Direct measurements of edge diffraction from soft underwater acoustic panels

Abstract

Direct measurements of edge diffraction arising from the interaction of an acoustic wave with an underwater panel that satisfies soft-body boundary conditions are reported. The measurements were obtained by utilizing a specially fabricated ‘‘airbox’’ sample, which is literally a ‘‘box of air,’’ fabricated using thin polycarbonate walls. The airbox theoretically would exhibit a typical insertion loss in excess of 60 dB (in the absence of edge diffraction), thus avoiding interference of the directly transmitted wave with the edge-diffracted wave of interest. The validity of the edge-diffraction measurements was established by demonstrating that the performance of a small sample panel fabricated from a closed-cell foam material can be deduced by adding (frequency-by-frequency) measurements obtained from an airbox to diffraction-free measurements obtained from a large sample of the same closed-cell foam. This procedure simulates (from direct experimental measurements) the combined edge-diffracted plus transmitted wave field that is present in the transmission region of the small sample. The results reported include the edge diffraction caused by the interaction of a spherically symmetric source with a soft sample panel and the edge diffraction caused by the interaction of an acoustic array with a soft sample panel. The frequency interval considered is 1–21 kHz.