Theoretical study of diffraction by straight and ragged edge noise barriers

This dissertation is a theoretical investigation of the diffraction of sound by straight and ragged edge barriers. Interest in the effect of making the edge of a barrier ragged has been motivated by experimental data that show that edge irregularity can cause a substantial degradation of the diffracted signal. In many cases the ragged edge improves the insertion loss by up to 5 dB. However, one experimental investigation showed that although the ragged edge changes the insertion loss by ;In the present work the theoretical investigation for diffraction by a ragged edge barrier begins with a study of diffraction by a straight edge barrier. A detailed survey of the existing methods for diffraction by a straight edge barrier is presented. The interrelations of the methods are studied, the results are re-interpreted, and common characteristics are revealed. Based on the observations a new method, termed Directive Line Source Model, is developed for predicting the sound field (pressure time waveform) due to diffraction by straight and ragged edge barriers. It is shown that the diffracted field is equivalent to re-radiation from the edge of the barrier, but modified by a certain directivity function. The straight or ragged edge of the barrier is modeled as a directive line source, straight or ragged, respectively.;The results obtained by the Directive Line Source Model are in good agreement with known analytical solutions and experimental results. Besides yielding predictions that agree favorably with measurements, the model presented here provides the following useful qualitative results: (i) The performance of a ragged edge barrier varies significantly with receiver position. (ii) The portion of the edge that intersects the shortest path between source and receiver plays an important role. (iii) The shape of the diffracted signal remains roughly the same when the receiver moves away from the barrier, but it changes appreciably when the receiver moves parallel to the edge. (iv) The diffracted field behind a ragged edge barrier approaches the field behind a straight edge barrier at receiver locations far from the barrier.;Furthermore, because the model is easy to apply, it is a promising tool for further research on ragged edge barriers. The ultimate goal is to be able to use the Directive Line Source Model in the design process to specify the edge profile of a barrier that would increase the insertion loss (relative to a straight edge barrier with the same average height) at a given set of receiver positions.