Control of diffracted sound by means of thnadners

Abstract

The diffraction of sound around barriers partially defeats efforts to control noise by interruption of line of sight. Recent legislation, which limits allowable highway noise in adjacent communities, has stimulated interest in this problem. Currently, solid walls are used to provide partial acoustical shadows. This approach is expensive, partly because of nonacoustical engineering requirements. A review of Fresnel diffraction theory suggests that either transparency gradients or phase velocity gradients or both may be used to provide deeper shadows. Elemental Fresnel theory is slightly modified to introduce transparency or phase gradients near the diffracting edge and parametric studies are reported which define configurations that deepen the shadows. Scale model tests are described which display the trends predicted by the theory. This agreement leads to the fascinating conclusion that the best way to improve the performance of a solid wall is to cut a substantial portion of it away. Structures are suggested for the practical implementation of the method into noise barriers. A literary analogy suggests that such structures be called Thnadners.