Design and experimental demonstration of broadband acoustic pressure enhancing passive metafluids.

This paper presents the design and experimental demonstration of a passive broadband acoustic pressure enhancing metafluid in air. The design is optimized for fabrication via three-dimensional (3D) printing and takes advantage of the property of acoustic pressure to enhance sound as the sound passes with minimal insertion loss from the background medium into a high impedance fluid. Numerical simulations and experimental measurements of the fabricated structure show that the metafluid enhances the sound pressure level by 7 dB in more than one octave without introducing sound distortions. Moreover, the metafluid is subwavelength in size and does not increase the aperture of the sensor. These results provide an excellent path toward improving the sensitivity of compact acoustic sensors without employing active elements.

Broadband sound barriers with bianisotropic metasurfaces.

Noise is a long standing societal problem that has recently been linked to serious health consequences. Despite decades of research on noise mitigation techniques, existing methods have significant limitations including inability to silence broadband noise and shield large volumes. Here we show theoretically and experimentally that acoustic bianisotropic materials with non-zero strain to momentum coupling are remarkably effective sound barriers. They surpass state-of-the-art sound isolators in terms of attenuation, bandwidth, and shielded volume. We implement our barriers with very compact active meta-atoms that owe their small size to their local response to external sound. Moreover, our active approach is not constrained by feedback stabilization requirements, in stark contrast with all traditional active sound control systems. Consequently, bianisotropic sound barriers have the potential to revolutionize noise control technologies and provide much needed solutions to an increasingly important and difficult challenge.