DETERMINATION OF THE DIRECTIVITY OF A PLANAR NOISE SOURCE BY MEANS OFNEARFIELD ACOUSTICAL-HOLOGRAPHY .3. MEASUREMENTS OF HOMOGENEOUS, PROFILED AND COMPOSITE PANELS

In two companion papers, a method has been presented for determining t he directivity of a planar noise source by using the technique of Near field Acoustical Holography [1, 2]. In this paper, a practical NAH mea suring system based upon a single scanning microphone is described. Th e system is employed to measure the directivity of a number of planar noise sources consisting of panels excited on one side by a reverberan t sound field. The noise sources include profiled panels which are str ucturally orthotropic and for which it has been suggested there should be two critical frequencies. Evidence for the existence of two critic al frequencies for quasi-sinusoidally profiled panels is presented wit hin, from the observed directivity pattern. Similar evidence for the e xistence of two critical frequencies for trapezoidally corrugated pane ls was not observed. This is believed to be due to the sharp bends in the profile of such panels acting as localized reflecting boundaries. (C) 1996 Academic Press Limited