Metasurface for Water-to-Air Sound Transmission

Effective transmission of sound from water to air is crucial for the enhancement of the detection sensitivity of underwater sound. However, only 0.1% of the acoustic energy is naturally transmitted at such a boundary. At audio frequencies, quarter-wave plates or multilayered antireflection coatings are too bulky for practical use for such enhancement. Here we present an acoustic metasurface of a thickness of only $\sim \lambda /100$, where $\lambda$ is the wavelength in air, consisting of an array of meta-atoms that each contain a set of membranes and an air-filled cavity. We experimentally demonstrate that such a meta-atom increases the transmission of sound at $\sim 700\mathrm{Hz}$ by 2 orders of magnitude, allowing about 30% of the incident acoustic power from water to be transmitted into air. Applications include underwater sonic sensing and communication.

Figure 1

(a) Subwavelength-diameter waveguide with a bare air-water interface, with acoustic incidence from the air side. (b) Analytically calculated absolute values of the real (solid blue line) and imaginary (dashed red line) parts of the normalized acoustic impedance $Z_l$ at $x=-l$, plotted on a log-linear scale vs $l/\lambda$. (c) Case in which a sliding thin mass is placed at $x=-l$. (d) As in (b), but in the presence of the meta-atom.

Figure 2

(a) Structure of the meta-atom, shown in exploded view, composed of an effective mass element and a cylindrical cavity element. (b) Diagram of the meta-atom installed in the waveguide, with acoustic incidence from the water side.

Figure 3

(a) Experimental and FEM transmission loss from water to air vs frequency. Dotted blue line (experiment) and solid blue line (FEM): case with the meta-atom. Red triangles (experiment) and dashed red line (FEM): case without meta-atom. (b) Experimental (blue circles), FEM (blue dotted line), and analytical (blue solid line) power transmission ($\tau$) from water to air vs frequency and corresponding plots for power reflection coefficient ($R$) (see legend). In FEM for (a) and (b), dissipation arising from the viscosities and heat conduction of air and water are included. In the analytical calculations, dissipation is included by fitting. (c) Experimental (black squares), FEM (black dotted line), and analytical (black solid line) total dissipated power ratio (dissipated power divided by the incident power) vs frequency. Also plotted is the contribution to the dissipated power ratio by heat conduction losses (red triangles) and by viscous losses (dashed green line). (Meta-atom, MTA; dissipation, diss.)