2P1-2
Multilayer Acoustic Metasurfaces for Broadband
Sound-Absorbing and Energy-Harvesting Devices
Akira Kojima1‡, and Kenji Tsuruta1* (1Okayama Univ.)
1. Introduction
As one of the measures against environmental
problems, Sound shielding becomes importanttechnology. In many applications materials withlightweight and thin, yet high sound absorption isrequired. In recent years, as an alternative toconventional sound absorbing materials, acoustic
metasurfaces have been attracting mush attentions.
Acoustic metasurfaces[1] are artificial materials thatcan control acoustic wave propagation, for example,absorption, reflection and transmission. In addition,
their dimensions are sub-wavelength order, which iscompatible to compact devices. For these reasons,various metasurfaces have been proposed. In thepresent study, we focused on an acoustic metasurface
with membrane resonators. Decorated Membrane
Resonator(DMR)[2] has been proposed for soundabsorbing device utilizing hybrid resonance between
membrane and air layer. Figure 1 shows a basic
structure of DMR. By using impedance-matching toairborne sound, DMR can absorb nearly 100% ofincident acoustic wave at resonant frequency.
In addition, DMR has been considered to be acandidate for an energy harvesting device. The
strong vibration of membrane at the resonant
frequency compresses air layer, thus converting theacoustic energy to electric power via piezoelectricelement (e.g., PZT) on the bottom of DMR. Acoustic
energy thus harvested is expected to be a powersource for sensor network in the IoT era.
We adopted frequency-response analysis by 3DFinite Element Method. The incident acoustic waveof 1Pa pressure is incident from the direction normal
to the surface of the structures to evaluate theabsorption as well as the electric power generation
within the frequency range from 500Hz to 4000Hz.

Figure 2 illustrates the 3D model of a DMR withPZT. By frequency domain analysis of the finiteelement method (FEM), we simulated soundabsorption and voltage generation in a single DMR
depicted in Fig.2.

Fig. 2 3D-model of the basic DMR
Figure 3 shows the result of the FEM simulation
for both sound absorption and electric voltage
generation. The figure clearly reveals that the DMR
has a characteristic of high-Q resonance resulting in
narrowband for sound absorbing and energy
harvesting properties. Since the spectrum expectedin an ambient noise has rather broad spectrum, thischaracteristic of DMR hampers effectiveness insound shielding application. For this reason, thepurpose of our research is designing and optimizing
the structure of DMR with broadband responses.

Fig. 3 Sound absorption and voltage generation
spectra of the DMR illustrated in Fig.2.

Fig. 1

Decorated Membrane Resonator

2. Sound absorption and voltage generation inDMR

3. Helmholtz-DMR multilayer structure
So far, several attempts have been reported fordesigning broadband DMR[3] by changing the shapeof membrane or the size of air layer. In recent years,
on the other hand, high performance metasurfaces
have been achieved by coupling with other resonantstructures.[4] Here we also examined a new approachwith a DMR based structure hybridized with anadditional layer that has the same effect as aHelmholtz resonator. Helmholtz resonator is themost classical acoustic resonator for soundproofing.
As the resonator is composed of just small neck andcavity, the combination with acoustic metasurfaces
is easy to design and fabricate.[5] Figure 4 shows thestructure we propose in the present study. As shownin the figure, the upper layer plays as a Helmholtz
resonator and the lower layer corresponds to a DMR.

Fig. 5 optimized structure of the HelmholtzDMR multilayer structure

Fig. 4 The schematic of the proposed
multilayered structure.
4. ResultUsing FEM simulations of sound absorption in
the proposed structure, we found two resonant
frequencies in the spectrum, which can be attributed
to a combined resonance mechanism. We further
optimized the structural parameters to obtain higher
performance in the spectra. Figure 5 illustrates theoptimized structure of the Helmholtz-DMR
multilayer structure. Figure 6 shows for theabsorption/voltage generation spectrum that thisstructure has two resonant frequencies with over
90% sound absorption and 2~3mV voltagegeneration.
5. Conclusion
We have proposed a multilayer Helmholtz-DMR
structure for a highly efficient sound-absorbingmetasurface with power-generation function applicableto the energy harvesting device. The analyses haverevealed that the device we designed have two
combined resonance frequency with over 90% soundabsorption and about 0.1 μW . An experimental
verification is also in progress. We plan to extend thepresent approach to broaden the frequency response byintroducing a periodic arrangement of the proposed
structure for becoming commonplace in loudenvironment.

Fig. 6 FEM result for absorption and voltage
generation spectrum of the multilayer
structure.
AcknowledgmentThis work was supported in part by the JSPS
KAKENHI Grant Numbers 21H05020.
References[1] B. Assouar, B. Liang, Y. Wu, Acoustic
metasurfaces, Nat. Rev. Mater. 3, 460 (2018).
[2] G. Ma, M. Yang, S. Xiao, Acoustic metasurface
with hybrid resonances, Nature Mater. 13, 873(2014).
[3] K. Watanabe, M. Fujita, and K. Tsuruta, Designof non-circular membranes metasurfaces forbroadband sound absorption, Jpn. J. Appl. Phys.
59, SKK06(2020).
[4] Y. Ming, Z. Cao, L. Jun, X. Chou, Recent
developments of acoustic energy harvesting: Areview, Micromachines 10, 48 (2019)[5] G.-S. Liu, G.-S. Peng, M.-H.Liu, X.Y. Zou, J.-C.
Cheng, Broadband acoustic energy harvestingmetasurface with coupled Helmholtz resonators,Appl. Phys. Lett. 113, 153505 (2018)