Grating lobes trapped

Furthermore, the surface waves here are not related to the well-known surface waves that can exist on infinite arrays in a stratified medium next to the elements. These will readily show up in PMM calculations. These are simply grating lobes trapped in the stratified medium and will consequently show up only at higher frequencies, typically above resonance but not necessarily so in a poorly designed array. In contrast, the surface waves associated with finite arrays will typically show up below resonance (20-30%) and only if the interelement spacing is <0.5)t. 

We have demonstrated the presence of surface waves that can exist only on a finite periodic structure. It is quite different from the well-known types of surface waves that can exist in a stratified medium next to a periodic structure often referred to as Type 1. These merely represent grating lobes trapped inside the stratified medium. Thus, they will readily manifest themselves in computations based on infinite array theory at frequencies so high that grating lobes can be launched. 

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