Shape-optimal design of graded index sonic crystal noise barriers with line defects

A graded index sonic crystal gives rise to upward refraction due to a negative sound speed gradient, which progresses with height. However, in addition to upward refraction, these cylinder formations can simultaneously benefit from band-gap phenomena. This paper presents a method to optimise graded index sonic crystals for broadband traffic noise by: (1) organising cylinders in complex formations using natural cubic splines and (2) introducing line defects. All variables were optimised with a multi-objective genetic algorithm, for structures based on horizontally oriented acoustically hard cylinders, located above a perfectly reflecting ground plane. A four-lane outdoor situation, with a traffic scenario consisting of 95 % light and 5 % heavy vehicles driving at 70 km/h has been studied. For such a configuration we obtained a spatially averaged mean reduction of 3.7, 4.3 and 5.4 dBA, with optimised structures covering an effective cross-sectional area of 0.69, 1.25 and 1.45 m2, respectively. It is found that the focussing performance of the studied structures is enhanced by increasing the number of scatterers as a function of height, which effectively represents a negative sound speed gradient. In addition, it is found that the effect of line defects is significant, and that the insertion loss among the studied traffic lanes is reasonably constant