Combined effects of admittance optimisation on both barrier and ground

The focus of this paper is on the problem of finite impedances on both ground and barrier. Using a boundary element approach the surface treatment of the barrier and finite parts of the ground have been optimised to yield maximum insertion loss at multiple frequencies simultaneously. A 1 m high T-shaped barrier optimised in this way gives up to 8 dB higher insertion loss than a rigid barrier of equal shape. Optimisation of the acoustical properties of the ground below the source as well as those of the barrier improves the insertion loss dramatically for all receiver heights. The ground close to the source is the part of the ground that influences the insertion loss most, and in such a way that the radiation properties of the source are altered, and the radiated sound power is reduced. Having an optimised admittance only on the ground close to the barrier gives only a minor effect. A barrier-ground combination with specialised treatment on the ground close to the source and on the barrier top gives an increase in insertion loss that is comparable to the optimised results. The main conclusion of this paper is that specialised surface treatments provide largest effect if they are used on the ground surface.