Acoustic transmission loss of pipes excited by internally propagating plane waves

Abstract

A control valve constitutes a source of mechanical vibration, nearfield imcompressible pressure fluctuations and radiated acoustic waves. An ideal mechanical vibration filter placed several wavelengths from the source will remove the influence of the first two excitations from the response of the downstream pipe. The internal propagation of acoustic energy, however, is unaffected and so the transmission loss of pipes to these propagating waves is of concern. In the plane-wave region, Morfey, Heckl, and others have measured acoustic radiation far greater than their estimates. This work has been directed towards explaining these differences. Experiments with plane acoustic excitation of steel and P.V.C. pipes were performed. Different gases were used inside the P. V. C. pipe, for which the wall response was forced, to investigate the radiation from subsonic and supersonic axisymmetric wave motion. Close agreement was found, in contrast to the case of steel pipes, where a strong resonant response occurred. This theoretically unpredicted resonant response (which was about 45 dB above the forced estimate) contained all resonant modes but was dominanted by the modes with the highest circumferential mode order. The acoustic radiation, however, was greater than the forced estimate by only 25 dB, since it is determined by the most efficient radiators—the n=1 “bending” modes. Only these latter modes are damped by the pipe supports; the measured Q of these modes was about 100. Similarly these modes are most strongly coupled to the excitation, and consequently the n=1 modes control the radiation. It was found that the application of surface damping treatments caused reductions of 10 to 20 dB in response, but only 2–5 dB in power radiated. This is because surface damping treatments are particularly effective on higher-order circumferential modes and much less effective on the n=1 modes, in which there are no changes in circumferential radius of curvature. It is concluded that a significant reduction in power radiated requires damping procedures directed specifically at the n=1 modes. Experiments in progress suggest that for the pipes studied, a Q less than 20 for n=1 modes is required to achieve the lower bound power radiated corresponding to a forced response.