Drive wave forms for the parametric array showing the ideal form of the demodulated signal.
Difference frequency spectra obtained using raised cosine bell modulated pulses with envelope frequencies of 20, 10, 5, and . The amplitudes are normalized to the maximum of the spectra.
Difference frequency beam profiles for frequencies of 5, 10, 20, and at a range of from the transducer for an array truncated at from the transducer.
Variation of generated difference frequency pulse amplitude as a function of hydrostatic pressure through two hydrostatic pressure cycles at for pulse center frequencies of 5 and . The sequence of results moves clockwise around the hysteresis loops as the pressure is varied, with the two upper lines representing the rising pressure phase and the two lower lines representing the falling pressure phase.
Schematic diagram of the parametric array in APV showing the location of the transducer, acoustic filter, test panel, and hydrophones (all dimensions in meters). Note that both receiving hydrophones are depicted, although only one is used at a time. The diagram is drawn to scale.
Examples of received hydrophone signals for a measurement of reflection loss. The wave forms shown were recorded for a pulse modulation frequency of . Top: signal with test panel present; center: reference signal with test panel absent; bottom: the signal obtained by subtracting the two signals, showing the reflected signal only.
Results obtained for reflection loss measurements for test objects 1 and 2 at ambient pressure and . (a) Results for test object 1 (steel plates separated by ) using and (b) results for test object 2 (PMMA plates separated by ) using .
Model predictions for test panel 1 at three values of hydrostatic pressure. (a) Reflection loss values calculated for and (b) transmission loss values for .
Measured results for test panel 1 at three values of hydrostatic pressure using a pulse envelope. (a) Reflection loss was measured at , whereas (b) transmission loss was measured at .
Model predictions for (a) reflection loss and (b) transmission loss of test panel 2 at for three values of hydrostatic pressure.
Measured results for test panel 2 at and three values of hydrostatic pressure. (a) Reflection loss was measured with a pulse generated using , whereas (b) transmission loss was measured with a pulse generated using .
Results for test panel 1 at and ambient pressure for pulses generated using envelope frequencies of 5, 10, 20, and showing agreement between frequency ranges.
Results for reflection loss of test panel 1 (left) at a pressure of and a temperature of and for test panel 2 at a pressure of and a temperature of . For panel 1, the measurement window was varied from in steps of ; for panel 2, it was varied from in steps of . The mean values of the calculated results are shown with the error bars calculated for a confidence level of 95%.
Test objects used to exercise the method.
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