Object and purpose of research. The study of the acoustic pulse changes regularities during its propagation in con-fined media is one of the fundamental problems of acoustics, which allows to pose and solve the inverse problem of determining the dissipative and resonant properties of these media. The physical processes occurring during the propagation of a pulse in a cylindrical waveguide with rigid walls were investigated. Materials and methods. To analyze the mechanism of dissipation, experimental studies of pulse propagation in a hy-droacoustic tube were carried out, and the theoretical description of the obtained results was carried out using analytical methods. The simulation of the propagating pulse in the finite element waveguide model was used to confirm the theoretical assessments and the experiment. Main results. Experimental studies of physical processes during the propagation of an acoustic pulse in confined medium of cylindrical waveguide bounded by walls with characteristics close to absolutely rigid are carried out. The data showed that it is possible to control changes in the phase velocity, amplitude, and waveform, which made it possible to quantify the impedance of the internal walls of the waveguide and the dissipation of acoustic energy with a sufficient degree of accuracy. The numerical model calculation, taking into account the theoretically obtained quantitative assessments of the dissipation values and the impedance value of the waveguide inner surface, showed a good correspondence between the model and experimental characteristics of the change in the propagating pulse. Conclusion. In the studies devoted to the propagation description of acoustic waves in waveguides, the issues of energy dissipation are usually not considered, especially in cases where it has a weak effect on the measurement result. The theoretical value of the research is to quantify the wave energy dissipation by the parameters that can be determined with sufficient accuracy in the experiment: the phase velocity, the pulse form. Further accuracy improvement of the experimental data, especially in a wide frequency range, will improve the theoretical model of dissipation by taking into account the mechanism of inhomogeneous viscous and thermal waves near the inner surface of the waveguide. The practical significance of the research is to increase the reliability of experimental data and to develop physical and mathematical models of underwater sound absorption due to a forced variable flow with a highly transformed velocity of a viscous liquid in a thin surface layer near the elastic wall.
Generation of Subpicosecond Pulse Trains in Fiber Cascades Comprising a Cylindrical Waveguide with Propagating Refractive Index Wave
