Extreme low-frequency ultrathin acoustic absorbing metasurface

Abstract

We introduce a multi-coiled acoustic metasurface providing a quasi-perfect absorption (reaching 99.99% in experiments) at extremely low-frequency of 50 Hz, and simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multi-coiled metasurface offers additional degrees of freedom capable to tune the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation and experimental demonstrations for this unique absorber concept, and discuss its physical mechanism which breaks the quarter-wavelength resonator theory. Furthermore, based on the same conceptual approach, we propose a broadband lowfrequency metasurface absorber by coupling unit cells exhibiting different properties.Due to the weak intrinsic dissipation of conventional materials in the low-frequency region, the perfect absorption of sound at low frequency (<100Hz) is still a scientific challenge [1]. To enhance the dissipation, it is necessary to increase the energy density inside the relevant material by some means like through resonances [2]. In the past two decades, the explosion of interest in developing artificial resonant structures like acoustic metamaterials [3][4][5][6] and metasurfaces [7][8][9][10][11][12] was drastically increased owing to their exotic capabilities of manipulating sound waves and their deep subwavelength thickness [13, 14]. The remarkable characteristics of acoustic metamaterials such as negative mass density, refractive index, double negativity, and controlled anisotropy have attracted massive research in the field of developing deep subwavelength sized acoustic devices for low-frequency applications such as negative refraction [13], deep subwavelength focusing/imaging [14], cloaking [15], perfect absorbers with ultrathin thickness [12,[14][15][16][17][18][19][20][21][22] etc. In related researches, perfect metasurface absorbers have received considerable attention, and variety of acoustic metasurfaces designs have been designed for the applications in noise control at low-frequency regime, which could be used in aircraft, locomotives, automobiles, machines, and buildings [13,[17][18][19][20][21][22][23][24].Ultra-thin acoustic absorber metasurface is generally based on a single or hybrid resonant system [5-12; 16-23]. The hybrid resonant system [25] usually has a broader bandwidth than a single resonant one. One interesting way to design the perfect acoustic absorber is to use an ultrathin decorated membrane with thin air chamber. Such a system was reported by Ma et al. [7] to form hybrid resonance capable of to realizing complete absorption at low-frequency range [8, 9] with a thickness of about ~1/133 of the operating wavelength. However, this design requires

Ultrathin Electromagnetic–Acoustic Amphibious Stealth Coats

“…Multiple resonant units are integrated into a supercell planarly to broaden the absorption bandwidth. [ 14–18,31–34 ] To satisfy the fabrication requirements of FPC, the thickness of PI t PI and copper patches t m are chosen as 0.053 and 0.035 mm, respectively. The copper patches P 1 –P 4 are designed to be ellipses to assure the polarization‐insensitive microwave absorption for normal incidence.…”

“…[ 29 ] In 2015, Leroy et al fabricated a ultrathin polydimethylsiloxane (PDMS) coats with microscale air bubble period array to achieve over 90% absorptivity at the frequencies ranging from 1.4 to 2.9 MHz. [ 30 ] To broaden the bandwidth of the acoustic metamaterial absorber for the audible sound, many researchers took great efforts to integrate multiple resonances in a supercell by adjusting the size of each resonant cavity planarly [ 31–34 ] or vertically [ 35 ] similar to the electromagnetic wave counterpart.…”

“…The Minnaert resonance of an air bubble metascreen in PDMS backed by a copper plate is employed to acquire the underwater acoustic absorption. [ 30 ] By incorporating four air bubbles with various diameters into a supercell planarly, [ 31–34 ] the working bandwidth of the absorber is greatly broadened. A thin flexible printed circuit (FPC) with copper patch array covered above the PDMS shall resonant at specific microwave frequencies and the microwave energy would be dissipated with the help of the dielectric loss of PDMS.…”

Dual frequency sound absorption with an array of shunt loudspeakers

“…Traditional absorbers usually require large back cavity or thick depth for low frequency sound absorption [1][2][3][4][5] . Acoustic metamaterials have been studied intensively due to their subwavelength size [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . For example, thickness of metamaterial absorbers can be significantly reduced with space coiling or folding [6][7][8][9] ; however, the absorption performance is hard to be tuned after they are manufactured 8,10 .…”

“…Acoustic metamaterials have been studied intensively due to their subwavelength size [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . For example, thickness of metamaterial absorbers can be significantly reduced with space coiling or folding [6][7][8][9] ; however, the absorption performance is hard to be tuned after they are manufactured 8,10 . Membrane based metamaterials [11][12][13] and resonance coupling metamaterials [14][15][16][17][18] require specific elastic properties or Q-factors to attain optimal absorption performance, but they are difficult to be manufactured for large-scale applications.…”