Detection of weak sound signals masked by strong noise background remains challenging in acoustic science and engineering. The major bottleneck of advancing this technology is the limited directivity and sensitivity of ordinary acoustic sensors. Here, we engineer acoustic metamaterials with a near-zero-index (NZI) in the form of a low-profile planarized acoustic antenna for combined highly directive-sensitive detection. The detectable incident angle can be substantially narrowed down by the directional selectivity of NZI acoustic metamaterials, while the detected pressure can be enhanced by deeply tunneling compression at the sound radiation vent. Magnification of signal amplitude more than 18 dB with a half-power beam width of mainlobe less than 5° is demonstrated both numerically and experimentally, which overcomes the detection limit of conventional acoustic sensing systems.
Transceiving ultra-weak sound typically relies on signal pre-amplification at the transmitting end via active electro-acoustic devices, which inherently perturbs the environment in the form of noise that inevitably leads to information leakage. Here we demonstrate a passive remote-whispering metamaterial (RWM) enabling weak airborne sound at audible frequencies to reach unprecedented signal enhancement without altering the detected ambient soundscape, which is based on the extraordinary scattering properties of a metamaterial formed by a pair of self-resonating subwavelength Mie meta-cavities, constituting the acoustic analogy of Förster resonance energy transfer. We demonstrate efficient non-radiative sound transfer over distances hundreds times longer than the radius of the meta-cavities, which enables the RWM to recover weak sound signals completely overwhelmed by strong noise with enhanced signal-to-noise ratio from −3 dB below the detection limit of 0 dB in free space to 17.7 dB.
A high-performance acoustic vortex beam generator (VBG) based on artificial micro-structured metamaterials is of great significance in acoustic communication. However, to date, the research on metamaterial VBGs mainly focused on their single frequency properties in the narrow band. Here, we propose a design strategy of broadband VBGs constructed by gradient coupled-resonant meta-atoms, all of which show near-unity transmission amplitudes, while covering 2π phase shifts linearly varied along with frequency throughout the desired overlapping frequency range. Moreover, the phase differences between adjacent meta-atoms are constant at regular intervals within this entire frequency range, allowing the unique wideband response of the proposed VBG. We demonstrate, both in simulations and in experiments, the efficient mode conversion from plane sound wavefronts into vortex beams with a topological charge of 1. Our study provides a platform to manipulate broadband wavefront conversion based on acoustic coupled-resonant metamaterial, which allows us to envision promising acoustic devices with versatile applications.
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