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This paper presents a theoretical, numerical, and experimental investigation of a deep-subwavelength absorber based on the concept of coiled-up space. By adjusting a partition panel in the cavity to form an unequal-section channel, it is found that the resonance frequency of the absorber is easily tuned and near-total absorption is acquired under a fixed deep-subwavelength thickness. The absorption mechanism induced by nearly critical coupling is revealed by graphically analyzing the reflection coefficient in the complex plane. In contrast to conventional techniques, near-total absorption can be adjusted over a wider frequency range. To further enhance the absorption, we demonstrate a broadband absorber with a relative bandwidth up to 33.3%. [ABSTRACT FROM AUTHOR]

This paper discusses the design of structured materials for broadband waterborne sound absorption. The structured materials are composed of a rubber matrix embedded periodically with cavities. To find the optimal distribution of cavities, an inverse design method based on topology optimization is proposed. Structured materials with novel hybrid cavities are thus designed. Efficient absorption over a wide frequency range between 600 Hz and 8000 Hz is achieved. The underlying mechanism behind the broadband absorption performance is revealed. Both the bending motion of the structured material and the translational motion of its steel backing affect the absorption in the low-frequency regime. Coherent coupling of local resonant modes together with the multiple scattering effects among cavities contribute to sound absorption in the mid-to-high frequency range. Moreover, a comparison with a conventionally structured material demonstrates the advantages of our design. Finally, an inverse design process with a different rubber matrix is performed. The results show that these cavities still exhibit similar topological features when the shear wave velocity of the rubber matrix is varied. [ABSTRACT FROM AUTHOR]

A new hybrid-mechanism metastructure combined resonances of locally resonant scatterers and air cavities is proposed for broadband waterborne sound absorption. In the design, the locally resonant scatterers are embedded into the backing plate of a rubber layer with air cavities. The results demonstrate significant absorption improvement in the low-frequency range using the locally resonant scatterers. Cavities of mixed sizes and locally resonant scatterers of mixed types can be used to achieve efficient absorption over an ultrawide band. This broadband absorption is found to be attributed to the hybrid-mechanism of the resonances of the cavities and the locally resonant scatterers. Furthermore, the absorption mechanism is illustrated by the displacement patterns, the absorption contribution decomposition analysis, and the effective medium theory. [ABSTRACT FROM AUTHOR]

We propose a hybrid acoustic metamaterial as a super absorber for a relatively broadband low-frequency sound based on a simple construction with deep-subwavelength thickness (5 cm). The hybrid metamaterial absorber is carefully designed and constructed based on a microperforated panel (MPP) and coiled-up Fabry–Pérot channels. It is demonstrated analytically, numerically, and experimentally that over 99% of acoustic absorption could be achieved at a resonance frequency (<500 Hz) with the working wavelength about 30 times larger than its total thickness. It is revealed that the superior absorption is mainly caused by the friction losses of acoustic wave energy in the MPP. The frequency of the absorption peak could be tuned by adjusting the geometry parameters of the MPP and the channel folding numbers. The relative absorption bandwidth could also be tuned flexibly (up to 82%) with a fixed deep-subwavelength thickness (5 cm). The absorber has wide potential applications in noise control engineering due to its deep-subwavelength thickness, relatively broad bandwidth, and easy fabrication. [ABSTRACT FROM AUTHOR]

The acoustic absorption of Alberich coatings is generally a topic attracting persistent interests. The present paper optimizes acoustic absorption of two types of Alberich coatings on a steel plate immersed in water using a differential evolution algorithm combined with a finite element method. Both the coatings contain cylindrical cavities of mixed sizes. We compare the optimized absorption for different arrangement, i.e., the both coatings on different surfaces or only on the outer surface of the steel plate. The results show that the different coatings on both surfaces of the steel plate can achieve better sound absorption from 1.3 kHz to 10.0 kHz. The power dissipation density and the displacement pattern are used to analyze the broadband absorption mechanism induced by the acoustic resonance and the acoustic coupling between the optimized coatings. The results validate the idea that it can effectively enhance the low-frequency and bandwidth of the absorption by laying different coatings on both surfaces of the steel plate. [ABSTRACT FROM AUTHOR]

Abstract: Backing effects on the underwater acoustic absorption of a viscoelastic polymer slab embedded with locally resonant scatterers are reported. The polymer slab is embedded with two layers of locally resonant scatterers, i.e. Al spheres coated by soft silicon rubber. Theoretical absorption coefficients of the polymer slab under different backings using a layer multiple scattering method show good agreement with the experimental results, which supports unambiguously the experimental observation. Then relations between the resonance modes and the low-frequency absorption peaks of the composite slab are clarified to address the absorption mechanisms. It shows that the mass of the steel backing affects evidently the low-frequency absorption, the absorption peak shifts to lower frequency range while increasing the backing mass. [Copyright &y& Elsevier]

Rubber layers with air-filled cavities or local resonance scatters can be used as anechoic coatings. A lot of researches have focused on the absorption mechanism of the anechoic coatings. As the anechoic coatings are bonded to the hull of submarine, the vibration of the hull should not be neglected when the analysis of the absorption characters is carried out. Therefore, it is more reasonable to treat the anechoic coating and the backing as a whole when the acoustic performance is analyzed. Considering the effects of the steel plate backing, the sound absorption performances on different models of anechoic coatings are investigated in this paper. The Finite Element Method is used to illustrate the vibrational behaviors of the anechoic coatings under the steel backings by which the displacement contours is obtained for analysis. The theoretical results show that an absorption peak is induced by the resonance of the steel slab and rubber layer. At the frequency of this absorption peak, the steel plate and the coating vibrates longitudinally like a mass-spring system in which the steel slab serves for mass and the coating layer is the spring. To illuminate the effects of the steel slab backing on the acoustic absorption, the thicknesses of the steel slab and the anechoic layer are discussed. Finally, an experiment is performed and the results show a good agreement with the theoretical analysis. [ABSTRACT FROM AUTHOR]

Recently, by introducing locally resonant scatterers with spherical shape proposed in phononic crystals into design of underwater sound absorption materials, the low-frequency underwater sound absorption phenomenon induced by the localized resonances is observed. To reveal this absorption mechanism, the effect of the locally resonant mode on underwater sound absorption should be studied. In this paper, the finite element method, which is testified efficiently by comparing the calculation results with those of the layer multiple scattering method, is introduced to investigate the dynamic modes and the corresponding sound absorption of localized resonance. The relationship between the resonance modes described with the displacement contours of one unit cell and the corresponding absorption spectra is discussed in detail, which shows that the localized resonance leads to the absorption peak, and the mode conversion from longitudinal to transverse waves at the second absorption peak is more efficient than that at the first one. Finally, to show the modeling capability of FEM and investigate shape effects of locally resonant scatterers on underwater sound absorption, the absorption properties of viscoelastic materials containing locally resonant scatterers with ellipsoidal shape are discussed. [ABSTRACT FROM AUTHOR]

We consider the absorptive properties of three-dimensional phononic crystal (PC) composed of steel spheres arranged in viscoelastic rubber. The mode conversions during the Mie scattering of a single steel sphere in unbounded rubber are analyzed in detail. Then the multiple scattering (MS) and absorption effects induced by the simple cubic lattice and the viscosity of the rubber are investigated by the MS method. The results show that the shear and viscoelastic properties of the rubber are crucial, and the destructive interface induced by MS below each Bloch frequency enhances the absorption. Finally, the acoustic properties of finite PC slabs variation with the filling fraction and the incident angle are discussed for a variety of cases. The results show that the PC can be used as underwater anechoic material. [Copyright &y& Elsevier]

We investigate a meta-absorber composed of impedance-matched rods for the broadband acoustic absorption. However, with a slow longitudinal wave speed, the absorber's density is high to keep the characteristic impedance matching with that of water. To obtain a lightweight high efficiency absorber, we analyze the influence of characteristic impedance and loss factor on absorption behaviors and find that the characteristic impedance matching is not necessary and the low characteristic impedance structure can also attain the efficient absorption performance, and the high dissipation can reduce the density required for an efficient absorption. Based on this, we propose a meta-absorber composed of low characteristic impedance rubber rods in parallel coated by a rubber cover layer and backed with a stepped resin base. By the combination of the transfer matrix method and transmission line impedance transfer equation, the acoustic performance of meta-absorber is analyzed. Compared with the characteristic impedance matching rods, the low characteristic impedance rods can release restrictions on design to attain better acoustic behaviors with a lower density. Further, the integration of rods with designed lengths can motivate coherently coupled weak resonances to obtain a broadband absorption. Finally, an optimized meta-absorber is tested in a water-filled impedance tube and the experimental results agree well with the theoretical results which can achieve a broadband low frequency quasi-perfect absorption (α > 0. 9) in a deep subwavelength region (λ /158 at 950 Hz) with the density close to that of water. Our work finds a new mechanism that low characteristic impedance absorbing materials can achieve the high efficiency absorption with a slow longitudinal wave speed and low density and promise broad applications in the sound stealth of marine vehicles. [Display omitted] • Low characteristic impedance structures achieving an efficient sound absorption. • High losses reducing the structure density required for an efficient absorption. • Slow sound speed of a slender rod decreasing the resonant frequency. • Coherently coupled weak resonances attaining a broadband strong absorption. • Experimental verifications of the broadband low frequency meta-absorber. [ABSTRACT FROM AUTHOR]

Abstract: The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings. [Copyright &y& Elsevier]

[Display omitted] • GANs newly used for topological design of metaporous materials for sound absorption. • Huge acceleration (~0.04s/design) for design process enabling instantaneous designing. • Successful designs of broadband sound absorption checked by simulation and experiment. • Creative configurations and rich local features generated in GANs-designed patterns. • AI-guided designing/optimizing as new possibility for AI-materials interdiscipline. The topological design and optimization of metaporous materials is one of the key challenges in the field of sound absorption. Limited by the expensive computational cost, it is particularly disadvantaged when instantaneous multiple designs are required. In recent years, an increasing number of research fields are harnessing machine learning approaches thanks to their experience-free manner and outstanding efficiency. Generative Adversarial Networks (GANs), as a type of machine learning algorithms, enjoy the special benefit of powerful generative capability, making them brilliantly suitable for designing purposes. Additionally, it can fully explore the data distribution space with enormous computational power and create brand new designs. In this work, GANs are newly employed for the topological design of metaporous materials for sound absorption. Trained with numerically prepared data, they successfully propose designs with high-standard broadband absorption performance, verified by simulation and experiment. The designing process is dramatically accelerated by hundreds of times using GANs (100 designs in 4.372 s). This allows GANs to easily provide more structures and configurations, and achieve instantaneous multiple solutions, giving designers more choices to satisfy various constraints such as mass or porosity. In addition, GANs are demonstrated remarkably capable of generating creative configurations and rich local features. This work proposes a new designing principle, illustrates the value of machine learning in guiding the designing and optimizing process in the mechanical world, and opens new possibilities for the future of AI-materials interdisciplinary research. [ABSTRACT FROM AUTHOR]

• Predict the sound absorption curve from 300 Hz to 3000 Hz (50 Hz interval) with one single deep learning network. • Build residual connection network blocks to obtain network models of different depths. • Determine the most suitable network hyperparameter via constantly monitoring the overfitting level of all the models. • Exploit cross-validation to train the network to the best performance. Obtaining the airborne sound absorption coefficient is essential for studying the sound absorption performance and sound absorption mechanism of acoustic metamaterials. The most commonly used method for numerical calculation of airborne sound absorption coefficient is Finite Element Method (FEM). However, when the number of samples is relatively large, especially when the internal geometric structure of the samples is complicated, the calculation cost of FEM becomes exponentially high. Compared with FEM, machine learning algorithms show great potential in efficiently and intelligently predicting material properties. Taking images representing the topological structure of acoustic metamaterials (along with their airborne sound absorption performance simulated by FEM) as input, we propose a deep convolutional neural network to predict the broadband airborne sound absorption curve of the metaporous materials from 300 Hz to 3000 Hz with the interval of 50 Hz. To avoid overfitting, the network hyperparameter with favorable generalization capability is determined via constantly monitoring the overfitting level of the network. In addition, cross-validation is exploited to train the network to the best performance. Designed in such a compact manner where only one network is sufficient to predict for a whole absorption curve with a large range, the network is marvelously computationally economic and efficient and shows excellent prediction accuracy. [ABSTRACT FROM AUTHOR]

[Display omitted] • SAP-net with CNN to predict the sound absorption coefficient of metaporous materials. • Enormous acceleration achieved (7ms/image) for the prediction process. • Trained SAP-nets show outstanding accuracy with MAE averagely smaller than 0.02. • SAP-net's ability to learn the underlying mechanism of topology-performance linkage. • New fast and accurate tool for investigating sound absorption and designing materials. Airborne sound absorption coefficient is the premise for investigating the sound absorption performance or mechanism of metaporous materials. The common numerical evaluation approach is FEM which is relatively computationally costly particularly when processing complex structures or a large batch of data. Rapidly developing deep learning algorithms, on the other hand, show a promising trend in the data-driven manner to learn and predict material parameters efficiently and precisely. We propose SAP-net based on deep convolutional neural network to predict the sound absorption coefficient at a specific frequency of an input image representing the topological structure of metaporous materials. Trained with FEM-prepared data for six frequency points, SAP-net demonstrates outstanding evaluation speed of 0.007 s/image and brilliant prediction accuracy with mean absolute errors all smaller than 0.019 (the smallest 0.008 at f = 1000 Hz). Meanwhile, the fact that SAP-net remains accurate when predicting for images that are essentially different from those in the training data shows its capability of learning and capturing the underlying physical mechanism linking the topological structure to the sound absorption performance. In conclusion, SAP-net provides an extraordinarily fast and accurate approach for the investigation of sound absorption performance, which is expected to accelerate the examination and design process of materials. [ABSTRACT FROM AUTHOR]