The propagation of sound from a monopole and directional source near a layered ground
Abstract
Sound propagation near a layered ground is systematically studied as a means of outdoor noise reduction in this dissertation. A variety of ground types and source characteristics, as well as the interference of the ground, are treated. The sound field of a monopole above an impedance ground is firstly reviewed. The direct numerical integral and the Fast Field Program for the numerical calculation are shown as a standard to check the accuracy of the developed methods. Then, close form asymptotic equations are derived for the sound field of a monopole near the semi-infinite rigid porous ground. They are evaluated along the steepest descent path with the method of subtraction of the pole. They are further modified as simplified formulas, which have more physical insight. Both analytical solutions are compared with the exact numerical calculations. It is shown that they work well for most of the general outdoor ground. To generalize the problem of sound propagation above a rigid porous layered ground, a new numerical scheme is developed along the steepest descent path with the supplement of subtraction of the pole. It avoids the inherent problems in the first two numerical methods and increases both the accuracy and the efficiency of the numerical computation. Following the above studies, the sound field above an impedance-backed porous ground is also investigated. Similar methods are applied to obtain an asymptotic solution. The numerical issues in the calculation are explained in detail. The relation between the various components in the asymptotic solution is discussed. It is shown the impedance-backed layer can improve the performance of noise reduction. For the study of the sound field above a semi-infinite ground, alternative solutions can be derived by heuristic approximation of the effective impedance. The effective impedance approach offers good agreement with the numerical integration as well as provides simpler physical insight of the problem. They are also validated by numerical integration and laboratory experimental results. They are further employed in the characterization of the acoustical properties of the ground. A new acoustical characterization method for the rigid porous materials is developed with the downhill simplex method. Application to glass beads in the laboratory is shown. Analytical asymptotic solutions are also obtained by using the effective impedance approach for the sound field of a multipole including a dipole and a quadrupole near a semi-infinite rigid porous ground. They are compared with the numerical calculations, and the results show very good agreement. Finally, the summary of the current study is given and future work is shown.
Degree
Ph.D.
Advisors
Li, Purdue University.
Subject Area
Environmental Studies|Mechanical engineering|Acoustics
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