The performance of a simple noise barrier is determined by its height relative to the heights of the noise source and receivers. There have been many attempts to overcome this limitation by modifications to the barrier top edge. These generally take the form of an absorbent top or the introduction of additional diffracting edges. However, relatively minor variations in geometry have sometimes been found to result in drastic changes to barrier performance. In this paper the Boundary Element Method is employed to calculate the sound field in the vicinity of complex barrier tops and sound field visualization techniques are used with these data to investigate how the sound field varies with frequency and dimensional changes. It is shown that for some top edge configurations local resonance effects have a significant effect on the attenuation afforded by the barrier.