Development of a technique for the experimental determination of the acoustic transmission and reflection characteristics of submerged plates
The acoustical reflection and transmission properties of flat plates in water are of relevance to the behaviour of submerged structures exposed to waterborne sound waves. The reflection and transmission coefficients are generally defined for plane wave incidence. Where the plate is simple and uniform these coefficients may be estimated from its elastic properties. When confirmation of the estimates is required or the plate is complex in its construction then an experimental measurement of these must be made. It is shown that the plane wave reflection and transmission coefficients may be estimated using an arbitrary field incident upon a panel of the material under test. The incident and reflected or transmitted field is mathematically modelled as the sum of a continuum of plane harmonic waves. The amplitude of each component may be found by spatially and temporally sampling the fields, and Fourier Transforming the results. Advantage is taken of the properties of circularly symmetric fields to simplify the three dimensional Fourier Transform to a two dimensional Fourier/Hankel Transform. Simple division of the amplitudes of the transmitted plane components by the amplitudes of the corresponding incident plane components yields the transmission coefficient as a function of wavenumber and frequency. The numerical implementation of a suitable transform is discussed, and the effects of mis-positioning of the measurement hydrophone upon the results considered. Acoustically compact transient sources are shown to be suitable for plate insonification, in that they are wideband in both wavenumber and frequency. Several types of sources are discussed and details of an underwater spark source given. Experimental results are presented for a 16 mm thick and a 32 mm thick aluminium plate. The measured position and amplitude of dominant features of the transmission coefficient, such as the grazing and coincident peaks, agree reasonably well with theory.