A technique for evaluating the underwater acoustic performance of panels under simulated ocean conditions in a laboratory test facility is described. The method uses a parametric array as a source of sound within a test vessel capable of simulating ocean depths down to 700 m and water temperatures from 2 to 35 degrees C. The reflection loss and transmission loss of the test panel may be determined at frequencies from a few kilohertz to 50 kHz. The use of the parametric array enables wideband measurements to be undertaken with short-duration pulses and reduces the effects of diffraction from the panel edges. An acoustic filter is used to truncate the array in order to provide a source-free measurement region and to simplify the measurement process. The difficulties of establishing a parametric array in the confined space of the vessel are outlined, and the experimental procedures adopted are described. The techniques were validated by undertaking measurements on two test objects that have predictable behavior. The potential of the technique is also illustrated with experimental results for test panels for hydrostatic pressures up to 2.8 MPa. An extensive discussion of the measurement limitations is included.
operating in the frequency range 100 kHz to 500 kHz are compared. In the first approach two-dimensional planar scans, with a spatial resolution of better than half a wavelength, are performed in the acoustic near-field using a small probe hydrophone. The measured two-dimensional data are propagated numerically using a Fourier Transform method to predict the far-field response. Alternatively the data can be back-propagated to reconstruct the pressure distribution at the source, a powerful diagnostic technique which can identify defects in transducers and array elements. The second approach uses a scanning laser vibrometer to measure the velocity of the transducer surface; with the resulting velocity data also being used to predict the far-field response by numerical propagation. Comparison of the propagated hydrophone near-field scan data with direct measurements at these ranges shows very good agreement, indicating the usefulness of the method for deriving far-field transducer responses from near-field measurements in laboratory tanks. The potential limitations introduced to the optical approach by the acousto-optic effect are discussed.
A description is given of the results of a Key Comparison of primary free-field standards for underwater acoustics at frequencies from 1 to 500 kHz. This is the first such Key Comparison exercise in the field of underwater acoustic calibration and measurement. Laboratories from UK, Germany, USA, Russia, China, Canada, and South Africa participated by calibrating three reference hydrophones, with project coordination provided by the National Physical Laboratory, UK. The agreement between the results obtained from the comparison was generally encouraging, with the calibration values reported by the laboratories agreeing within quoted uncertainties over the majority of the frequency range, and the results generally lying within a ±0.5-dB band for frequencies up to 300 kHz. A discussion is given of the general sources of uncertainties in the calibrations, in particular those which are thought to have contributed to the differences in the results between laboratories. The results of the participants have been used to estimate the equivalence of national measurement standards within this field.
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