3Pb1-1
Low-frequency sound absorbing metasurface using
multiple split resonatorsከ㔜ศ๭⟶ඹ᣺ჾ࡟ࡼࡿప࿘Ἴ྾㡢࣓ࢱ⾲㠃
Shota Takasugi1†, Keita Wattanabe1, Masaaki Misawa1 and Kenji Tsuruta1(1Okayama Univ.)㧗ᮡ຾ử 1†㸪Ώ㎶ᩗኴ 1㸪୕⃝㈼᫂ 1㸪㭯⏣೺஧ 1 (1 ᒸᒣ኱㝔 ⮬↛㸧

1. Introduction
Sound absorption/insulation technology has
always been required in many places such as houses
and public facilities. In particular, low-frequency
sounds are not only harmful to objects, but also tomental health, and countermeasures are necessary. In
the environment that requires high soundabsorption/insulation performance, thick and heavysound absorbing materials have often been usedinspite of their costfullness. It has thus been desired
to develop a new class of materials that canabsorb/shield the low-frequency sound without theheavy mass and large thickness.
In previous researches, we have been focusing
on the Decorated Membrane Resonator (DMR) [1,2],
which is one of the acoustic metasurfaces that are
sufficiently smaller in size than the wavelength ofsound waves. It has been shown that the DMR canachieve nearly 100% sound absorption at theresonance frequency. It is, however, difficult toachieve the efficiency in low-frequency range below
1000Hz by the metasurface unless the size of theresonator is large enough. Other metasurfaces thanDMR, such as a three-dimensional single-port
labyrinthine structure [3] and split tube resonators[4], have been proposed to overcome this drawback.
Those structures are still hampered to be a practicalapplication by their narrow operating range. In thepresent study, we propose a device that can absorb
and block low-frequency sound over a wide range bymultiplexing split tube resonators.

was about 66 cm, and the thickness of the soundabsorbing structure including the split tube and thealuminum plate was 2.5 cm. The high soundabsorption characteristics are thereby exhibited evenin low-frequency sounds where sound absorption isdifficult due to the long wavelength. The principle ofabsorbing sound is based on conversion of the soundenergy into heat energy by mutual vibration of thesplit tube and the air layer [5]. Despite that itachieves nearly 100% sound absorption at theresonant frequency, the sharp peak at the resonant
frequency hampers to utilize the structure in practicalapplications that have ambient sound environmentswith relatively wide frequency spectra.

Fig. 1The basic structure of a split tube.

2. Acoustic absorber based on split tube
resonators
The low-frequency tunable acoustic absorberbased on split tube resonators originally proposed inRef. [4] is a polylactide (PLA) elliptic cylinder witha side cut so that the cross section becomes a split
ring. The aluminum plate has the role of reflecting
sound waves which resonate with the sound absorber,
as well as the role of preventing sound transmission.
Fig. 1 shows the basic structure of a split tube, andFig. 2 shows its sound absorption coefficient. The
wavelength of the sound wave at 516 Hz, at which
the sound absorption peak in Fig. 2 was obtained,

Fig. 2 Sound absorption coefficient of the basic
structure of a split tube.

Proceedings of Symposium on Ultrasonic Electronics, Vol. 41 (2020)25-27 November, 2020

3. Multiple split tube structure
To solve the problem of narrow frequency
band, we focus on multiplexing the split tubes withdifferent size to broaden the absorption spectrum.
Fig. 3 shows the structure of a six-fold split pipe,
which is one of the multiple split structures proposedin the present study.

The sound absorption coefficient for
2- to 6-layered split pipes.

Fig. 4

6. Conclusion

Fig. 3The structure of a six-fold split pipe.

4. Simulation method
A series of numerical simulations on themetasurface model was performed using COMSOLMultiphysics® [6], a commercial 3D finite-element
method (FEM) software developed by COMSOL AB.
To evaluate the absorption characteristics, wecalculated the absorption coefficient based on thetransfer-function method [7]. This method needs tomeasure the sound pressure at two locations with amicrophone, which is simulated by measuring
pressure at two different points in the FEM
calculations.
5. ResultFig. 4 shows the sound absorption coefficientfor 2- to 6-layered split pipes. It is shown to increase
the number of sound absorption peak by increasing
the number of layers. The result indicates, however,that the increase of the absorption peak is not simply
due to the superposition of the absorption peaks ofindividual pipe.
We analyzed the modes at each resonant
frequency with sound absorption peaks. At
frequencies lower than about 500 Hz, soundabsorption peaks are obtained by the single mode inwhich a split tube vibrates at the corresponding
frequency. At the higher frequencies than 500 Hz, on
the other hand, all the tubes vibrate correctively toform “hybrid modes”. Such a hybridization of themode emerges in the cases more than 4 layers.

We have verified the sound absorption of a
split tube in the frequency band below 1000Hz andthe multiplexing of sound absorption peaks createdby superposing multiple split tubes. We thus
succeeded in multiplexing sound absorption peaks inthe frequency band below 1000 Hz. We further foundthe multiplexing is not only by simple superpositionof each mode but also by hybridization of mode thatexhibits a collective motion of the tubes.
Acknowledgment
This work was supported in part by the JSPS
KAKENHI Grant Number 17K19035.
References
1. G. Ma, et al., Nat. Mater. 13 (2014) 873.
2. M. Fujita, K. Manabe, K. Tsuruta, T. Hada, and N.
Yorozu, Proc.symp. Ultrasonic Electronics(USE2018) 39 (2018) 3P1-3.
3. C. Zhang and X. Hu, Phys. Rev. Appl. 6 (2016)
064025.
4. X. Wu et al, Appl. Phys. Lett. 109 (2016) 043501.
5. C. R Liu et al, Appl. Phys. Express 12 (2019)
084002.
6. http://www.comsol.com.
7. ISO 10534-2:1998 “Acoustics–Determination ofsound absorption coefficient and impedance inimpedance
tubes–Part2:
Transfer-function
method”.