ANALYSIS AND CONTROL OF MULTIPLE REFLECTIONS BETWEEN PARALLEL TRAFFIC NOISE BARRIERS

This dissertation presents the development, validation, and use of an algorithm and computer program to predict the degradation in insertion loss of a traffic noise barrier when a second parallel barrier is added on the opposite side of the highway.;The algorithm combines the emission, propagation and barrier attenuation functions of the Federal Highway Administration Highway Traffic Noise Prediction Model with image source creation, reflection and absorption functions. It was validated against available mathematical and acoustical scale model data, as well as field measurement data provided by other researchers.;The resulting computer program, called IMAGE-3, meets requirements for use on Federally-funded highway design projects and was developed as a flexible and easily used analysis and absorptive barrier design tool. For highway design projects, IMAGE-3 would be used in conjunction with current traffic noise barrier design optimization programs.;A parameter analysis showed degradation to be very dependent on receiver height. For depressed highways, degradation was also very dependent on barrier height; for elevated and at-grade highways, it was only slightly dependent on barrier height. Sound absorption played a major role in controlling degradations. Degradation was greater for low source heights (automobiles) than for high heights (trucks). Finally source position between the barrier also affected degradation, depending on receiver position.;An extensive literature review examines American and foreign research on the subjects of urban traffic noise propagation and multiple sound reflections between parallel highway noise barriers. The review supports the hypotheses that parallel barrier insertion loss degradation can be significant, that geometrical acoustics or "image theory" is a valid technique for its analysis, and that sound scattering and ground reflections are only important in certain situations. Control of degradation may be accomplished through use of sound absorptive barrier surfaces or inclining the barriers from the vertical plane.;The program was used to design over one-half mile of sound absorptive barriers for Interstate 440 in Nashville, Tennessee. Its use is illustrated through two detailed examples. Finally, appendices present degradations curves as functions of the various parameters, and details on IMAGE-3 use.