Keywords: noise barriers, shape optimization, evolutionary algorithms, boundary element method.

Noise Barriers are widely used for environmental protection on the boundaries of high traffic roads near population centres in order to reduce the noise impact. Shape optimum design of M-shaped noise barriers (MNB) is carried out using the boundary element method (BEM) for modelling and evolutionary algorithms for optimization. The sound level is calculated being known: the source position, the receptor position, the barrier shape, and the sound frequency. Shape optimum design of noise barriers have been carried out previously using this methodology [1].

New advances are introduced here, applying the previous methodology to MNB and performing an optimum design that allows lower effective height designs. The fitness function to minimize is the sum of squared differences corresponding to the insertion loss (IL) throughout a set of frequencies belonging to the one-third octave band spectra (fourteen values are considered) of two barriers: the candidate M-barrier design and a reference noise barrier design (a simple barrier with higher effective height than the maximum constrained value). Therefore, the design fits a IL reference curve corresponding to a higher effective height simple barrier and obtains a M-shape design whose IL curve performance fits this reference. A maximum limit to the effective height of the barrier is imposed. Four independent runs of evolutionary search were executed in each case. Among them the best result was selected. A population size of 100 individuals and 3% mutation rate were used in a Gray coded steady-state genetic algorithm with uniform crossover.

Results are detailed in terms of IL values and barrier shape designs, numerically and graphically. The higher the straight barrier height equivalence to be achieved, the harder it is for the evolutionary algorithm to obtain lower fitness function values. This shows that the acoustic efficiency physical limitations due to a constrained maximum effective height in the M-shape design. This methodology allows a physical image of the acoustic barrier efficiency to be obtained relating it with the efficiency of a common straight barrier. Also the values of IL of the M-shape optimum barrier designs for a standard road traffic spectrum are represented.

A methodology for optimum design of MNB has been presented with successful results. It is based in the BEM modelling coupled with evolutionary computation, solving an inverse problem to obtain the barrier shape design that corresponds to a known IL curve at a certain number of frequencies. M-shape barriers are obtained with constrained maximum effective height not higher than 3.0 m, lower than two cases of straight barriers (3.5 and 4.0 meters height) with the same acoustic efficiency. Therefore, it is possible to obtain MNB designs with the same attenuation efficiency as simple barriers with higher effective height. Lower environmental and visual impact is achieved.

1

D. Greiner, J.J. Aznárez, O. Maeso and G. Winter, "Shape Design of Noise Barriers Using Evolutionary Optimization and Boundary Elements", in Proceedings of the Fifth International Conference on Engineering Computational Technology, B.H.V. Topping, G. Montero and R. Montenegro, (Editors), Civil-Comp Press, Stirlingshire, United Kingdom, paper 43, 2006. doi:10.4203/ccp.84.43

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