Conventional acoustic absorbers are used to have a structure with a thickness comparable to the working wavelength, resulting in major obstacles in real applications in low frequency range. We present a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in an extremely low frequency region. The metasurface possessing a deep subwavelength thickness down to a feature size of ∼λ/223 is composed of a perforated plate and a coiled coplanar air chamber. Simulations based on fully coupled acoustic with thermodynamic equations and theoretical impedance analysis are utilized to reveal the underlying physics and the acoustic performances, showing an excellent agreement. Our realization should have an high impact on amount of applications due to the extremely thin thickness, easy fabrication, and high efficiency of the proposed structure.

We report the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies. This is achieved through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption. Two examples, both using decorated membrane resonators (DMRs) as the basic units, are presented. The first is a flat panel comprising a DMR and a pair of coupled DMRs, while the second one is a ventilated short tube containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both examples, near perfect absorption, up to 99.7%, has been observed with the airborne wavelength up to 1.2 m, which is at least an order of magnitude larger than the composite absorber. Excellent agreement between theory and experiment is obtained.Total absorption of sound using subwavelength structures or materials has always been a challenge, since the linear dynamics of dissipative systems dictates the fractional power to be linearly proportional to the elastic deformation energy [1], which is negligible in the sub-wavelength scale. To enhance the dissipation, it is usually necessary to increase the energy density, for example, through resonances. However, in an open system, radiation coupling to resonances is an alternative that can be effective in reducing dissipation. In previous studies, by utilizing localized subwavelength resonances, membrane-type metamaterial [2][3][4][5][6][7], containing decorated membrane resonator (DMR) with tunable weights, has shown efficient and flexible capability in low frequency sound absorption [8]. A balance between dissipation and scattering at resonance has been found for optimum absorption [9]. More recently, a perfect absorber has been realized by hybridizing DMR's two resonances through coupling via a thin gas layer. Through interference, waves reflected from such DMR have been shown to completely cancel that from a reflective wall placed a short distance (about 1/133 of airborne wavelength) behind the DMR [9,10]. Meanwhile, the coherent perfect absorber (CPA) in optics shows that the scattering waves at resonance can be cancelled when another counterpropagating coherent light wave, with specific phase and intensity, interferes with the incident beam, thereby leading to total absorption [11][12][13][14][15][16]. Recent efforts have also been made for its analogy in acoustics [17][18][19][20]. However, except for some theoretical attempts in acoustic [21] and numerical studies in optics [22], up to now no perfect absorber has been experimentally realized that intrinsically eliminates all the scattered waves, thereby realizing total absorption regardless of the incident direction, and with no need for a control wave.In this article, we advance the idea of creating a total acoustic absorption unit comprising a monopole (symmetric under mirror reflection) and a dipole (anti-symmetric) resonator that are re...

Acoustic perfect absorption via a structure with deep subwavelength thickness is of great and continuing interest in research and engineering. This study analytically and experimentally investigates acoustic systems based on Helmholtz resonators which have embedded-apertures. The strategy of embedding apertures greatly improves the ability to manipulate the impedance of the systems. Based on the inverted configuration, perfect absorption has been realized (reaching 0.999 in experiments) via a design whose thickness is only ∼1/50th of the operating wavelength. Moreover, a tunable resonant frequency (137–300 Hz) and tunable absorption frequency bandwidth (22%–46%) can be achieved while preserving the perfect absorption performance and constant external shape. In tuning the perfect absorbers having a constant thickness, a conservation factor is revealed experimentally and then verified analytically, which could guide absorbers' design and facilitate the tuning. In addition, the distinct features of the proposed design were evaluated and validated and were compared with those of a related structure, a metasurface with a coiled backing cavity. The results have the potential to help with the design of highly efficient, thin, and tunable acoustic absorbers.

We theoretically report on an innovative and practical acoustic energy harvester based on a defected acoustic metamaterial (AMM) with piezoelectric material. The idea is to create suitable resonant defects in an AMM to confine the strain energy originating from an acoustic incidence. This scavenged energy is converted into electrical energy by attaching a structured piezoelectric material into the defect area of the AMM. We show an acoustic energy harvester based on a meta-structure capable of producing electrical power from an acoustic pressure. Numerical simulations are provided to analyze and elucidate the principles and the performances of the proposed system. A maximum output voltage of 1.3 V and a power density of 0.54 μW/cm3 are obtained at a frequency of 2257.5 Hz. The proposed concept should have broad applications on energy harvesting as well as on low-frequency sound isolation, since this system acts as both acoustic insulator and energy harvester.

The metascreen-based acoustic passive phased array provides anew degree of freedom formanipu-latingacoustic waves due to their fascinating properties, such as afully shifting phase, keeping impedance matching, and holding subwavelength spatial resolution. We develop acoustic theories to analyze the transmission/reflection spectra and the refracted pressure fields of a metascreen composed of elements with four Helmholtz resonators (HRs) in series and a straight pipe. We find that these propertiesare also valid under oblique incidence with large angles, with the underlying physics stemming from the hybrid resonances between the HRs and the straight pipe. By imposing thedesired phase profiles, the refracted fields can be tailored in ananomalous yet controllable manner. In particular, two typesof negative refractionare exhibited,based on two distinct mechanisms: one is formed fromclassical diffraction theory and the other is dominated by the periodicity of the metascreen. Positive (normal) and negative refractions can be converted by simply changing the incident angle, with the coexistence of two typesof refractionin a certain range of incident angles. 1 2 . The symbols p i , p r , and p t represent the obliquely incident, reflected and transmitted sound waves, respectively. (b) A single HR is illustrated for the purpose of the corrected acoustic impedance of the cavity. (c) A single HR is located in front of a hard boundary (dashed line) to obtain the corrected acoustic radiation impedance at the junction between the neck and the straight pipe.New J. Phys. 18 (2016) 043024 Y Li et al

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In this work, we analytically and experimentally present perfect acoustic absorbers via spiral metasurfaces composed of coiled channels and embedded apertures. Perfect absorption (reaching 0.999 in experiments) is realized with an ultra-thin thickness down to ∼1/100th of the operating wavelength. Owing to the superior impedance manipulation provided by the embedded apertures, perfect absorption with tunable frequencies is demonstrated. Our results would contribute to paving a way towards designing thin and light absorbers for the low frequency absorption challenge.

Carbon monoxide (CO), a crucial gas message molecule, plays an important role in the regulation of physiological and pathological process. Hypoxia-induced CO is involved in modulating various cellular activities, including signal transduction, proliferation, and apoptosis. However, tracking CO fluctuation in the hypoxic cells is still a challenge due to lack of straightforward, visualized, and noninvasive tools. In this work, based on metal palladium-catalyzed reaction, we present the rational design, synthesis, and biological utility of an azobenzene-cyclopalladium-based fluorescent probe, ACP-2, for CO monitoring. ACP-2 exhibits capacity of detecting CO in aqueous buffer solution and live cells with high sensitivity and specificity. Utilizing ACP-2, we displayed a direct and visual evidence of endogenous CO up-regulation in live cells induced by hypoxia. Moreover, CO up-regulation during oxygen-glucose deprivation/reperfusion (OGD/R) was also imaged and certified by ACP-2.