Low frequency acoustic sinks

An impedance-matched surface has the property that an incident wave generates no reflection. Here we demonstrate that, by using a simple construction, an acoustically reflecting surface can acquire hybrid resonances and become impedance-matched to airborne sound at tunable frequencies, such that no reflection is generated. As there can be no transmission, the impedance-matched acoustic wave is hence either completely absorbed at one or multiple frequencies,or converted into other forms of energy, such as an electrical current. A high acoustic-electrical energy conversion efficiency of 23% has been achieved. Each resonant cell of the metasurface is deep-subwavelength in all its spatial dimensions. With its thickness less than the peak absorption wavelength by two orders of magnitude, the resonant cell itself can be treated as an acoustic sink, acting as a time-reversal counterpart for an acoustic point source.