Free Vibration Analysis of Orthotropic Thin Rectangular Plates

The paper presents a comparative modal analysis of thin rectangular orthotropic plates with different surface treatments, to determine the natural frequencies and mode shapes, using the experimental method. The samples analyzed are from resonance spruce wood, a material used in the construction of musical instruments. The dynamic response of the boards was analyzed for two qualities of wood - class A (for maestro instruments) and class D (for school type musical instruments). The modal analysis was performed on unvarnished and varnished wooden boards with 5, 10 and 15 layers of varnish. The convergence and accuracy of the dynamic parameters were examined, as well as the quantitative and qualitative differences between different types of boards. The method consisted of hitting the plates at different points and acquiring the signals with the help of accelerometers. Signal processing consisted of time and frequency analysis of the signals for each board using a program developed in MatLab. Thus, the resonance frequency spectrum was obtained, the values being correlated with the anatomical descriptors of the wood for each analyzed category. The novelty of the study consists in the comparative analysis of plates from resonance spruce covered with layers of varnish applied according to the violin manufacturing technology.