Road traffic noise barrier design : measurements and models concerning multiple-walls and augmented earth mounds

This research programme is concerned with the design of road traffic noise barriers, in particular, the use of multiple-walls on the ground and on top of earth mound type barriers.

As part of this research, a comprehensive up-to-date review of the research carried out on noise barriers was undertaken. A number of areas requiring further research were identified. The discussion of these resulted in the proposal of a simplified noise barrier selection method which would be of use particularly to non-acousticians. This method indicated that acoustic information available for the design of earth mounds was limited, although this barrier type is commonly used in practice and is known to have a number of non-acoustic benefits. Initial investigations showed that the performance of an earth mound could be enhanced by the use of multiple-walls on its top.

A detailed investigation was undertaken into the acoustic performance of multiple-walls both on the ground and on top of earth mounds. Both physical and numerical modelling techniques were used for this purpose. The physical scale modelling experiments were carried out both under uniform field conditions and in two different semi-anechoic chambers in the presence of a continuous noise source, using a model scale of 1: 10. The numerical modelling was applied using indirect boundary element method formulation. The commercial software named SYSNOISE was employed for the computations. It was found that numerical modelling results and the semi-anechoic chamber experiments generally agreed very well. The level of accuracy of the uniform field experiments depended on the choice of source and receiver locations as well as the size of the model geometry.

This investigation resulted in acoustic advice on the use of multiple walls both on their own and on top of earth mounds. Under favourable conditions, the multiple-wall configurations were shown to provide substantial attenuations of up to 26dB. The physical parameters involved in their design and their noise attenuation mechanisms were identified. In addition to long-wave scattering and diffraction effects, it was identified that surface wave generation mechanisms and interference effects played a role in attenuating noise. The acoustic advice for the design of earth mounds was extended to the applications of single, double and multiple-walls on their top.

This work also showed that uniform field conditions in conjunction with a continuous noise source could be used for physical modelling. It was found that for small-sized geometries good agreements were observed between physical modelling (both types) and numerical simulations. There were lesser agreements between the sets of data for larger geometries.

The multiple-wall configurations investigated as part of this research programme could be used as noise mitigating measures in central reservations of dual carriageways. However, further research would be required into their acoustic performance and engineering design. The results obtained from this investigation have led to the identification of a number of research areas which could be undertaken in the future.