Internal acoustic fields in pipes as generated by control valves, for example, interact with the pipe walls such that very large drops in transmission loss are experienced at a series of discrete frequencies, the coincidence frequencies. This paper develops the analysis for the calculation of the pipe vibration amplitude at the coincidence frequencies so that the radiation external to the pipe can be estimated. External acoustic loading and material damping are considered. The analysis was applied to experiments which were performed on a somewhat idealized test fixture. Combining the results of the analysis with the test results permitted the estimation of the material damping for the particular material used. Results show that structural damping for the low carbon steel varies from 0.04 at high frequency to a high of 0.20 at the lowest frequency which agrees fairly well with the results from other research on flat steel plates. The rather large values of the estimated damping coefficients may well be caused by the visco-elastic damping materials used at the ends of the pipe’s tee section.

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