Study for Contactless Detection of Number of Thin Sheet Material by Sound

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This report deals with the development of a new method for detecting the number of thin sheets of porous material without making contact. It is necessary to detect one or more of sheets to avoid mishandling. This detecting sensor utilizes the variation of the phase or magnitude of the transfer function within an entire acoustic system. The incident impedance of the porous material varies with the number of sheets. Characteristic acoustic impedance and complex wave number define the particular characteristics of the material. In this application it is useful to know characteristic impedance of the porous material. The measured impedance of sheets by the transfer function method was included in the theoretical analysis and FEM analysis. The results of the experiments, theoretical analysis and FEM show good coincidence.In the next part, the normal incidence absorption coefficient for porous multilayer between different grain diameters with arbitrary layer thicknesses can be calculated. This calculation process is comparatively lengthy, so the experiments for real multilayer and the calculations were compared. Result of the calculations agreed to the experiments. Consequently, calculation result of normal incidence absorption coefficient for multilayer between different grain diameters is useful.Moisture is an important affector for acoustic characteristics of the porous materials. So we investigated also about relationship between percentage of moisture content and absorption coefficient of granular (as porous) material.In the last research period we developed a system including an impedance tube with 4 microphones to measure transmission loss of porous thin film.