. It is suggested that sensitivity to ITDs in the envelope of high-frequency sounds is restricted both by peripheral processing and also by an upper fm above which sensitivity to ITDs does not occur. For these reasons, the physiological relevance of sensitivity to ITDs in the envelope of high-frequency may be limited.

This thesis is concentrated on mathematical modeling of high frequency pulsations in pump turbines, which are the source of high-cycle continuous stress of the spiral casing cover, wicket gates and runner. There are proposed the solutions using the transfer matrix for the tube with a constant and conical cross-section. The paper compares variations of cylindrical and conical tubes, changes in boundary conditions. There are the models of PSPP Dlouhé Stráně made only of cylindrical tubes comparing to the model with cylindrical and conical tubes

付記する学位プログラム名: 霊長類学・ワイルドライフサイエンス・リーディング大学院
Kyoto University (京都大学)
0048
新制・課程博士
博士(理学)
甲第22298号
理博第4612号
新制||理||1661(附属図書館)
京都大学大学院理学研究科生物科学専攻
(主査)教授 幸島 司郎, 教授 平田 聡, 教授 伊谷 原一
学位規則第4条第1項該当

La surveillance par acoustique passive est un outil d'importance croissante pour l'étude des mammifères marins. Cette thèse pose des nouveaux modèles pour l'étude du plus grand d'entre eux, la baleine bleue (Balaenoptera musculus), qui émet de très basses fréquences. Pour ce faire, nous avons enregistré un corpus inédit dans l'archipel de Humboldt au nord du Chili. Nos données révèlent un chant caractéristique de la baleine bleue 'chilienne', dont nous étudions la structure pulsée et l'évolution au cours des dernières décennies. Le classement en signal tonal ou non-tonal nous permet, en nous focalisant sur la fréquence fondamentale mise à jour, de quantifier la baisse en fréquence des chants et d'effectuer une comparaison des signaux au niveau mondial. Notre troisième contribution est une méthode de localisation mono-hydrophone basée sur des simulations de propagation par éléments spectraux. C'est à notre connaissance le premier modèle de ce type, implémenté en milliers d'heures de calcul haute performance. Nous concluons en soulignant l'intérêt des méthodes en bioacoustique comme moyen de suivi et de connaissance du milieu marin
Passive acoustic monitoring has a growing importance in marine mammals studies. This work is concerned with the largest of marine mammals, the blue whale (Balaenoptera musculus). We obtained a new corpus of acoustic data in the northern part of Chile, in the Humboldt archipelago. We show the presence of a song characteristic of the 'Chilean' blue whale, formerly described in southern Chile and Galapagos islands. Based on this sang type, we propose new methods of analysing and classifying pulsed sounds. Using the fundamental frequency thus obtained, we analyse the blue whale's sang, showing a general evolution of the frequency on a decadal scale. We also construct a method of mono­hydrophone source localisation based on high performance simulation of the acoustic wave field, by spectral elements methods. We conclude emphasizing on the importance of bioacoustic for monitoring the marine world

Muchos tipos de señales que encontramos a diario pertenecen a la categoría de sinusoides no estacionarias. Una gran parte de esas señales son sonidos que presentan una gran variedad de características: acústicos/electrónicos, sonidos instrumentales harmónicos/impulsivos, habla/canto, y la mezcla de todos ellos que podemos encontrar en la música. Durante décadas la comunidad científica ha estudiado y analizado ese tipo de señales. El motivo principal es la gran utilidad de los avances científicos en una gran variedad de áreas, desde aplicaciones médicas, financiera y ópticas, a procesado de radares o sonar, y también a análisis de sistemas. La estimación precisa de los parámetros de sinusoides no estacionarias es una de las tareas más comunes en procesado digital de señales, y por lo tanto un elemento fundamental e indispensable para una gran variedad de aplicaciones. Las transformaciones de tiempo y frecuencia clásicas son solamente apropiadas para señales con variación lenta de amplitud y frecuencia. Esta suposición no suele cumplirse en la práctica, lo que conlleva una degradación de calidad y la aparición de artefactos. Además, la resolución temporal y frecuencial no se puede incrementar arbitrariamente debido al conocido principio de incertidumbre de Heisenberg. \\ El principal objetivo de esta tesis es revisar y mejorar los métodos existentes para el análisis de sinusoides no estacionarias, y también proponer nuevas estrategias y aproximaciones. Esta disertación contribuye sustancialmente a los análisis sinusoidales existentes: a) realiza una evaluación crítica del estado del arte y describe con gran detalle los métodos de análisis existentes, b) aporta mejoras sustanciales a algunos de los métodos existentes más prometedores, c) propone varias aproximaciones nuevas para el análisis de los modelos sinusoidales existentes i d) propone un modelo sinusoidal muy general y flexible con un algoritmo de análisis directo y rápido.
Many types of everyday signals fall into the non-stationary sinusoids category. A large family of such signals represent audio, including acoustic/electronic, pitched/transient instrument sounds, human speech/singing voice, and a mixture of all: music. Analysis of such signals has been in the focus of the research community for decades. The main reason for such intense focus is the wide applicability of the research achievements to medical, financial and optical applications, as well as radar/sonar signal processing and system analysis. Accurate estimation of sinusoidal parameters is one of the most common digital signal processing tasks and thus represents an indispensable building block of a wide variety of applications. Classic time-frequency transformations are appropriate only for signals with slowly varying amplitude and frequency content - an assumption often violated in practice. In such cases, reduced readability and the presence of artefacts represent a significant problem. Time and frequency resolu

The interest in applications of nanomaterials for acoustic absorption purposes is growing rapidly with advances in nanotechnology. A need also exists for a simulation framework that is applicable for modelling acoustic absorption in nanomaterials in order to develop an understanding of nanoscopic acoustic absorption mechanisms. The current study investigates the acoustic absorption characteristics of a carbon nanotube (CNT) acoustic absorber to develop an understanding of the absorption behaviour and mechanisms of the CNTs. This task involves undertaking an exploratory study of the absorption characteristics of a CNT forest and modelling the absorption effects of the CNT at the nanoscale. The absorption characteristics of the CNTs were explored by studying the normal incidence absorption coefficient of 3mmand 6mm-long vertically aligned CNT arrays measured experimentally using the two-microphone impedance tube method, while the modelling of the absorption effects was performed using a non-continuum particle-based method. The experimental investigation showed promising results for the acoustic absorption capability of CNT acoustic absorbers and suggests that the absorption performance could be enhanced with longer CNTs and a lower spatial density of the nanotube arrays. The study of absorption using a theoretical model based on classical absorption mechanisms indicated that the absorption behaviour of nanomaterials is likely to deviate from continuum behaviour emphasising the necessity of acoustic modelling at the nanoscale using non-continuum methods. An examination of the physical phenomena that are likely to be relevant for simulating acoustic wave propagation in the presence of CNTs revealed that the modelling of such a system would be a multi-physics problem involving heat transfer and dynamic interaction of particle vibrations. A study of various particle approaches of non-continuum methods indicated that molecular dynamics (MD) is the method best suited to simulate and study the acoustic absorption of CNTs at the nanoscale. A survey of previous molecular simulations demonstrated that the MD simulations carried out thus far have not simultaneously accounted for all relevant aspects of the multi-physics problem required for modelling the acoustic absorption effects of CNTs. Hence, three independent validation studies were performed using MD simulations for modelling a subset of the relevant phenomena, namely fluid/structure interactions, bi-directional heat transfer, and acoustic wave propagation. Each of these MD simulations were performed for a model incorporating Lennard-Jones (LJ) potentials for the non-bonded interactions of gas and CNT atoms and the REBO potential for the CNT, and the results validated against the reference case studies. A molecular system was then designed to study acoustic wave propagation in a simple monatomic gas in a domain containing a 50nm-long CNT opposite to the sound source and parallel to the direction of the acoustic wave propagation. The simulation domain was modelled using argon gas as the wave propagation medium, a piston made of solid argon layers as a sound source, and a specular wall as the termination wall. MD simulations were also performed without the CNT present for comparison. The characteristics of the acoustic field were studied by evaluating the behaviour of various acoustic parameters and comparing the change in behaviour with frequency. The attenuation of the acoustic wave was estimated using thermodynamic exergy concepts and compared against standing wave theory and predictions from continuum mechanics. Similarly, the acoustic field characteristics and attenuation due to the CNT were studied using MD simulations incorporating the CNT. A standing wave model, developed for the domain with the CNT present, was used to predict the attenuation by the CNT and verified against estimates from exergy concepts. Comparison of the simulation results for acoustic wave propagation with and without the CNT present demonstrated that acoustic absorption effects in the presence of CNTs can be simulated using the developed MD simulation setup although the degree of absorption was not sufficient for the CNTs simulated to investigate absorption mechanisms. The modelled MD system can also be used to study deviations from continuum theory in the characteristics of high frequency sound. The study suggests that the investigation of absorption mechanisms in nanomaterials can be conducted using the developed platform for MD simulations, however further investigations are required to capture the loss mechanisms involved in the molecular interactions between the acoustic wave and the CNT. Additionally, to permit simulations in the audible frequency range, it is necessary to speed up the computational process by modifying the system model such as by employing a hybrid model with molecular dynamics coupled to a continuum domain.
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2016.