3P1-7
Fabrication and evaluation of sound-absorbing metasurface with coincidence-effect suppression
Tomoya Ishikawa1͋, Masaaki Misawa2, and Kenji Tsuruta1*(1Grad. School Nat. Sci. & Technol., Okayama Univ. 2Fukuoka Institute ofTechnology)
1. Introduction
In recent years, noise problems associated withtechnological developments, such as transport andfactories, has become an important issue. In thepresent study we focus in particular on thephenomenon of the coincidence effect as one of theessential keys to solving the issue. The coincidence
effect[1] is an effect where the transmission ofincident sound through a wall is enhanced when thewavelength of the wall deflection (lamb) wave
coincides with the wavelength of the incident soundwave. In this study, we adopted designing an acoustic
meta-surface[2-5] that is thinner than the wavelengthof sound, but exhibits the sound insulation for wide
range of frequency by suppressing the coincidence
effect without additional sound absorbing materials
or changing the wall thickness.
In our previous report, [6] we have designed ameta-surface structure to provide both soundinsulation and sound absorption by periodicallyarranging holes in a plate and creating air inlets
therein. The periodic structure can be regarded as aphononic crystalline structure on the plate surface.
The phononic crystal[7] is an artificial material whose
response properties to incident sound waves can be
designed in terms of its dispersion relations(phononic band).
Simulations were carried out using a twodimensional finite element method (FEM), after
which the structures were actually fabricated andtested.
2. Acoustic Meta-Surface based on PeriodicStructureFirst, the sound transmission of aluminum plateswith and without a periodic structure are comparedby numerical simulations. In order to reduce thecoincidence effect in thin plates, we have employeda grating structure by applying aluminum tapes
periodically on one face of an aluminum plate. The
unit cell of the proposed structure is shown in Fig. 1.
When a plane wave of 1 Pa is incident on a flat 3
mm thick aluminum plate without the periodic
structure at an angle of incidence of 60°, thetransmission loss drops significantly at 5310 Hz. As
shown in the inset of Fig. 2, the plate is significantly*e-mail: tsuruta@okayama-u.ac.jp

displaced resonating to the incident wave, indicatingthat a coincidence effect occurs.

Fig. 1 Cross sectional view of unit cell in the proposed
acoustic meta-surface (w/ periodic structure).

Figure 2 also shows that the transmission loss isrecovered to high dB in the frequency for the platewith periodic structure. The inset of Fig. 2 depicts
that the displacement in the periodic structure at thecoincidence frequency is very small compared to thedisplacement in the plate without the structure. Thisimplies that the existence of gap frequencies where
Bragg scattering of Lamb waves takes place in thethin plate, thus blocking sound near the frequency ofthe coincidence effect.

Fig. 2 Transmission loss spectra and (inset) displacement
distributions in the Al plates with and without periodic
structure at 5310 Hz.

3. Experimental measurement
The structure shown above was fabricated and themagnitude of the vibrations was measured using a
Laser-Doppler vibrometer[8].Ө As the coincidence
effect causes the plate to vibrate significantly when
it occurs, the presence or absence of thisphenomenon was confirmed by measuring themagnitude of the vibration using the vibrometer.ӨThealuminum plate has the dimension of 200 mm high,
300 mm wide, and 3 mm thick. The periodic
structure proposed in this study was fabricated byapplying eight aluminum tapes of 0.1 mm thickness.Ө
Figure 3 shows the results of the vibrometermeasurements. The peak around 5300 Hz of thespectrum, where the significant plate oscillations

was observed, indicates that the coincidence effecthas taken place in the plate without the periodic
structure. On the other hand, the reduction of thepeak is clearly observed in the plate with periodicstructure, implying the coincidence effect wassuppressed efficiently by the proposed structure.

(referred as “periodic MR”) shown in Fig. 4. The
vibrational mode profile for each of the three peaksin the spectrum is also shown in the inset of thefigure. These figures indicate that the energy of theincident sound wave is converted into thermalenergy through the vibration of the membrane,
leading to the efficient sound absorption. This can be
regarded as a similar mechanism elaborated in thedecorated MR. The frequency at which soundabsorption occurs depends on the deformation modeof the membrane. We thus find that the addition ofPP membranes to the grating structure in analuminum plate not only provides coincidence-effectsuppression, but also a sound-absorbing effect.
Conclusion

Fig. 3 Experimental results for the magnitude of vibrationof normal plate and Structure 1.

4. Design of sound-absorbing meta-surface
In addition to the sound insulation, we also aimed
for implementing a sound-absorbing function in themeta-surface. As shown in Fig. 4, a polypropylene
membrane was applied to cover the holes between
the aluminum tapes. This structure was adopted
based on a sound-absorbing device called adecorated Membrane Resonator (MR)[3], where theenhanced vibration of the membrane is induced
through the normal mode of the membrane and its
hybrid mode with the air layer.

Using FEM simulations, an acoustic meta-surface
structure with periodic aluminum tapes was designed
to prevent the occurrence of coincidence effects.
Experimental measurements for the proposed
structure actually fabricated were carried out with alaser-doppler vibrometer. The results showed that thecoincidence effect was effectively suppressed. Thisapproach can be applied to various structures wherehigh sound insulation performance is required in alimited space and weight.
It was also confirmed by FEM analysis that addinga PP film onto the periodic structure could produce asound absorbing effect. The fabrication of the soundabsorbing structure and the experimental approach toan examination of the sound absorption effects will
also be discussed in the presentation.
Acknowledgement
This work was supported in part by the JSPS
KAKENHI Grant Numbers 21H05020.

Fig. 4 Sound-absorbing structure with PP membrane(periodic MR).

Fig. 5 Sound absorption spectrum of periodic MR andmode profiles of the membrane at the peaks indicated by
the red arrows.

Figure 5 shows the simulation results of the soundabsorption of the proposed meta-surface structure

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