Remote whispering metamaterial for non-radiative transceiving of ultra-weak sound

Efficiently transferring weak sound is crucial for various acoustic technologies, including sonar, speech communication, medical imaging, and surveillance. In conventional methods (Fig. 1a, b), sound intensity diminishes with distance due to geometric spreading and losses, necessitating strong sources for detectable signals, especially in noisy environments. Whispering aims for privacy, but weak signals are easily lost in noise. Active pre-amplification using electro-acoustic devices is often necessary but introduces noise and compromises secrecy. Acoustic metamaterials, artificial materials with unusual wave properties, offer potential for sound transfer enhancement. While prior research explored metamaterials for sound emission, propagation, or reception, few addressed transceiving processes or the interaction with the environment. Non-radiative wireless electromagnetic power transfer via magnetic resonance coupling inspires this work. This paper proposes a remote whispering metamaterial (RWM) (Fig. 1c) using coupled Mie resonant objects near the source and receiver to concentrate wave energy and enhance weak sound transfer while maintaining a quiet environment. The RWM is predicted and demonstrated using artificial Mie resonant meta-cavities fabricated from a subwavelength maze-like structure, achieving non-radiative sound transfer over distances up to 32.5 times the meta-cavity radius with over 40 dB signal enhancement and significantly reduced ambient sound leakage.

Extensive research has been dedicated to acoustic metamaterials for manipulating sound emission, propagation, and reception. Studies demonstrated emission power enhancement via the acoustic Purcell effect, directional sound radiation using phononic crystals or locally resonant metamaterials for beamforming, and signal amplification through anisotropic tapered metamaterials. However, most focused on single operations, neglecting the complete transceiving process and environmental interaction. Non-radiative wireless electromagnetic power transfer using magnetic resonance coupling showed promise for applications like powering implantable devices. In acoustics, energy exchange between near-field resonant objects at the same frequency is efficient, yet the acoustic analog of non-radiative wireless power transfer remained unexplored until this research.

The study begins with a theoretical model (Fig. 2a) of two subwavelength cylindrical objects (emitter and receiver) with high refractive indices, separated by a large distance. Using coupled-mode theory and rigorous acoustic scattering theory, a model predicts pressure enhancement (Equation 1). The model considers the continuity of pressure and particle velocity at interfaces, involving wavenumbers, Hankel functions, and transfer matrices (Supplementary Note 1). Finite element method (FEM) simulations using COMSOL Multiphysics validated the theoretical predictions (Fig. 2b, c). The simulations showed significant pressure amplification (52-59 dB) for different refractive indices and distances, with the maximum enhancement increasing with refractive index while shifting to lower frequencies. The strong monopole-monopole resonance interaction between the emitter/receiver pair plays a key role, similar to Förster Resonant Energy Transfer. The physical RWM (Fig. 3a) comprises two 3D-printed epoxy resin meta-cavities with a maze-like structure (inset) to achieve high refractive index (n ≈ 4.55). Experiments involved a balanced armature speaker as the source. Measurements of pressure enhancement with either one or both meta-cavities (Fig. 3c-e) confirmed the theoretical and simulation results, showing amplification exceeding two orders of magnitude (48 dB) with both cavities. The deviation from ideal values is attributed to thermo-viscous losses. A rigorous acoustic scattering theory yielded analytical expressions for pressure field distributions (Supplementary Note 5). In the near-field, enhancement with both meta-cavities exceeded the product of individual enhancements due to strong monopole-monopole interaction. In the far-field, enhancement approximated the product of individual efficiencies. The meta-cavities showed robustness against obstacles and scattering layers in the sound path (Supplementary Note 8). Multi-targeted remote whispering was also demonstrated (Supplementary Note 9). The enhancement efficiency, while decaying with distance, offers a trade-off between steady amplification and available acoustic energy.

The RWM system demonstrated exceptional performance in several key areas: 1. **Significant Sound Pressure Enhancement:** The RWM achieved over two orders of magnitude (48 dB) pressure amplification at a distance 32.5 times the meta-cavity radius, exceeding 100 times the free-space signal. This substantially enhances the ability to detect extremely weak sounds. 2. **Non-Radiative Sound Transfer:** The system exhibited non-radiative characteristics, focusing amplified acoustic signals to specific remote locations while preserving a quiet soundscape in the surrounding environment (Fig. 4a, b). This demonstrates a considerable reduction in environmental noise interference. The reduction is quantified as more than 40 dB enhancement in the detected signal and -20 dB reduction in the ambient sound leakage compared to ordinary setups. 3. **Robustness against Environmental Factors:** The RWM's performance proved robust against various environmental disturbances including solid obstacles and randomly distributed scattering layers in the sound path. The system also successfully transferred sounds to multiple targets (Supplementary Notes 8, 9). 4. **Anti-Interference Capability:** In a noisy environment with a strong noise source masking a weak signal, the RWM effectively restored and amplified the weak signal (Fig. 5). The signal-to-noise ratio increased by more than 20 dB, rendering the weak signal detectable (from below the detection limit to well above it). This showcases the RWM’s superior anti-jamming capabilities. 5. **Theoretical and Experimental Agreement:** The experimental results aligned well with both theoretical predictions and numerical simulations, strengthening the validity and reliability of the findings.

The study successfully addressed the research question of achieving efficient non-radiative sound transfer of ultra-weak signals. The RWM system demonstrates a new paradigm in acoustic signal transmission, offering significantly improved signal-to-noise ratio and reduced noise pollution compared to conventional methods. The non-radiative nature preserves privacy and minimizes environmental disturbance, while the robust anti-interference capability ensures reliable signal recovery in noisy conditions. The system's ability to transfer sound to multiple locations simultaneously extends its potential applications. The close agreement between theoretical, simulation, and experimental results validates the underlying physics and design principles of the RWM.

This research introduced a novel remote-whispering metamaterial (RWM) system enabling efficient, non-radiative transfer of weak sound signals over considerable distances. The RWM’s unique features, including significant signal enhancement, quiet operation, robust anti-interference capability, and multi-target capability, open promising avenues for advanced acoustic technologies. Future work could explore 3D Mie resonator configurations for enhancing communications and expanding the RWM concept to underwater or microwave applications.

While the study demonstrates significant advancements, limitations exist. The current prototype uses a 2D cylindrical structure. Extending to more complex 3D structures may improve performance but adds design complexity. Thermo-viscous losses in the high-refractive-index metamaterials impact the achievable amplification levels, suggesting a trade-off between enhancement and material properties. The efficiency of the RWM system is sensitive to the alignment and distance between the meta-cavities, indicating the need for precise positioning and control for optimal performance.