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The impetus for examining ventilated acoustic metamaterials stems from the demand for increasingly efficient and adaptable noise reduction and sound wave manipulation techniques. However, for the present reconfigurable metamaterial absorbers, their absorption band and ventilation rate have not yet attained practical application at low frequencies (<1000 Hz). A theoretical and experimental demonstration of a reconfigurable ultra-sparse ventilated metamaterial absorber for low frequencies with highly sparse ventilation and tunability is presented. The acoustic absorber is based on triadic cylindrical Helmholtz resonators, and a coupled mode theory for three resonators is implemented to elucidate the underlying physics. The absorber achieves a sparsity level of more than 80%, with a measured wind velocity ratio higher than 95%. Furthermore, tunability is accomplished in the frequency range of 600 to 950 Hz through reconfiguration. Due to its simple structure, this metamaterial absorber can be easily fabricated in mass in the future, providing promising solutions for various acoustic engineering applications. [ABSTRACT FROM AUTHOR]

It is desired to achieve broadband noise reduction while supporting heat and fluids transmission simultaneously. However, attenuating sound energy with simultaneously achieving excellent heat-transferring performance in a fluid environment has yet to be explored. Here, we demonstrate an ultra-broadband absorber with outstanding thermal convection under a fluid environment, whose thickness is only λ/15 (λ is the airborne wavelength). By linearly extending the embedded apertures, the silencer can achieve sound absorption in the range of 455–585 Hz and can block more than 90% of sound energy in the ultra-broadband range of 455–1600 Hz. In addition, the outstanding thermal convection performance of the metamaterial absorber is experimentally demonstrated. [ABSTRACT FROM AUTHOR]

An automatically adaptive metamaterial sound absorber is designed, which can absorb tunable low‐frequency (<500 Hz) sounds under ventilated conditions by designing a feedback circuit to actively detect the noise signals and adjust the sliders on the reconfigurable absorbers. The automatically adaptive ventilated absorber provides an intelligent route to adapt for different low frequencies, through adjusting the sound absorption units directly in accordance with the external environment while retaining high‐efficiency absorption and ventilation. The intelligent sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators is employed to understand its mechanism. In the future, the absorber that is fabricated can be adjusted to adapt for different working frequencies in stunning applications such as ventilated smart windows, which not only effectively solves the influence of external noise but also provides a fresh and bright working environment. The automatically adaptive absorber should also find promising applications in ducts, where the frequencies of noises can vary time by time, and it is often difficult and inaccurate to manually tune an absorber in such scenarios. [ABSTRACT FROM AUTHOR]

For most acoustic metamaterials, once they have been fabricated, their operating frequencies and functions cannot be adjusted, which is an intrinsic barrier for the development of realistic applications. The study to overcome this limit has become a significant issue in acoustic metamaterial engineering. Although with the advance of metamaterials in the past two decades, a series of methods such as electric or magnetic control have been proposed, most of them can only work in the condition of no fluid passage. Some metamaterials with large transmission losses have been proposed, but the sounds are essentially reflected rather than absorbed. Here, to overcome this intrinsic difficulty, we propose a ventilated sound absorber that can be manually tuned in a large range after being manufactured. During the tuning that is achieved through an intricately designed slider, high-performance absorption and ventilation are both ensured. The tunable ventilated sound absorber is demonstrated experimentally and the effective model of coupled lossy oscillators can be employed to understand its mechanism. The manually tunable ventilated metamaterial has potential application values in various complicated pipe systems that require frequency adjustment and it also establishes the foundation for future development of active tunable ventilated acoustic metamaterials. [ABSTRACT FROM AUTHOR]

We propose a multi-band metamaterial perfect absorber with a deep-subwavelength thickness and near-unity absorption coefficient at microwave frequencies. This planar metamaterial is composed of periodically arranged concentric metallic rings and a continuous metallic ground plate separated by one or two lossy dielectric layers. The origin of the induced multi-band perfect absorption is attributed to the multiple impedance-matching to free space and can be achieved through tuning the effective constitutive relations by interactions of several electromagnetic resonances. Numerical simulations reveal the electric field and surface current distributions at each absorption peak, and further analysis indicates that the absorption mainly arises from the dielectric loss. Moreover, the absorption could be enhanced and broaden by overlapping the modes of different rings. The absorption is identical for arbitrary polarizations due to the rotational symmetry of the subwavelength unit. It also remains over 90% energy absorption for a wide angle of incidence up to 45 $$^\circ$$ , which is confirmed by the experiment. The designed multi-band absorber is also easy to manufacture by the mature printed circuit board technology, which is favorable for a wide variety of applications. [ABSTRACT FROM AUTHOR]

The remarkable properties of acoustic metamaterials have attracted massive researches and applications, especially on low-frequency sound absorptions. Currently, most of the acoustic metamaterial absorbers employ resonances in plastic cavities, and their structural strengths are important in many circumstances, especially in harsh environment. However, studies of metamaterials including this point are very scarce. Here, we propose an acoustic metamaterial for low-frequency (<500 Hz) absorptions, composed of three nested square split tubes with inverted opening directions. The efficiency of the absorber is investigated both numerically and experimentally, and absorptions at the peeks are found to exceed 90% and the frequency can be effectively adjusted by tuning its geometric parameters. We further test its yield strength under compression and confirm its buckling behavior happens from the outmost layer. This tunable acoustic metamaterial with a fairly good mechanical strength may lead to broad applications in noise reduction. • We propose a split-tube metamaterial absorber with a sensitive dependence on geometric parameters. • Simulation and experiment results confirm its high-efficiency and customizable absorptions. • Its compressive strength is theoretically analyzed and agreement with experimental measurement reveals its buckling behavior. • The metamaterial absorber has potential applications for noise reduction in both buildings and harsh environment. [ABSTRACT FROM AUTHOR]