Near field measurement of the complex radiation impedance presented to a vibrating plate in a reverberant room containing a rotating diffuser

Abstract

The radiation impedance presented by the field of a reverberant room to a rectangular plate mounted in a large baffle and vibrating in its fundamental simply supported edge mode is experimentally investigated. It is shown that a large diffuser rotating at 30 rpm causes large variations in both the real and imaginary parts of the radiation impedance but the average values agree very well with values obtained in the free field of an anechoic room. The real part of the radiation impedance is in good agreement with theoretical prediction. The experimental method makes use of a time average hologram of the mode stored in computer memory, a single point acceleration measurement on the plate, and a surface pressure scan. The method allows investigation of side band frequencies generated by the plate in the presence of the rotating diffuser. It is shown that these side band frequencies contain negligible energy and that they are the result of the cyclic amplitude modulation of the power radiated by the plate.