Is there really a sound line limit for surface waves in phononic crystals?

When phononic crystal were first introduced in the early 1990's, their ability to prohibit acoustic wave propagation was first demonstrated for bulk waves. Since then, it has been shown that these artificial materials offer unprecedented ways of steering the course of any type of elastic waves, bulk or guided. A series of works has then focused on investigating the effects these artificial materials could have on already confined surface-guided waves, an interest clearly driven by the prominent position surface acoustic waves and their combination with piezoelectric solids occupy in the vast field of wireless telecommunication systems. Theoretical reports stated that complete surface wave band gaps could be obtained in perfect 2D structures. Experimental demonstrations did not live up to one's expectations, though: significant energy loss was observed for frequencies supposedly lying above the bandgap and coupling of the acoustic energy to the bulk substrate was blamed. The radiation of these modes located above a sound line - defined by the dispersion relation of the bulk mode with the lowest velocity - seemed to cast a genuine stumbling block on the development of phononic structures relying on surface waves. Yet, if losses are unavoidable there, configurations do exist that can make them acceptable. In this paper, we will focus more closely on recent theoretical and experimental results that show, through the simulation, fabrication and characterization of a hypersonic phononic crystal, not only that bandgaps can be obtained at near-GHz frequencies, but also that a clear transmission of the signal can be observed even for modes lying within the sound cone.