Acoustic Metamaterials for Noise Control and Cloaking

Acoustic metamaterials represent a class of engineered materials with subwavelength structures designed to manipulate sound waves in ways not typically found in conventional materials. By precisely controlling parameters like effective density and bulk modulus, these materials offer unprecedented capabilities for sound absorption, redirection, and even concealment. This has opened exciting avenues for addressing persistent challenges in noise control across various sectors and exploring the more exotic concept of acoustic cloaking, rendering objects effectively invisible to sound waves. While traditional noise control often relies on bulky materials effective only at high frequencies, and acoustic cloaking has remained largely theoretical, metamaterials promise compact, frequency-tailored solutions, pushing the boundaries of acoustic engineering.

This article reviews the fundamental principles governing acoustic metamaterials and explores their burgeoning applications in noise control and acoustic cloaking. We delve into the diverse mechanisms enabling wave manipulation, discuss specific designs and their performance, highlight the challenges associated with bandwidth and practicality, and survey the cutting-edge design and fabrication techniques driving the field forward. Furthermore, we speculate on the convergence of passive and active control, the potential of illusion acoustics, and the development of multifunctional acoustic systems.

Mechanisms for Acoustic Wave Manipulation

Acoustic metamaterials derive their unique properties not from their constituent materials' chemistry, but from their engineered subwavelength structure. Several key mechanisms are exploited:

• Local Resonance: Perhaps the most common mechanism, local resonance involves incorporating subwavelength resonant elements (e.g., Helmholtz resonators, mass-spring systems, membrane resonators) within a host medium. Near the resonance frequency, the effective dynamic mass density or bulk modulus of the metamaterial can become negative, leading to strong wave attenuation and the formation of bandgaps where sound propagation is forbidden (Hyun et al., 2024; Liu et al., 2025). This is particularly effective for low-frequency noise control.
• Bragg Scattering: In periodic structures with lattice constants comparable to the wavelength, coherent scattering from the periodic elements leads to Bragg bandgaps. Unlike local resonance gaps, these depend on the periodicity and lattice symmetry (Li et al., 2025). While effective, achieving low-frequency Bragg gaps requires large structures.
• Space Coiling and Slow Sound: By forcing sound waves to travel along complex, folded pathways within subwavelength unit cells (e.g., labyrinthine structures), the effective path length is increased, slowing down the sound speed and reducing the effective bulk modulus. This allows for compact designs exhibiting strong absorption or phase manipulation (Chojnacki et al., 2025).
• Transformation Acoustics: Analogous to transformation optics, this theoretical framework uses coordinate transformations to design materials with specific anisotropic and spatially varying properties (density and modulus) that can guide sound waves along desired paths, such as around an object, effectively cloaking it (Huang et al., 2025 references the concept in application).
• Scattering Cancellation: This approach aims to eliminate the scattered field from an object by introducing a carefully designed cloak that generates an opposing scattering signature, destructively interfering with the object's scattering (Liu et al., 2024 discusses this in the context of illusion acoustics; Yang & Huang, 2024 mention the theory for diffusion).

These mechanisms, often employed in combination, form the basis for designing metamaterials tailored for specific acoustic functionalities, ranging from perfect absorption to invisibility.

Metamaterials for Noise Control and Sound Absorption

Controlling unwanted noise is a critical challenge in environments ranging from industrial settings and transportation to building acoustics. Acoustic metamaterials offer significant advantages over traditional bulky porous or fibrous materials, particularly at low frequencies where conventional solutions are often ineffective or impractical.

Locally resonant metamaterials, often based on arrays of Helmholtz resonators or membrane-type structures, excel at creating narrow-band absorption peaks or transmission stop-bands at targeted low frequencies (Hyun et al., 2024). Designs incorporating added mass elements can further lower the frequency of operation and potentially widen the bandgap (Liu et al., 2025). Space-coiling structures provide another route to low-frequency absorption in compact form factors by effectively slowing sound waves (Chojnacki et al., 2025). A major challenge remains achieving broadband absorption, especially in the low-frequency regime.

Strategies to overcome this include coupling multiple resonators with different frequencies, incorporating dissipative elements, or using optimization techniques to design complex multi-resonant unit cells. Furthermore, integrating metamaterials with other functionalities is an active area of research, such as creating structures that provide both noise reduction and ventilation, or combining noise control with vibration energy harvesting using piezoelectric elements integrated into resonant structures (Hyun et al., 2024; Jean et al., 2024; Ismaili et al., 2025).

Acoustic Cloaking and Illusion

The concept of rendering an object invisible to sound waves, or acoustic cloaking, represents a fascinating application of metamaterial principles. Transformation acoustics provides a theoretical blueprint but often requires extreme material properties (anisotropy, spatial gradients) that are difficult to realize physically (Huang et al., 2025). Scattering cancellation offers an alternative, aiming to eliminate the scattered waves from an object, but is typically limited to specific object shapes and frequencies.

A more recent and potentially powerful approach is illusion acoustics. Instead of making the object disappear entirely, illusion metamaterials manipulate the scattered waves to mimic a different object or even flat space, effectively camouflaging the hidden object (Liu et al., 2024). Liu et al. (2024) demonstrated an ultra-broadband illusion cloak based on guiding waves through subwavelength tunnels, recreating the incoming wavefront on the exit side and effectively cancelling scattering below a certain frequency limit. Challenges remain, particularly in extending these concepts to three dimensions robustly (Huang et al., 2025 explores a simplified 3D approach) and dealing with inherent losses and fabrication tolerances. Hybrid active/semi-active approaches, using materials like piezoelectric composites and smart viscoelastic layers controlled adaptively, are being explored, especially for complex environments like underwater acoustic camouflage (Hasheminejad & Kasaeisani, 2024).

Design, Fabrication, and Emerging Frontiers

The design of acoustic metamaterials often involves complex geometries and requires sophisticated tools. Inverse design and topology optimization methods, increasingly powered by machine learning and artificial intelligence (AI), are becoming indispensable for discovering novel structures with desired acoustic responses (Dedoncker et al., 2025; MacNider et al., 2025; Qian et al., 2025). Evolutionary algorithms and generative models can explore vast design spaces to find high-performance solutions (Dobrykh et al., 2025; Dedoncker et al., 2025).

Fabrication, especially for high-frequency or miniaturized applications, poses significant challenges. Additive manufacturing (3D printing) is widely used, but precision, material properties, and potential defects can impact performance (Chojnacki et al., 2025). Techniques like two-photon polymerization are enabling the fabrication of micro-acoustic metagratings, but thermoviscous losses become significant at these scales and must be incorporated into the design process (Melnikov et al., 2024). The integration of active components (e.g., piezoelectric actuators, speakers) controlled electronically offers a pathway towards adaptive and reconfigurable metamaterials (Malléjac et al., 2025; Zhang et al., 2024). Such active metasurfaces can dynamically shape complex sound fields, enabling functionalities like multi-user sound communication isolation in reverberant rooms (Zhang et al., 2024). Exploiting nonlinearities in metamaterials also presents opportunities for novel wave control, such as shock mitigation or pulse shaping (MacNider et al., 2025).

Conclusion

Acoustic metamaterials have rapidly evolved from theoretical concepts to tangible engineered structures capable of unprecedented control over sound waves. They offer promising solutions for low-frequency noise control, surpassing the limitations of traditional materials, and provide pathways towards the realization of acoustic cloaking and illusion devices. Key mechanisms like local resonance, space coiling, and scattering management are continuously refined, while advanced design tools based on optimization and AI accelerate the discovery of novel functionalities.

Significant challenges remain, including the development of truly broadband and efficient devices, practical implementation in complex environments (e.g., with flow, Colombo & Iemma, 2025), scalability, and cost-effective fabrication. Future directions point towards hybrid active-passive systems for adaptive control, the development of multifunctional metamaterials integrating noise control with structural integrity, sensing, or energy harvesting (Hyun et al., 2024; Feng & Li, 2025), and the exploitation of time modulation and nonlinearity for advanced wave manipulation. The ultimate goal is shifting towards creating intelligent acoustic environments where sound propagation can be dynamically sculpted on demand, transforming applications in acoustics, communication, and beyond.

References

Chojnacki, B. et al. (2025). Accuracy of 3D printing of the samples in the prototyping of cavity-based metamaterials. Scientific Reports. https://doi.org/10.1038/s41598-025-98391-4
Colombo, G. & Iemma, U. (2025). Assessment of the performances and limitations of spacetime convective corrections for acoustic metacontinua design. Scientific Reports. https://doi.org/10.1038/s41598-025-92264-6
Dedoncker, S. et al. (2025). Generative inverse design of multimodal resonant structures for locally resonant metamaterials. Engineering with Computers. https://doi.org/10.1007/s00366-025-02130-2
Dobrykh, D. et al. (2025). 3D evolutionarily designed metamaterials for scattering maximization. Communications Engineering. https://doi.org/10.1038/s44172-025-00349-x
Feng, H. & Li, L. (2025). Review of damping composite materials and structures involving self-healing constituents. Frontiers of Mechanical Engineering. https://doi.org/10.1007/s11465-025-0833-7
Hasheminejad, S. M. & Kasaeisani, A. (2024). A novel smart hybrid multimorph piezoelectric spherical shell cloak for broadband near-perfect underwater acoustic camouflage applications. Scientific Reports. https://doi.org/10.1038/s41598-024-63201-w
Huang, J-H. et al. (2025). A simplified approach to 3D spherical cloaking using shell based dielectric metamaterials. Scientific Reports. https://doi.org/10.1038/s41598-025-94344-z
Hyun, J. et al. (2024). Simultaneous low-frequency vibration isolation and energy harvesting via attachable metamaterials. Nano Convergence. https://doi.org/10.1186/s40580-024-00445-2
Ismaili, Z. et al. (2025). Converting industrial noise into useful electrical energy: a review and case study on acoustic energy harvesting in district cooling plants. Sustainable Energy Research. https://doi.org/10.1186/s40807-025-00156-0
Jean, F. et al. (2024). Advancement in piezoelectric nanogenerators for acoustic energy harvesting. Microsystems & Nanoengineering. https://doi.org/10.1038/s41378-024-00811-4
Li, X. et al. (2025). Analytical and numerical study on bandgap and transmission of periodical truss structures based on phononic crystal theory. Advances in Bridge Engineering. https://doi.org/10.1186/s43251-025-00161-9
Liu, C. et al. (2024). Ultra-broadband illusion acoustics for space and time camouflages. Nature Communications. https://doi.org/10.1038/s41467-024-49856-z
Liu, Y. et al. (2025). Enhanced broadband low-frequency performance of negative Poisson’s ratio metamaterials with added mass. Scientific Reports. https://doi.org/10.1038/s41598-025-98598-5
MacNider, B. et al. (2025). Customizable wave tailoring nonlinear materials enabled by bilevel inverse design. Nature Communications. https://doi.org/10.1038/s41467-025-58630-8
Malléjac, M. et al. (2025). Active control of electroacoustic resonators in the audible regime: control strategies and airborne applications. npj Acoustics. https://doi.org/10.1038/s44384-025-00006-9
Melnikov, A. et al. (2024). Shape optimized acoustic metagratings for anomalous refraction under strong thermoviscous effects. Scientific Reports. https://doi.org/10.1038/s41598-024-76240-0
Qian, C. et al. (2025). A guidance to intelligent metamaterials and metamaterials intelligence. Nature Communications. https://doi.org/10.1038/s41467-025-56122-3
Yang, F-B. & Huang, J-P. (2024). Particle Diffusion Process with Artificial Control: Diffusion Metamaterials. In: Diffusionics. Springer, Singapore. https://doi.org/10.1007/978-981-97-0487-3_17
Zhang, H. et al. (2024). Optimizing multi-user indoor sound communications with acoustic reconfigurable metasurfaces. Nature Communications. https://doi.org/10.1038/s41467-024-45435-4
Zhuang, P. (2024). Diffusionics: Basic Theory and Theoretical Framework. In: Diffusionics. Springer, Singapore. https://doi.org/10.1007/978-981-97-0487-3_1