Acoustic Surface Waves on Metasurfaces

Acoustic metamaterialsAcoustic metamaterial and metasurfaces are typically comprised from periodicPeriodic arraysArray of subwavelengthSubwavelength resonantResonant elements to endow them with some designed material properties or functionality. Often the metamaterial principles that underpin the physics of these systems rely on these resonantResonant elements being coupled to form some collective response: in this chapter we explore how resonant cavities can be utilised to form trapped acousticAcoustic surface wavesWaves and thus manipulate soundSound. We outline the fundamentals of acoustic surface waves; the interaction of sound with rapidly-varying geometry changes is discussed, such as when incident on an open or close-ended tube cavity arrayArray, which can induce a finite impedanceImpedance boundary condition that leads to trapped surface-waveWaves-like features, termed ‘acousticAcoustic surface waves’. Following this, to illustrate the physics introduced, we present and discuss measurements of the acoustic surface wavesWaves supported on a simple 2D arrayArray of open-ended cavities. We show that the metasurface array supports a highly anisotropicAnisotropic surface waveWaves over a narrow frequencyFrequency band, which leads to the ‘acoustic beaming’ phenomenon, where acoustic power flow is confined to specific and predictable directions.