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

Micro-perforated sound absorbers with sub-millimeter size holes can provide high absorption coefficients. Various types of micro-perforated absorbers are now available in literature for different applications. This thesis presents results of work on the development of an effective single layer micro-perforated sound absorber from the commercial composite material Parabeam with micro diameter holes drilled on one side. Parabeam is used as a structural material made from a fabric woven out of a E-glass yarn and consists of two decklayers bonded together by vertical piles in a sandwich structure with piles (thick fibers) woven into the decklayers. The thesis includes, the analytical model developed for prediction of absorption coefficients, finite element solution using commercial software MSC.ACTRAN and experimental results obtained from impedance tube measurements. Different absorption characteristics can be achieved by variations in hole diameter and hole spacing. Based on the developed models, an optimization is performed to obtain an efficient absorber configuration. It has been anticipated that several different and interesting applications can be deduced by combining structural and sound absorption properties of this new micro-perforated absorber along with conventional fibrous absorbers.