Date of Award
6-1-2007
Document Type
Thesis
Degree Name
M.S. in Engineering Science
Department
Engineering Science
First Advisor
James Chambers
Second Advisor
Dr. Jack Seiner
Third Advisor
Dr. Richard Raspet
Relational Format
Dissertation/Thesis
Abstract
Experiments were performed to determine the patterns of sound propagation past a berm with various sizes of roughness. Previous research had shown a +6 to -20 dB change in the insertion loss over such a surface when roughness was added. This previous data led to advances in modeling propagation over various rough surfaces, but the previous work was limited to one berm size and two roughness sizes. The work presented here is intended to expand upon the previous efforts by adding more roughness sizes and additional berms in order to ascertain the influence of the roughness and to validate the previously developed model with more data sets.
The work confirmed the previous conclusions for the limited data sets and was able to effectively apply those conclusions about the influence of the roughness and the applicability of the model to a larger set of data. The conclusions for the influence are as follows: when the product of the roughness and frequency is small, more sound (less insertion loss) is created in the shadow zone. As either the roughness or frequency grows, there is more sound in the deep shadow zone and less near the apex of the berm as compared to a smooth surface. Finally as the product of the roughness and the frequency leaves this transition region, an increase of insertion loss is noted at all locations in the shadow zone of a berm when compared to a smooth surface. Seen in the experiments was also a phenomenon where the insertion loss is often higher just above the surface than right on it. This may suggest that some sort of creeping boundary wave is present.
The recent modeling efforts developed based upon an analogy between the roughness and impedance produced good qualitative results. Unfortunately, quantitative results were not as precise leaving future work to be performed on the model.
Also, work was performed on berms of the same radius but with different geometrical shapes, cylindrical and spherical. This effort was chosen to help lay ground work for future research. The premise behind the work was to correctly determine the applicability of the propagation model for upward refraction as it is designed to mirror propagation past a cylindrical berm, but the actual real world propagation is more properly described by propagation past a hemispherical berm. The results showed a slight, 1-2 dB discrepancy between propagation past a semi-cylinder and hemisphere with more sound (lower insertion loss) behind the hemisphere. These modest differences may be significant and more research utilizing added geometries will confirm their importance.
Recommended Citation
Whelan, Andrew, "Examination on the diffraction and scattering of sound by curved, rough surfaces and experimental analysis into propagation around a spherical berm" (2007). Electronic Theses and Dissertations. 3454.
https://egrove.olemiss.edu/etd/3454