This paper presents the experimental and theoretical results of applying resonant acoustic spectroscopy (RAS) to determine elastic parameters and losses in such consolidated granular materials as rock and building bricks. First, the theoretical aspects of the RAS method are outlined. A computer code for the rectangular and cylindrical samples was developed and tested. The results of experiments on specimens of rock and ceramic brick are then described. Finally, a modification of the previously published RUS algorithm is presented which permits a significant reduction in computing time for elongated samples.
The paper describes nonlinear effects due to a biharmonic acoustic signal scattering from air bubbles in the sea. The results of field experiments in a shallow sea are presented. Two waves radiated at frequencies 30 and 31-37 kHz generated backscattered signals at sum and difference frequencies in a bubble layer. A motorboat propeller was used to generate bubbles with different concentrations at different times, up to the return to the natural subsurface layer. Theoretical consideration is given for these effects. The experimental data are in a reasonably good agreement with theoretical predictions.
The difference-frequency sound generation as a result of interaction of two high-frequency harmonic waves in a bubble layer in water is investigated both theoretically and experimentally. Because the sound speed in the layer is less than that outside, the layer has resonance properties. As was shown before, this can considerably increase the efficiency of the nonlinear frequency transformation. However, unlike the cases considered before, the layer resonance is practically achievable only at the low (difference) frequency, whereas the high-frequency signal (pump) resonates at individual bubbles and then it strongly dissipates. Here the results of an experiment with a bubble layer with a thickness of about 10 cm in an anechoic tank are presented. One of the incident (primary) wave frequencies was 60 kHz, whereas the other could be varied, thus providing the low-frequency signal in the range of 0.8–14.8 kHz. Due to the first-mode layer resonance, this secondary signal had a pronounced maximum at a frequency of 2.4 kHz. The high attenuation of pump waves was due to resonant bubbles. A theory which agrees with the experimental results reasonably well, is developed for this type of interaction.
The method of velocity of elastic waves hodograph, aimed at non-destructive testing of structurally heterogeneous composite materials and products based on them, as well as multilayer products and constructions, is considered.
The theoretical basis for determining the propagation velocity of elastic waves in a multilayer medium by the hodograph method is given. Based on the studies, recommendations are given for determining the propagation velocity of elastic waves in each individual layer of a multilayer medium, which allows non-destructive testing of the physicomechanical characteristics of each layer of a multilayer medium.
It is shown that in addition to simple multiple reflections in a homogeneous medium, in a multilayer medium with parallel interfaces consisting of two or more layers, complex types of multiple reflected waves and mixed waves (reflected-refracted and refracted-reflected) can arise.
The main task of applying the low-frequency ultrasonic method is to determine the acoustic parameters of the propagation of elastic waves (velocities, amplitudes, spectra). The main methods for determining the elastic wave velocities are considered, based on the hodograph equation of the indicated reflected waves in a multilayer medium.
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