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The variable reluctance motor (VRM) is known for its structural simplicity, lower cost and capability of higher specific outputs compared with the AC induction motor. These favourable attributes arise from the way the torque is generated. An asymmetric half bridge would commonly be used as the controller for the VRM. It is also a relatively simple circuit because unlike the requirement with an AC induction motor, the imput current for a VRM would only need to be able to travel in one direction.

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

Magnetic resonance imaging (MRI) has revolutionized the field of cognitive neuroscience as it allows researchers to noninvasively map brain function in response to stimulus or task demands. However, the acquisition of MR images generates substantial acoustic noise, so that imaging studies of speech, language and hearing are problematic. One proven solution for reducing acoustic noise in MRI scanners is the use of micro-perforated panels placed in the bore of the scanner. They can be applied to existing scanners with minimal cost and are suitable for sterile environments. Although these panels result in quantifiably lower noise levels, measured with microphones in an empty MRI, the improvement has not been quantified with a patient in the scanner bore, which dramatically affects the acoustic noise field. This thesis tested the reduction of noise inside the MRI environment using a previously designed micro-perforated acoustic absorber panel. These panels resulted in quantifiably lower noise levels with a volunteer in the scanner bore, however the reduction was not sufficient for significant differences in volunteer perceptions. Volunteers were generally unable to perceive a difference in noise between scans with and without absorbers and no reduction of fatigue was observed. Also no significant change in cortical activity was found between scans done with and without absorbers during an auditory function MRI study. Further testing could include designing a micro-perforated acoustic absorber for a specific scan sequence for maximum attenuation. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-09-25 16:54:41.118

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Switched reluctance motors (SRMs) possess many desirable qualities for the long-term sustainability of electrified transportation such as cheap production costs and simple, robust configurations. However, high acoustic noise and torque ripple are two performance imperfections that have prevented the widespread implementation of SRMs. This thesis investigates design techniques to reduce the acoustic noise produced by an 8/6 SRM while also analyzing the impact each design has on the motor’s performance. The fundamentals of SRMs are discussed including the operating principles, modelling and control strategies. The multiphysics finite element analysis (FEA) toolchain used to accurately model acoustic noise and vibrations of SRMs is described. Using the network of FEA tools, nodal forces and natural frequencies of a four phase 8/6 SRM are analyzed to study the acoustic noise and vibration behaviours. The FEA process is validated experimentally by matching measured vibration modes and acoustic noise sound pressure level (SPL) with FEA numerical results. Through inspiration from an extensive literature review, various design techniques are applied to a baseline four phase 8/6 SRM and compared for both acoustic noise reduction and EM performance criteria. The investigated designs were split into two categories, stator-housing modifications that aim to increase the stiffness of the assembly and rotor modifications that aim to reduce the magnitude of radial forces while preserving performance. The best design strategies as determined by the comparative analysis were then further optimized to combine the best techniques together for the 8/6 SRM. The proposed structural improvements included the modifications of the stator yoke shape along with increasing the number of fastening components involved in the assembly. Additionally, an iterative procedure for the parametric modelling of windows introduced to the rotor poles is outlined. The best design considerations are combined to create the design of a novel 8/6 SRM which significantly reduces the acoustic noise produced by the motor with little impact to performance. / Thesis / Master of Applied Science (MASc)

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Darbo tikslas - aptarti akustinės taršos problemas ir ištirti triukšmo sulaikančių užtvarų saugos zonose aplinką psichologinio ir garso energijos ¬¬ sklaidos požiūriu. Akustinė aplinka triukšmą sulaikančių užtvarų saugos zonose apibūdinančios garso lauko amplitudinės dažninės charakteristikos kinta dėl garso bangų difrakcijos. Jos laipsni apibudinanti Fresnelio skaičių lemią užtvaros geometrinių dydžių ir atstumo iki šaltinio bei stebėjimo taškų santykis. Tyrimų rezultatų analizė rodo jog dėl garso bangų difrakcijos akustinio šešėlio zonoje 7-15 m nuotolyje nuo triukšmo sulaikančios užtvaros garso lygis sumažėja atitinkamai 11-9 dBA, o nuo želdinių 8-5 dBA. Dėl atstumo padidėjimo garso lygis už 7-15 m nuo triukšmą sulaikančių sienelių sumažėja 6-8 dBA ,o už želdinių 6-9 dBA. Dėl garso bangų difrakcijos už akustinio šešėlio zonos ribų 50 m nuotolyje nuo sienelės garso lygis sumažėja 5 dBA, o nuo kelio iškasos sumažėja 3 dBA, tuo tarpu dėl atstumo padidėjimo garso lygis sumažėja 14-15 dBA . Taip pat nustatyta, jog kokybės vertinimo požiūriu ir prieš ir už triukšmą sulaikančių užtvarų aplinka palankiai nepriskirtina, nes ženkliai viršijamas kokybės vertinimo indeksas 22-10>5 dBA. Pastebima neženkli akustinės aplinkos pagerėjimo tendencija už akustinio šešėlio ribų. Taigi galima teigti, kad aplinkosauginis triukšmą sulaikančių užtvarų efektyvumas didžiausias, kai jos įrengiamos atstume ne toliau kaip trigubas sienelės aukštis iki apsaugomo objekto. / Purpose of this workwas to research the acoustic climate in the safety zones of lightweight noise barriers from energy’s dissipation and psychological point of view. Acousticenvironment behind the noise barriers is usually described by amplitude frequency characteristic of sound fields at different distances which are caused mainly due to diffraction of sound waves. Diffraction effects are characterized by the Fresnel’s number, calculation of which includes the shape of the barrier, its dimensions, position of noise source and the listening point. Research results showed that diffraction effects were most significant up to 30 m radius from the noise barrier in its acousticshadow zone. Sound pressure level ∆L_pAat 10-15 m distance from noise barrier reduced 9…11 dBA while behind plantings ∆L_pAwas 5…8 dBA. Effect of increasing distance decreased the sound pressure level ∆L_pA6…8 dBA for noise barriers and 6…9 dBA for plantings respectively. At the non-acoustic shadow zone (50 maway from noise barrier) diffraction level was reduced by some 4…5 dBA and more noticeably from the excavation. At the distance of 10 meters from the edge of the excavation the reduction of 7 dBA in sound pressure level was found, and the decrement because of increased distance for noise barrier was 15 dBA and excavation 11 dBA respectively. It was also ascertained that acoustic environment in the shadow zones of noise barrier is not attributed to favourable. Calculated quality assessment index (QAI) was... [to full text]