Broadband Acoustic Hyperbolic Metamaterial

In this Letter, we report on the design and experimental characterization of a broadband acoustic hyperbolic metamaterial. The proposed metamaterial consists of multiple arrays of clamped thin plates facing the $y$ direction and is shown to yield opposite signs of effective density in the $x$ and $y$ directions below a certain cutoff frequency, therefore, yielding a hyperbolic dispersion. Partial focusing and subwavelength imaging are experimentally demonstrated at frequencies between 1.0 and 2.5 kHz. The proposed metamaterial could open up new possibilities for acoustic wave manipulation and may find usage in medical imaging and nondestructive testing.

Figure 1

Snapshots of the AHMM. (a) Physical structure of the AHMM. Each of the two frames have a separation distance of $d=2\mathrm{cm}$ and the thickness of the frame is $t=0.16\mathrm{cm}$. The frames in the $y$ direction, therefore, have a periodicity of $D=d+t=2.16\mathrm{cm}$. The width of the square plate is $a=2\mathrm{cm}$. To study the effective acoustic properties, the 2D AHMM can be decoupled into two waveguides in each direction: one contains periodically arranged plates ($y$ direction) and the other does not ($x$ direction). (b) Photo of the fabricated AHMM sample. The size of the sample is 30.2 by 27 cm.

Figure 2

Material properties of the AHMM and schematic of the experimental setup. (a) Predicted effective density along the $x$ and $y$ directions. (b) Calculated EFC at two selected frequencies which are both below the cutoff frequency. The dispersion curves are clearly hyperbolic, and the EFCs become flat at low frequencies. Solid line: lumped model. Circle mark: retrieved from numerical simulations. (c) A loudspeaker mimicking a point source is placed 170 mm away from the front face of the sample. Sound absorptive materials are placed on the edges of the 2D waveguide and two sides of the AHMM sample to minimize the reflection and sound field interference behind the AHMM, respectively.

Figure 3

Simulated and measured acoustic field showing partial focusing. (a) Acoustic pressure field at 2440 Hz. Top two figures show the simulation results for the entire domain. The left one is with AHMM and the right one is without AHMM. The bottom figures compare the simulation and measurement in the scanning area. The solid white box, dashed black box, and solid black box denote absorbing foam, AHMM, and scan area, respectively. (b) Normalized pressure magnitude distribution on the exiting surface of the AHMM. A focused profile can be clearly observed. Blue (solid), measurement with AHMM; blue (dashed), measurement without AHMM; red, simulation (real structure); black, simulation (effective medium).

Figure 4

Measured acoustic fields demonstrating subwavelength imaging. (a) Imaging performance of the AHMM at 1.1 kHz. At this frequency, the resolved resolution is about $1/4.7$ of the wavelength. The normalized acoustic intensity distribution along the exiting surface of the AHMM clearly shows two peaks, while the control case (without AHMM) shows a single peak. (b) The broadband performance of subwavelength imaging of the AHMM. Two peaks are resolved within a broad frequency band (1–1.6 kHz).