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This thesis describes the challenges involved in detecting sources of acoustic noise using an autonomous underwater vehicle (AUV) in real world environments. The initial part of this thesis describes the developments made for redesigning an acoustic sensing system that can be used to estimate the relative bearing between a source of acoustic noise and an AUV. With an estimate of the relative bearing, the AUV can maneuver toward the source of noise. The class of algorithms that are used to estimate bearing angle are known as beamforming algorithms. A comparison of the performance of a variety of beamforming algorithms is presented. When estimating the bearing to a source of noise from a small AUV, the noise of the AUV, especially its propulsor, pose significant challenges. Toward the goal of active cancellation of AUV self-noise, we propose placing an additional hydrophone inside the AUV in order to estimate the AUV self-noise that appears on the exterior hydrophones that are used for bearing estimation. / Master of Science / A real world application using an autonomous underwater vehicle (AUV) is presented in this thesis. The application deals with detecting and estimating the relative location (bearing angle) between sources of acoustic noise and the AUV. The thesis starts by describing design changes made to target data sensing system inside the AUV for collecting and estimating the bearing angle. The estimation of bearing angle is done with a class of algorithms called beamforming algorithms whose performance comparison is presented on real world data. Operating the AUV propulsor yields inaccurate bearing angle estimations and thus presents a huge challenge for bearing estimation. We propose measuring AUV self-noise using additional sensors to move towards the goal of cancelling AUV self-noise and recovering target signal for accurate bearing estimation.

Understanding speech in noisy environments is a challenge for normal-hearing and impaired-hearing listeners alike. However, it has been shown that speech intelligibility can be improved in these situations using a strategy called the ideal binary mask. Because this approach requires knowledge of the speech and noise signals separately though, it is ill-suited for practical applications. To address this, many algorithms are being designed to approximate the ideal binary mask strategy. Inevitably though, these algorithms make errors, and the implications of these errors are not well-understood. The main contributions of this thesis are to introduce a new framework for investigating binary masking algorithms and to present listener studies that use this framework to illustrate how certain types of algorithm errors can affect speech recognition outcomes with both normal-hearing listeners and cochlear implant recipients.

The sound transmission loss characteristics of plywood based sandwich panels were investigated. Measurements were made of the sound transmission loss of a range of materials and used as a baseline for comparison while a sound transmission loss optimisation method was developed. A unique test rig was built and calibrated to determine selected mechanical properties of materials of interest. The results of sound transmission loss and material properties measurements were used to select an appropriate prediction model, which was then used in conjunction with a mathematical optimisation model to determine combinations of materials and panel parameters which result in improved sound transmission loss. An effort was made to reproduce these predictions in experimental testing by constructing several prototype panels.

The Dielectric Barrier Discharge (DBD) plasma actuators have received a significant attention of many researchers in the last few decades. The main focus of these studies has been on the flow control areas such as turbulent boundary layer separation and turbulent skin friction reduction. Little attention has been paid on the effect of the DBD plasma actuators on the aerodynamic noise reduction. In this regard, the aim of the current work is to investigate the effect of the DBD plasma actuator driven at relatively low voltages on vortex-induced noise. The first part of the current work includes an extensive assessment of the effect of the DBD plasma actuator on the narrowband tonal noise radiated from a flat plate with blunt trailing edge and an airfoil (NACA 0012) with blunt and cut-in type serrated trailing edge. The measurements were carried out at Reynolds numbers between 0.75 x 10 to the power of 5 and 4 x 10 to the power of 5. It is found that the DBD plasma actuator effectiveness depends on the direction of the generated electric wind. For example, a high reduction in the narrowband tonal noise level is achieved when a direct streamwise electric wind is injected into the wake region. However, using a plasma actuator, which can induce streamwise vortices into the wake region, shows more superior noise reduction capability at lower voltages. Flow measurement results revealed that the mechanism responsible for the narrowband tonal noise reduction when the electric wind is directly injected into the wake is not due the momentum injection into the wake deficit. Rather, the streamwise jet isolates the two separated shear layers and prevents the interaction between them. On the other hand, it is found that the break-up of the spanwise coherence of the vortex shedding is responsible for the significant reduction in the tonal noise level when the spanwise actuation is used. The second part of the current work comprises the effect of the DBD plasma actuator on both the narrowband tonal noise and interaction broadband noise radiated from both single and tandem cylinder, respectively. The experiments were conducted at subcritical Reynolds number ReD = 1.1 x 10 to the power of 4. The actuators were positioned at different azimuthal angles 27° ≤ θj ≤ 153°. For the single cylinder case, the acoustic results show the DBD plasma actuator that is positioned at θj = 133° leads to a more reduction in the narrowband tonal noise level when compared to the other angles. It is found that the streamwise jet produced by the plasma actuators plays an important role in prevention of the interaction between the shear layers that separates from the cylinder. For the tandem cylinders case, the acoustic results show that the simultaneous actuation of both the upstream and the downstream cylinders leads to more reduction in both the narrowband tonal noise and the interaction broadband noise level compared with the case where only the upstream or the downstream cylinder is actuated. The mechanism responsible for this noise reduction is found to be mainly due to the streamwise jet induced by the upstream cylinder activation, which delays the vortex shedding formation and reduces the turbulence intensity in the near wake region. On the other hand, the plasma induced jet against the main-flow direction works as a virtual fluidic barrier which displaces the wake produced by the upstream cylinder away from the downstream cylinder.

Accurate assessment of the electromagnetic excitation forces and the vibration behaviour of stators is essential to arrive at a suitable design for quiet operation of an electrical machine. These electromagnetic forces are produced on the surfaces of the stator and rotor along the air-gap periphery. The physical mechanisms underlying the production of the electromagnetic forces, and the characteristics of the ensuing vibrations are described in this thesis. <p>In this thesis, comprehensive analytical methods are developed for the determination of the radial electromagnetic forces in squirrel-cage and wound-rotor induction machines. Various magnetic fields are generated due to the distribution of the windings in slots, the slotting of the stator and rotor surfaces, and the magnetic saturation of the iron. The air-gap fields are determined using the permeance method by expressing the MMF and the air-gap permeance as waves. <p>In the analyses, the mutual interactions between the stator and rotor are incorporated. Asthe load increases, these interactions become prominent and influence significantly the nature of the air-gap field. Their effects on the electromagnetic forces are discussed from the perspective of the production of vibrations. Extensive investigations were conducted on a 7.5 kW squirrel-cage induction motor and a 70 kW wound-rotor induction motor to verify the validity of the analyses. The comparisons made between the experimental and the analytical results prove the general validity of the analytical methods. <p>Conclusions are drawn with a view to determine the actual role played by the harmonic air-gap fields on the production of the electromagnetic forces and the ensuing vibrations. In the course of these investigations, an experimental set-up which is particularly suited for the measurement of magnetic fields, magnetic forces, resonant frequencies, vibrations and noise was developed. A fundamental study on the vibration behaviour of electrical machine stators using an experimental modal-analysis is presented in the thesis. Modal-analysis is a process of forcing a structure to vibrate predominantly at a selected resonance. In order to achieve this, distributed electromagnetic forces are used.<p>Detailed investigations were conducted on the stator models of a 120 hp induction motor to study their vibration behaviour, and to critically examine the damping present in them. The physical interpretations of the results given in the thesis would help in acquiring a better understanding of the vibration behaviour of stators in relation to the problem of electromagnetic acoustic noise in electrical machines. This work was supported by the National Sciences and Engineering Research Council of Canada through grant no. OGP0004324.

With MEG, the tiny magnetic fields produced by neuronal currents within the brain can be measured completely non-invasively. But the signals are very small (~100 fT) and often obscured by spontaneous brain activity and external noise. So, a recurrent issue in MEG data analysis is the identification and elimination of this unwanted interference within the recordings. Various strategies exist to meet this purpose. In this thesis, two of these strategies are scrutinized in detail. The first is the commonly used procedure of averaging over trials which is a successfully applied data reduction method in many neurocognitive studies. However, the brain does not always respond identically to repeated stimuli, so averaging can eliminate valuable information. Alternative approaches aiming at single trial analysis are difficult to realize and many of them focus on temporal patterns. Here, a compromise involving random subaveraging of trials and repeated source localization is presented. A simulation study with numerous examples demonstrates the applicability of the new method. As a result, inferences about the generators of single trials can be drawn which allows deeper insight into neuronal processes of the human brain. The second technique examined in this thesis is a preprocessing tool termed Signal Space Separation (SSS). It is widely used for preprocessing of MEG data, including noise reduction by suppression of external interference, as well as movement correction. Here, the mathematical principles of the SSS series expansion and the rules for its application are investigated. The most important mathematical precondition is a source-free sensor space. Using three data sets, the influence of a violation of this convergence criterion on source localization accuracy is demonstrated. The analysis reveals that the SSS method works reliably, even when the convergence criterion is not fully obeyed. This leads to utilizing the SSS method for the transformation of MEG data to virtual sensors on the scalp surface. Having MEG data directly on the individual scalp surface would alleviate sensor space analysis across subjects and comparability with EEG. A comparison study of the transformation results obtained with SSS and those produced by inverse and subsequent forward computation is performed. It shows strong dependence on the relative position of sources and sensors. In addition, the latter approach yields superior results for the intended purpose of data transformation.

Στην παρούσα διπλωματική εργασία παρουσιάζεται αρχικά η θεωρία της ψηφιακής επεξεργασίας σημάτων, βάσει της οποίας δημιουργήθηκαν οι τέσσερις αλγόριθμοι αποθορυβοποίησης ηχητικών σημάτων που μελετήθηκαν. Συγκεκριμένα μελετήθηκε η γραμμική φασματική αφαίρεση, ο αλγόριθμος Boll, ο αλγόριθμος Berouti και ο αλγόριθμος του Τσουκαλά, στον οποίο έγιναν και κάποιες τροποποιήσεις για βελτιστοποίηση της ποιότητας του αποθορυβοποιημένου σήματος. Πραγματοποιήθηκαν μετρήσεις SNR, NMR για τέσσερις διαφορετικές τιμές SNR ενθόρυβου σήματος: -5 , 0 , 5 και 10 dB. Το ενθόρυβο σήμα δημιουργήθηκε τεχνητά από ένα καθαρό σήμα ομιλίας σε ανηχοϊκό περιβάλλον με την χρήση λευκού θορύβου. Μέσω των μετρήσεων αυτών έγινε μια σύγκριση της απόδοσης των αλγορίθμων. / This thesis presents initially the theory of digital signal processing, in the bases of which the four noise reduction algorithms that are analysed, were created. In particular the algorithms that are analysed are the linear spectral subtraction, the Boll algorithm, the Berouti algorithm and the algorithm of Dionisios Tsoukalas, to which some optimizations were made for better quality of the noise reduced signal. SNR and NMR measurements were taken for four different noisy signal SNR values: -5, 0, 5 and 10 dB. The noisy signal was artificially created from a clean speech signal in an anechoic environment with the use of white noise. The performance of the algorithms has been compared through this measurements.

Baigiamajame darbe nagrinėjamas konvejerių keliamas triukšmas bei galimybės, kaip jį sumažinti. Apžvelgiama dažniausiai maisto pramonėje naudojamų konvejerių tipai, veikimo principai. Aprašomos pagrindinės triukšmo kilimo priežastys grandininiuose konvejeriuose bei kaip galima sumažinti kylantį triukšmą. Atlikti trys eksperimentiniai tyrimai. Aprašyta tyrimų eiga ir sąlygos, pateikti gauti rezultatai. Darbo pabaigoje surašytos išvados, pateiktas literatūros sąrašas. Darbo apimtis 60 A4 formato lapai bei 4 A4 formato lapai priedų. Baigiamasis darbas parašytas remiantis 36 bibliografiniais šaltiniais. / In this graduation thesis Conveyor noise and possibilities to reduce it are investigated. Common conveyors in food industry, their types and operating principles are reviewed. Main factors causing noise in chain conveyors and ways of reducing them are described. Three experimental investigations were completed. Investigation process, conveyoring line conditions are described, results presented. There are conclusion and used literature list written down at the end of thesis. Thesis includes 60 A4 format pages and 4 A4 format pages of supplement. 36 bibliographical sources are used.

A study of methods for reducing the acoustic noise in magnetic resonance imaging (MRI) scanners is presented in this thesis. The structural-acoustic coupling mechanism of MRI scanners was investigated using a method of structural-acoustic modal analysis. Mathematical expressions of generalized radiation impedances of gradient coil ducts with perforated panel inserts were developed and the effects of the perforated panel inserts on the acoustic noise in the duct were discussed. The possibility of using micro-perforated panel (MPP) absorbers in MRI scanners to reduce the acoustic noise was then investigated through analytical and computational modeling. A comprehensive experimental study was conducted after the analytical and computational investigation. Finally, design methods and procedures were developed specifically for the MPP absorbers in MRI scanners. Design considerations and recommendations were given as well. Several major conclusions can be made from this research. Firstly, the method of structural-acoustic modal analysis is effective for finding the structural-acoustic coupling modes which should be avoided in the design of MRI scanners. Secondly, a perforated panel insert produces significant effects on the radiation impedance of gradient coil ducts and MRI scanner bores. This attribute partly contributes to its capability of reducing the acoustic noise in a duct. Thirdly, the effectiveness of MPP absorbers in MRI scanners can be accurately predicted using a combination of theoretical analysis and computational modeling. Moreover, it has been proved that well designed MPP absorbers are effective in reducing the acoustic noise in MRI scanners. Lastly, the presented design methods and recommendations for the MPP absorbers can be relatively easily used by MRI designers or engineers to tackle the acoustic noise problem in MRI scanners. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2010-07-13 11:46:03.585