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To prevent noise pollution, a hexagonal acoustic metamaterial cell combining multiple parallel Helmholtz resonators with optional apertures is proposed. There were 6 trapezoidal chambers and 6 triangular chambers, and each front panel had 6 different apertures, which meant that there were 6 12 = 2176782336 possible permutations. The distribution of sound pressures obtained by acoustic finite element simulation revealed the acoustic absorption mechanism, which provided effective guidance to alter the absorption capacity. For certain scenarios, the acoustic absorption performance was optimized by the joint combination of artificial neural network and acoustic finite element simulation. Through manufacturing and testing the sample, actual average acoustic absorption coefficients were achieved at 0.6733, 0.7296, 0.8785 and 0.7065 for the target frequency ranges 350–950 Hz, 400–1000 Hz, 500–800 Hz and 350–700 Hz, respectively, with total thickness 40 mm. The tunable acoustic absorption property proved that the hexagonal acoustic metamaterial cell was appropriate for noise reduction with variable frequency ranges.

This study designs an underwater acoustic absorption metamaterial based on a multi-cavity diaphragm structure. The acoustic performance is carefully modeled and examined through simulations in COMSOL Multiphysics finite element software (v.6.1). First, a multilayer periodic unit model consisting of a main cavity and sub-cavities is constructed. A corresponding acoustic-structure coupled finite element model is established by incorporating diaphragm thickness and pre-tension parameters. The frequency domain analysis method is then employed to simulate sound wave transmission and resonance absorption within the structure, calculating the relationship between the acoustic absorption coefficient and frequency. Based on parametric sensitivity analysis, the study examines the influence of key parameters, including main cavity depth, slit width, sub-cavity depth, diaphragm thickness, and pre-tension, on acoustic absorption performance. The mechanisms by which these parameters regulate the absorption peak and bandwidth are revealed. The simulation results show that this metamaterial provides effective broadband acoustic absorption from 200 Hz up to 3000 Hz. The effective bandwidth with an absorption coefficient (α > 0.5) reaches 770 Hz, with a maximum absorption peak of 0.96 and an average absorption coefficient of 0.74, indicating excellent low-frequency underwater acoustic absorption capability. This study provides theoretical foundations and design guidelines for underwater noise control and related engineering applications.

Tunable perfect acoustic absorption at subwavelength thickness has been a prominent topic in scientific research and engineering applications. Although metamaterials such as labyrinthine metasurfaces and coiling-up-space metamaterials can achieve subwavelength low-frequency acoustic absorption, efficiently realizing tunable absorption under uniform and limited size conditions remains challenging. In this paper, we introduce a folded slit to enhance the micro-slit acoustic absorber, effectively improving its low-frequency acoustic absorption performance and successfully achieving a perfect acoustic absorption coefficient of 0.99 at a thickness of only 3.1 cm. By adjusting just two parameters of the folded area, we can efficiently achieve a tunable resonant frequency ranging from 525 to 673 Hz and a tunable acoustic absorption bandwidth of 56.5% to 60.2%, simultaneously maintaining uniform external dimensions. Additionally, the folded-slit absorber demonstrates a broader acoustic absorption bandwidth at lower frequencies, enhancing broadband absorption capabilities in the low-frequency domain. These results hold significant potential for the design of highly efficient, thin and tunable acoustic absorbers.

In the last decade, various asphalt paving materials have undergone investigation for sound attenuation purposes. This research aims to delve into the innovative design of sustainable road pavements by examining sound absorption in rubber-modified asphalt mixtures. More specifically, the impact of alternative sustainable materials on the sound absorption of asphalt mixtures across different temperatures, precisely crumb rubber (CR) derived from recycling of end-of-life tires, was investigated. The acoustic coefficient and its Gaussian fit parameters (Peak, BandWidth, and Area Under the Curve) were evaluated. Five different types of asphalt mixtures were studied, encompassing dense, discontinuous, and open mixtures with 0%, 0.75%, and 1.50% CR incorporated through the dry process (DP). The results of sound absorption indicated a slight influence of crumb rubber at temperatures ranging from 10 °C to 60 °C, particularly in mixtures with high void content. On the other hand, as expected, the void content proved to be highly correlated with sound absorption. These findings facilitated the establishment of predictive models that correlate acoustic absorption spectra with the characteristics of asphalt mixtures. As a result, these models will be valuable in the design of the next generation of sound-absorbing pavements.

In this study, sound insulation materials with a high sound absorption coefficient were prepared. In this paper, using cellulose (CEL) and graphene oxide (GO) as the main raw materials and epichlorohydrin as the cross-linker, the CEL-GO composite aerogels were prepared via lyophilisation. The structure, molecular bonding, and acoustic absorption mechanisms of the composite aerogel were characterised and analysed using SEM, FTIR, XRD, BET, and Raman. In addition, corresponding molecular structure models were constructed. The acoustic attenuation of the CEL-GO composite aerogel was measured using a standing wave tube acoustic attenuation tester. The results show that the chemical bond between the GO and CEL composite is established, and the addition of graphene makes the pores of the composite more advanced, which is more favorable for sound absorption, and the acoustic absorption coefficient can reach up to 0.87.

- In this study, an experimental procedure is developed in order to elaborate an acoustic absorber based on date palm fibers. This kind of material has a double interest: The first is due to its great importance in environment protection by recycling wastes; the second is the fact that these materials are used in the treatment of a second environmental problem which is the noise pollution by reducing it. In this work, wastes from the date palm were used to manufacture an acoustic absorber. In fact, agriculture wastes are increasingly used in the acoustic field. In this work, wastes from date palm were used to manufacture acoustic absorbers. To achieve this goal, date palm waste is acoustically evaluated by an experimental characterization based on measuring the acoustic absorption coefficient of the studied material in the frequency band [0-4000 Hz] using the Kundt tube. The obtained results are encouraging with acoustic absorbent coefficient reaching 0.9 in some frequencies.

The passive suppression of combustion instability by quarter wavelength tube was hereby studied to absorb the oscillation pressure with large amplitudes caused by combustion instability. The suppression effects of quarter wavelength tube on combustion instability were systematically analyzed by combining the acoustic numerical simulation and the experimental research methods. Firstly, the influence of quarter wavelength tube on the acoustic characteristics of the system was analyzed using acoustic numerical simulation; and then, the acoustic absorption characteristics to external acoustic disturbance and the suppression effects on the self-excited combustion instability were experimentally studied. The results show that the quarter wavelength tube can effectively absorb the acoustic pressure when the dominant frequency of acoustic pressure is close to the resonance frequency of the system, and can effectively suppress the combustion instability under acoustic resonance. However, given that the quarter wavelength tube adds adjoint dominant frequencies after eliminating the original resonant frequency of the system, and the combustion instability is stabilized on the adjoint dominant frequencies, combustion instability suppression is different from noise suppression. In addition, the diameter of wavelength tube exercises obvious effects on the above characteristics. All these make it necessary to determine the best parameters and the maximum suppression efficiency by combining numerical simulation and experiments. The research results of this paper provide theoretical and technical supports for the suppression of combustion instability by the quarter wavelength tube.

Advanced ceramic sponge materials with temperature-invariant high compressibility are urgently needed as thermal insulators, energy absorbers, catalyst carriers, and high temperature air filters. However, the application of ceramic sponge materials is severely limited due to their complex preparation process. Here, we present a facile method for large-scale fabrication of highly compressible, temperature resistant SiO 2 -Al 2 O 3 composite ceramic sponges by blow spinning and subsequent calcination. We successfully produce anisotropic lamellar ceramic sponges with numerous stacked microfiber layers and density as low as 10 mg cm −3 . The anisotropic lamellar ceramic sponges exhibit high compression fatigue resistance, strain-independent zero Poisson’s ratio, robust fire resistance, temperature-invariant compression resilience from −196 to 1000 °C, and excellent thermal insulation with a thermal conductivity as low as 0.034 W m −1 K −1 . In addition, the lamellar structure also endows the ceramic sponges with excellent sound absorption properties, representing a promising alternative to existing thermal insulation and acoustic absorption materials. Temperature-invariant highly compressible ceramic sponges are attractive for thermal insulators and energy absorbers, but development is limited by complex preparation processes. Here the authors report large-scale fabrication of silica-alumina composite ceramic sponges via blow spinning and calcination.

One of the fundamental hurdles in plasmonics is the trade-off between electromagnetic field confinement and the coupling efficiency with free-space light, a consequence of the large momentum mismatch between the excitation source and plasmonic modes. Acoustic plasmons in graphene, in particular, have an extreme level of field confinement, as well as an extreme momentum mismatch. Here, we show that this fundamental compromise can be overcome and demonstrate a graphene acoustic plasmon resonator with nearly perfect absorption (94%) of incident mid-infrared light. This high efficiency is achieved by utilizing a two-stage coupling scheme: free-space light coupled to conventional graphene plasmons, which then couple to ultraconfined acoustic plasmons. To realize this scheme, we transfer unpatterned large-area graphene onto template-stripped ultraflat metal ribbons. A monolithically integrated optical spacer and a reflector further boost the enhancement. We show that graphene acoustic plasmons allow ultrasensitive measurements of absorption bands and surface phonon modes in ångström-thick protein and SiO2 layers, respectively. Our acoustic plasmon resonator platform is scalable and can harness the ultimate level of light–matter interactions for potential applications including spectroscopy, sensing, metasurfaces and optoelectronics.The momentum mismatch between far-field light and acoustic graphene plasmons can be largely overcome by a two-stage coupling scheme for sensitive protein detection in sub-10-nm films.

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled. Willis coupling is an additional degree of freedom, which can enhance acoustic metamaterials, by coupling monopole and dipole excitations. Here, the authors experimentally demonstrate a meta-atom with Willis coupling approaching the theoretical maximum, which is robust to thermo-viscous losses.