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Thin flexible narrow tube structures have a wider area of applications such as, in medical devices, mechanical structures, wire protections, marine applications, food handling applications and superconducting transmission lines. Vibration analysis of thin narrow flexible structures is necessary to understand the dynamic behavior under specified boundary conditions and external load. Primarily, dynamic characteristic of structure is expressed in terms of modal parameters such as natural frequencies, mode shapes and damping. The flexible narrow tube structures have very thin walls and measuring modal parameters is quite challenging using contact type excitation methods such as shaker or impact hammer. Similar challenges exist for measuring vibration response on thin walls using contact type sensor such as accelerometer. So, the present study discusses the acoustic source as vibration excitation and laser vibrometer as a vibration response measurement device. An experimental test setup is developed to measure the modal parameters using an impedance tube. The impedance tube consists of a speaker at one end, and another end was exposed to thin narrow tubes. The acoustic excitation was generated by using a speaker in the interested frequency range. Laser vibrometer was used to pick up response of the tube wall at the same instance. The experiments were carried out under free-free boundary conditions, and the surrounding environment was maintained quite to minimize the effect of background noise. The modal parameters were extracted from the measured Frequency Response Function (FRF). The natural frequency and mode shapes obtained from the experimental method for thin flexible narrow tubes are validated with Finite Element Method (FEM) as well as analytical results. FEM analysis was performed using Block-Lanczos modal extraction method. Goldenveizer-Novozhilov theory was used to derive transfer matrix and subsequently to estimate the natural frequencies of the system. The systematic parametric studies such as the effect of shape and size were performed to characterize the vibration response of different flexible narrow tubes. The results shown in this paper will help to characterize the vibration response of the thin structures like mechanical filters and thin acoustic metamaterials, where the traditional contact-type excitation, as well as contact-type sensors were not feasible to use.
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