Interactive auralization based on hybrid simulation methods and plane wave expansion

The reconstruction and reproduction of sound fields have been extensively researched in the last decades leading to an intuitive approach to estimate and evaluate the acoustic properties of enclosures. Applications of auralization can be found in acoustic design, subjective tests, virtual reality and entertainment, among others. Different methodologies have been established to generate auralizations for room acoustics purposes, the most common of them, the use of geometrical acoustics and methods based on the numerical solution of the wave equation to synthesize the room impulse responses. The assumptions and limitations of each approach are well known, which in turn, restrict their application to specific frequency bands. If the aim is to reconstruct accurately the sound field in an extended range of frequencies, a combination of these methodologies has to be performed. Furthermore, recent advances in computational power have enabled the possibility to generate interactive atmospheres where the user is able to interact with the environment. This feature, although it expands the applications of the auralization technique, is nowadays mainly based on geometrical acoustics or interpolation methods. The present research addresses the generation of interactive broadband auralizations of enclosures using a combination of the finite element method and geometrical acoustics. For this, modelling parameters for both simulation methods are discussed making emphasis on the assumptions made in each case. Then, the predicted room impulse responses are represented by means of a plane wave expansion, which in turn, enables interactive features such as translation and rotation of the acoustic fields. An analytical expression is derived for the translation in the plane wave domain. Furthermore, the transformation of the plane wave representation in terms of spherical harmonics is also explored allowing the acoustic fields to be rotated. The effects of assuming a plane wave propagation within small enclosures and the consequences of using a finite number of plane waves to synthesize the sound fields are discussed. Finally, an implementation of an interactive auralization system is considered for different reference cases. This methodology enables reconstruction of the aural impression of enclosures in real-time with higher accuracy at low frequencies compared to only geometrical acoustics techniques. The plane wave expansion provides a convenient sound field representation in which the listener can interact with the acoustics of the enclosure. Furthermore, the sound reconstruction can be performed by implementing several sound reproduction techniques extending the versatility of the proposed approach

Micromachined Ultrasonic Transducers for 3-D Imaging

Real-time ultrasound imaging is a widely used technique in medical diagnostics. Recently, ultrasound systems offering real-time imaging in 3-D has emerged. However, the high complexity of the transducer probes and the considerable increase in data to be processed compared to conventional 2-D ultrasound imaging results in expensive systems, which limits the more wide-spread use and clinical development of volumetric ultrasound. The main goal of this thesis is to demonstrate new transducer technologies that can achieve real-time volumetric ultrasound imaging without the complexity and cost of state-of-the-art 3-D ultrasound systems. The focus is on row-column addressed transducer arrays. This previously sparsely investigated addressing scheme offers a highly reduced number of transducer elements, resulting in reduced transducer manufacturing costs and data processing. To produce such transducer arrays, capacitive micromachined ultrasonic transducer (CMUT) technology is chosen for this project. Properties such as high bandwidth and high design flexibility makes this an attractive transducer technology, which is under continuous development in the research community. A theoretical treatment of CMUTs is presented, including investigations of the anisotropic plate behaviour and modal radiation patterns of such devices. Several new CMUT fabrication approaches are developed and investigated in terms of oxide quality and surface protrusions, culminating in a simple four-mask process capable of producing 62+62-element row-column addressed CMUT arrays with negligible charging issues. The arrays include an integrated apodization, which reduces the ghost echoes produced by the edge waves in such arrays by 15:8 dB. The acoustical cross-talk is measured on fabricated arrays, showing a 24 dB reduction in cross-talk compared to 1-D arrays for 2-D imaging. Volumetric imaging is successfully demonstrated using a beamformer specifically developed for row-column addressed arrays. Furthermore, a technique for estimating flow velocities in all three dimensions is presented. Based on the developed techniques, a complete hand-held 3MHz λ/2-pitch ultrasound probe for volumetric imaging with 62+62 elements and in-handle electronics is produced and used on a commercial bk3000 scanner from BK Medical. The scanner is made for conventional 2-D ultrasound imaging, proving that the developed technology enables realtime volumetric ultrasound imaging with a total system cost and complexity equivalent to that of 2-D ultrasound imaging systems