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

The study addresses how to efficiently transfer and detect ultra-weak acoustic signals over distance without increasing environmental noise or causing information leakage. Conventional approaches rely on stronger sources or active pre-amplification, which radiate and elevate ambient noise. The research question is whether an acoustic analogue of non-radiative wireless power transfer can be realized to concentrate and transfer sound energy between resonant elements while keeping the surrounding soundscape quiet. The authors propose a remote whispering metamaterial (RWM) composed of a pair of coupled subwavelength Mie resonant cavities that can multiply emission and reception enhancements through strong resonant coupling, akin to Förster resonance energy transfer, enabling silent and efficient point-to-point transceiving of weak sound.

The paper builds on advances in acoustic metamaterials that enable unusual control of sound emission, propagation, and reception, such as emission enhancement via the acoustic Purcell effect, beamforming with phononic crystals and locally resonant metamaterials, and pressure amplification through anisotropic tapered structures. Prior work generally optimized single stages (emission or reception) rather than end-to-end transceiving and did not address radiative coupling to the environment. In electromagnetics, non-radiative mid-range wireless power transfer via resonant coupling has been demonstrated, inspiring the exploration of an acoustic analog. While near-field resonant acoustic objects can exchange energy efficiently, a general framework and practical demonstration for non-radiative, distant, and quiet acoustic transfer had been lacking.

Theory: The authors develop a rigorous acoustic scattering model for two identical cylindrical high-index resonant objects (radius R) in air, separated by a distance d1 much larger than R. An ultra-weak monopole source (A) is enclosed by the emitting object (S) and a microphone (B) by the receiving object (D). Ensuring continuity of pressure and particle velocity at interfaces, they derive a closed-form expression for the pressure enhancement η at the receiver relative to free space, involving air wavenumber k0, Hankel function H0, and transfer matrices T and TG from the scattering formulation. The model applies across near- and far-field regimes and captures monopole–monopole resonance coupling. Finite element simulations (COMSOL Multiphysics, pressure-acoustic and thermo-acoustic modules) validate the theory. Predicted enhancement spectra are examined for effective refractive indices n = c0/c1 = 40, 50, 60 at R = 4 mm and d1 = 1.3 m, including distance dependence and higher-order monopolar modes. Theoretical analysis also distinguishes near-field strong coupling (enhancement exceeding the product of emission and reception) from far-field cascaded behavior (approximately multiplicative efficiencies), and discusses trade-offs due to thermo-viscous losses as n increases.

Experimental setup: Two epoxy-resin 3D-printed meta-cavities (labyrinthine, eightfold zig-zag channels) act as Mie resonators. Each has outer radius R and inner radius r2; channel width w = 0.08R and wall thickness t = 0.02R. The structure yields an effective refractive index n ≈ 4.55 by coiling up the acoustic path. The transfer distance is d1 = 1.3 m, corresponding to 32.5R with R = 0.04 m; operating wavelength λ ≈ 0.61 m. A balanced armature speaker (Knowles DWFK-31785-000, 5 × 2.7 × 3.9 mm^3) serves as the monopole source; a standard condensed microphone records received pressure. Measurements include three configurations: source cavity only (S), detector cavity only (D), and both (S and D). The amplitude of the source is adjusted so that direct free-space transmission is negligible. Pressure enhancement spectra Ps/P0, PD/P0, and PSD/P0 are measured and compared with simulations including and excluding thermo-viscous losses. Spatial field maps and line profiles are acquired to assess ambient leakage. Anti-interference tests use Gaussian-modulated pulses centered at 563 Hz with 75 Hz bandwidth under strong broadband white-noise interference; time-domain and FFT analyses quantify SNR within 525.5–600.5 Hz, with SNR defined as (W−W0)/W0 based on spectral power with source on/off. Robustness is examined with obstacles and scattering layers, and multi-target whispering is demonstrated (details in supplementary notes).

• Theory and simulation: For n = 40, 50, 60 at d1 = 1.3 m, maximal enhancement peaks reach approximately 418, 646, and 945 at 761, 614, and 518 Hz, corresponding to amplified detected signals of about 52, 56, and 59 dB. Enhancement increases with higher n and shifts to lower frequencies. Enhancement versus distance oscillates in the near-field and becomes steady in the far-field.
• Single-cavity enhancement: With only the source cavity S or only the detector cavity D, measured pressure amplification at the receiver 1.3 m away reaches about 15.8-fold (~24 dB) at f0 = 563 Hz, consistent with simulations.
• Coupled-cavity RWM: With both S and D, predicted non-radiative transfer yields amplification over two orders of magnitude (≈259, ~48 dB). Accounting for losses reduces the expected factor to 157, while experiments measure a 66-fold enhancement at resonance.
• Remote whispering performance: The RWM enables more than 40 dB enhancement of detected signals and an average −20 dB reduction of ambient sound leakage compared to ordinary free-space setups at the same received level (60 dB at 1.3 m). To achieve 60 dB at the receiver, the source intensity can be reduced by a factor of 157 with RWM relative to free space at far-field distances; the ratio oscillates in the near-field and stabilizes when separated further.
• Anti-interference capability: Under strong broadband noise masking a weak 563 Hz pulse train, the free-space SNR is below unity (≈0.5, −3 dB) and the signal is undetectable. With RWM, the signal is amplified by >100× without distortion and SNR increases to ≈58.4 (≈17.7 dB), i.e., an improvement of about 20.7 dB, recovering the weak signal above the detection limit. The improvement is consistent across different noise conditions and challenging geometries.
• Mechanism: Strong monopole–monopole resonance coupling between meta-cavities enables non-radiative transfer concentrating acoustic energy within the resonators while maintaining low ambient radiation. Near-field behavior shows pronounced oscillatory enhancement; far-field behavior becomes steady and approximates cascaded emission-reception gains.

The RWM addresses the challenge of transmitting ultra-weak sound without elevating environmental noise by leveraging resonant coupling between paired subwavelength Mie cavities. This coupling acts as an acoustic analogue of non-radiative energy transfer, enabling the receiver to capture and amplify the desired signal while suppressing radiation into the surrounding space. The findings validate that in the near-field, strong resonant interaction yields enhancement exceeding the product of individual emission and reception gains, whereas in the far-field the total transfer efficiency approaches the multiplicative combination of the two. Experimentally, the system achieves substantial detected-pressure amplification (>40 dB) and marked reduction of ambient leakage (≈−20 dB), thus maintaining privacy and minimizing information leakage. The RWM also significantly improves SNR under strong noise, restoring signals otherwise below detection limits. Robustness to obstacles and capability for multi-target delivery suggest practical viability in complex environments. Trade-offs arise from thermo-viscous losses in high-index, fine-feature designs and from the 1/d decay of measured pressure with distance, indicating an optimal operating range balancing enhancement and available acoustic energy.

The work introduces and experimentally validates a remote whispering metamaterial composed of coupled Mie resonators that enables efficient, non-radiative transfer of ultra-weak airborne sound over extended distances while preserving a quiet ambient soundscape. The RWM provides large detected-pressure enhancements (>40 dB), reduces ambient leakage (~−20 dB), and dramatically boosts SNR, allowing recovery of signals otherwise hidden by noise. Theoretical scattering analysis and full-wave simulations agree with measurements and clarify near- vs far-field behavior. Future directions include implementing three-dimensional resonators (spherical or cubic coiled structures) for enhanced communication, extending the concept to underwater acoustics, and exploring analogous implementations for microwaves, as well as optimizing designs to mitigate thermo-viscous losses and tailoring multi-target configurations.

• Thermo-viscous losses in narrow, high-index labyrinthine channels limit achievable amplification; measured enhancements (e.g., 66×) fall below ideal lossless predictions (e.g., ~259×).
• Enhancement oscillations in the near-field due to strong resonant coupling can complicate consistent performance at very short separations.
• Despite steady enhancement in the far-field, the absolute received pressure decreases with distance (∝1/d), requiring a trade-off between enhancement and usable signal power.
• Current demonstrations focus on two-dimensional cylindrical geometries and airborne sound; generalization to 3D structures and other media requires further development.
• Device reconfiguration is limited by fixed physical structures, though some limited tuning is possible; performance depends on precise resonance matching between emitter and receiver.
• Effective index increases may be constrained by fabrication tolerances and increased dissipation in finer features.