Recent Developments of Acoustic Energy Harvesting: a Review

micromachines

Review Recent Developments of Acoustic Energy Harvesting: A Review

Ming Yuan 1,* , Ziping Cao 2,*, Jun Luo 2 and Xiujian Chou 3,*

1 School of Automation, Nanjing University of Posts and Telecommunications, Nanjing 210023, China 2 School of Telecommunications and Information Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China; [email protected] 3 Science and Technology on Electronic Test and Measurement Laboratory, North University of China, Taiyuan 030051, China * Correspondence: [email protected] (M.Y.); [email protected] (Z.C.); [email protected] (X.C.); Tel.: +86-025-858-66526 (M.Y.)

Received: 14 December 2018; Accepted: 7 January 2019; Published: 11 January 2019

Abstract: Acoustic energy is a type of environmental energy source that can be scavenged and converted into electrical energy for small-scale power applications. In general, incident sound power density is low and structural design for acoustic energy harvesting (AEH) is crucial. This review article summarizes the mechanisms of AEH, which include the Helmholtz resonator approach, the quarter-wavelength resonator approach, and the acoustic metamaterial approach. The details of recently proposed AEH devices and mechanisms are carefully reviewed and compared. Because acoustic metamaterials have the advantages of compactness, effectiveness, and ﬂexibility, it is suggested that the emerging metamaterial-based AEH technique is highly suitable for further development. It is demonstrated that the AEH technique will become an essential part of the environmental energy-harvesting research ﬁeld. As a multidisciplinary research topic, the major challenge is to integrate AEH devices into engineering structures and make composite structures smarter to achieve large-scale AEH.

Keywords: acoustic energy harvesting; energy conversion; wave manipulation; smart materials and structures

1. Introduction Acoustic sound waves are mechanical waves that possess energy and can be generated by many noise sources. When the sound wave is undesired, we refer to it as noise. Common noise sources include airplanes, vehicles, high-speed trains, power plants, loudspeakers, machines, and expressways. Nowadays, we live in a noisy world, and acoustic energy is an indispensable environmental energy source. The environmental sound levels can be higher than 100 dB, and low-frequency noise is dominant in the frequency spectrum [1–5]. Generally, acoustic energy is ultimately dissipated into thermal energy at the propagation stage, and low- and midfrequency sound waves have attracted the most attention. One reason is that this frequency band of noise is usually a signiﬁcant component of the spectrum. The other reason is that in this frequency range, the corresponding sound wavelength is long, making it difﬁcult to absorb or isolate it using most engineering structures. Many approaches have been developed to effectively absorb or isolate low- to midfrequency acoustic sound waves, and these include passive approaches [6–12] and active approaches [13–17]. At present, the target is to convert sound energy into electrical energy rather than dissipate it. Compared to other environmental energy types (solar, wind, and hydroelectric), acoustic energy is less

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less influenced by the weather and can be harvested with fewer installation restrictions. The influenced by the weather and can be harvested with fewer installation restrictions. The drawback drawback of acoustic energy is that its power is low. This means that, when acoustic energy is of acoustic energy is that its power is low. This means that, when acoustic energy is converted into converted into electrical energy, the power level cannot meet the demand of high power-consuming electrical energy, the power level cannot meet the demand of high power-consuming applications. applications. On the other hand, with the rapid development of modern electrical technologies, the On the other hand, with the rapid development of modern electrical technologies, the power power requirements of embedded chips are substantially decreasing, so that the harvested power can requirements of embedded chips are substantially decreasing, so that the harvested power can be be supplied to low-power devices [18]. supplied to low-power devices [18]. Acoustic waves belong to one kind of mechanical waves and utilizing ambient energy sources has Acousticalways wavesbeen a belong hot topic to one in kind recent of mechanical years. For waves instance, and utilizing an up-to-date ambient review energy sourcesarticle hascomprehensively always been a hotsummarized topic in recent vibration-based years. For instance,energy-harvesting an up-to-date approaches review articlefrom various comprehensively of energy summarizedsources [19]. vibration-basedIn this well-organized energy-harvesting review, vibratio approachesn sources are from classified various into of energydifferent sources categories. [19]. InFurthermore, this well-organized it emphasized review, that vibration resonance sources is an are essential classified element into differentto realize categories.energy harvesting Furthermore, from itcontinues emphasized vibrations that resonance or acoustic is an waves. essential With element respect to to realize the acoustic energy harvestingenergy-harvesting from continues (AEH) vibrationstechnique, or because acoustic environmental waves. With respect acoustic to the waves acoustic belong energy-harvesting to weak excitation (AEH) sources, technique, the because AEH environmentaldevice’s design acoustic is different waves from belong that toof weakthe vibration-based excitation sources, energy the harvester. AEH device’s Acoustic design resonance is different is fromcrucial that in of AEH the vibration-baseddesign, which can energy be used harvester. to amplify Acoustic the incident resonance sound is crucial wave and in AEH sufficiently design, excite which canenergy be used conversion to amplify part. the incident sound wave and sufficiently excite energy conversion part. WithWith the the help help of AEH of AEH technology, technology, distributed distribu smartted sensing smart andsensing condition and monitoringcondition applicationsmonitoring areapplications anticipated are for anticipated innovative for applications. innovative Inapplications. addition, the In harvestedaddition, the acoustic harvested energy acoustic can provide energy a triggercan provide function a trigger and be function utilized and as an be alarm. utilized as an alarm. ToTo successfully successfully convertconvert acousticacoustic energyenergy into electricalelectrical energy, energy, an an AEH AEH apparatus apparatus is is required. required. TheThe incident incident acousticacoustic energy can be be focused focused by by such such a adevice device and and piezoelectric, piezoelectric, electromagnetic, electromagnetic, or ortriboelectric triboelectric effects effects can can be be used used for for energy energy conversi conversion.on. After After conversion, conversion, a power-managing a power-managing circuit circuit is isused used for for voltage voltage rectification rectification [20], [20 ],regulation, regulation, an andd impedance impedance matching. matching. At Atthe the peripheral peripheral side, side, a apower-saving power-saving unit, unit, such such as as a a supercapac supercapacitor,itor, is is used used for for energy energy storage. storage. TheThe schematic schematic diagram diagram of of anan AEHAEH systemsystem isis shownshown inin FigureFigure1 1..

Figure 1. Schematic diagram of an acoustic energy-harvesting (AEH) system. Figure 1. Schematic diagram of an acoustic energy-harvesting (AEH) system. In this review, recently established AEH approaches are reviewed, which aims to pave the way In this review, recently established AEH approaches are reviewed, which aims to pave the way for their further development. To provide a better understanding of this technology, we ﬁrst present for their further development. To provide a better understanding of this technology, we first present the basic elements of audible acoustics. Subsequently, the AEH mechanism is described because it is the basic elements of audible acoustics. Subsequently, the AEH mechanism is described because it is an essential part of AEH device design. In Section4, a detailed review of AEH studies is presented, an essential part of AEH device design. In Section 4, a detailed review of AEH studies is presented, and the performances of the AEH devices are summarized in Section5. At the end of the article, and the performances of the AEH devices are summarized in Section 5. At the end of the article, we we provide comments on current issues and perspectives concerning AEH research. provide comments on current issues and perspectives concerning AEH research. 2. Audible Acoustic Fundamentals 2. Audible Acoustic Fundamentals 2.1. Sound Propagation in Air 2.1. Sound Propagation in Air Here, the frequency range of interest is 20 Hz–20 kHz, which is audible to humans. In the air, soundHere, is propagated the frequency in a longitudinalrange of interest wave is format,20 Hz–20 and kHz, the which mathematical is audible equation to humans. for plane In the wave air, sound is propagated in a longitudinal wave format, and the mathematical equation for plane wave sound propagation is: sound propagation is: ∂2 p 1 ∂2 p − = 0 (1) ∂∂22pp1 ∂x2 c2 ∂t2 −=0 (1) ∂∂xct222

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Propagation speed c in the air is 343 m/s (20 ◦C) and the relationship between sound speed, wavelength, and frequency is expressed as:

c = λ·f, (2) where f is the frequency and λ is the wavelength. In general, we treat sound propagation as adiabatic and inviscid. However, when sound is propagated in narrow tubes or small-scale places, the viscous loss considerably contributes to acoustic damping. The thickness of viscous layer δv has a close relationship with the frequency, which is expressed as [21]: s 2µ δv = (3) ρ0ω where µ is the dynamic viscosity, ρ0 is the air density, and ω is the angular frequency. For 100 Hz sound in air, the thickness of the viscous layer can be up to 0.22 mm. Therefore, to harvest acoustic energy in low frequencies, decreasing the inﬂuence of viscous layer is favorable.

2.2. Sound Pressure Level and Sound Power Audible sound pressure greatly varies and the metric logarithmic-scale sound-pressure level (SPL) is usually used as an effective measurement index. The detailed expression of the SPL is given as [22]:

SPL = 20 log10(p/pre f ) (4) where p is the measured root-mean-squared sound pressure, and pre f is the reference sound pressure, which equals 20 µPa in the air. The SPL’s unit is decibel. Sound pressure p can be linked to sound intensity I (acoustic power per unit area in the perpendicular direction) in a further step. For the plane wave propagation case, the relationship is given in Equation (5): I = p · v = p2/Z (5) where the particle velocity is v and the speciﬁc acoustic impedance is Z. Then, sound power Pow (to avoid confusion with sound pressure p), which is the product of sound intensity I and incident surface area A, can be represented as:

Pow = I · A = (p2 · A)/Z (6)

Table1 list typical values of sound pressures and the corresponding SPL and sound intensity values.

Table 1. Typical relationships of sound pressure, sound-pressure level (SPL), and sound intensity.

Sound Pressure (Pa) SPL (dB) Sound Intensity (W/m2) 0.2 80 0.0001 1 94 0.0025 2 100 0.01 10 114 0.25 100 134 25 2000 160 10000

According to the sound-power definition, sound power is proportional to the squared sound pressure value. Furthermore, because of the logarithmic scale, it has been shown that sound intensity can significantly vary even if SPL differences are small. Micromachines 2019, 10, x FOR PEER REVIEW 4 of 21 Micromachines 2019, 10, 48 4 of 21 Micromachines 2019, 10, x FOR PEER REVIEW 4 of 21 3. Acoustic Energy Harvesting (AEH) Mechanism 3. Acoustic3. Acoustic Energy Energy Harvesting Harvesting (AEH) (AEH) Mechanism Mechanism 3.1. Acoustic-Wave Manipulation 3.1. Acoustic-Wave Manipulation 3.1. Acoustic-WaveIn general, Manipulation environmental sound pressure is low. For instance, when the SPL value equals 100 dB,In thegeneral,In general,corresponding environmental environmental sound sound pressure sound pressure pressure equals is isloonly low.w. Forto For 2instance, instance,Pa. If no when when measures the the SPL SPL are value value taken, equals such 100weak dB, dB,excitationthe the corresponding corresponding cannot sufficiently sound sound pressure pressure excite equals small-scale equals only only tostructures. 2to Pa. 2 IfPa. no If measures no measures are taken, are taken, such weak such excitationweak excitationcannotTherefore, cannot sufficiently sufficiently in AEH excite studies, small-scale excite ansmall-scale acoustic structures. resonatorstructures. is usually used to augment sound pressure to achieveTherefore,Therefore, strong in excitationAEH in AEH studies, studies, of the an structure. acoustic an acoustic resonator resonator is usually is usually used used to augment to augment sound sound pressure pressure to to achieveachieve Forstrong stronginstance, excitation excitation the classical of the of thestructure. Helmholtz structure. acoustic resonator provides an effective method to amplify incidentForFor instance, sound instance, the waves. theclassical classical The HelmholtzHelmholtz Helmholtz acoustic resonator acoustic reso cons resonatornatorists provides of providesa neck an and effective an a effective cavity, method methodand theto amplify toresonator amplify incidentwallsincident aresound soundassumed waves. waves. to The be rigid. TheHelmholtz Helmholtz The acoustic resonator resonator system cons is consistsists equivalent of a ofneck a to neck and a single and a cavity, adegree cavity, and of and freedomthe theresonator resonator (SDOF) wallsmechanicalwalls are areassumed assumed system to be tofor rigid. be the rigid. lowerThe The acoustic mode. acoustic systemA schematic system is equivalent is equivalentdiagram to of a to singlea aHelmholtz single degree degree resonatorof freedom of freedom is (SDOF) shown (SDOF) in mechanicalFiguremechanical 2. system system for forthe thelower lower mode. mode. A schematic A schematic diagram diagram of a of Helmholtz a Helmholtz resonator resonator is shown is shown in in FigureFigure 2. 2.

Figure 2. Schematic diagram of a Helmholtz resonator and its mechanical analogy. FigureFigure 2. Schematic 2. Schematic diagram diagram of a ofHelmholtz a Helmholtz reso resonatornator and andits mechanical its mechanical analogy. analogy. The SDOF system indicates that this device can achieve strong sound-pressure amplification at a specificTheThe SDOF SDOFfrequency, system system indicateswhich indicates indicates that that this the this device occurrence device can can achieve of achieve acoustic strong strong resonance. sound-pressure sound-pressure According amplification amplification to the lumped at at a a specificelementspecific frequency, method, frequency, its which whichfundamental indicates indicates acoustic the the occurrence occurrence resonant of frequency ofacoustic acoustic resonance. can resonance. be calculated According According as: to the to thelumped lumped element method, its fundamental acoustic resonant frequency can be calculated as: element method, its fundamental acoustic resonantcs frequency can be calculated as: f = π + γ (7) csc2()r Vls a ff == (7) (7) 2()2ππ VlV+(lγ+ aγa) where c is the sound speed, s is the neck’s cross-sectional area, l is the neck length, V is the γ wherecavitywhere c volume,isc isthe the sound sounda is the speed, speed, radius s sof is the thethe open neck’sneck’s neck, cross-sectionalcross-sectional and is a correction area,area, llis is the factor,the neck neck which length, length, is assumedV isV theis the cavity to be cavity0.82volume, volume, [23]. a is a the is the radius radius of the of the open open neck, neck, and andγ is aγ correction is a correction factor, factor, which which is assumed is assumed to be 0.82to be [ 23]. TM 0.82 [23]AA. finite-element finite-element simulation simulation of of a Helmholtz resonator was developed using COMSOLCOMSOLTM toto demonstratedemonstrateA finite-element the the SPL simulation and velocity of distribution, a Helmholtz as resonator shown in was Figure developed3 3.. using COMSOLTM to demonstrate the SPL and velocity distribution, as shown in Figure 3.

FigureFigure 3.3. SoundSound pressure pressure and and velocity velocity distribution distribution of a Helmholtzof a Helmholtz resonator resonator at the resonant at the frequency.resonant Figurefrequency. 3. Sound pressure and velocity distribution of a Helmholtz resonator at the resonant frequency.

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At the acoustic resonance state, sound pressure increases gradually from the inlet to the cavity, and sound-pressureAt the acoustic resonancedistribution state, inside sound the cavity pressure is nearly increases uniform. gradually To achieve from the AEH, inlet the to thebottom cavity, of theand Helmholtz sound-pressure resonator distribution is usually inside flexible. the cavity This is generates nearly uniform. sufficient To achievepressure AEH, difference the bottom to the of ambientthe Helmholtz sound resonator pressure, is which usually is flexible. favorable This in generates the AEH sufficient design. pressureIn addition, difference particle to thevelocity ambient is substantiallysound pressure, higher which at isthe favorable neck (Figure in the 3), AEH and design. this contributes In addition, to particlemajor viscous velocity loss, is substantially which was statedhigher atin theSection neck (Figure2.1. The3), andviscous this contributesdamping toca majoruses the viscous resonator’s loss, which measured was stated sound-pressure in Section 2.1. amplificationThe viscous damping ratio to causesbe smaller the resonator’sthan the theoretical measured value. sound-pressure amplification ratio to be smaller thanAnother the theoretical commonly value. used acoustic resonator, which is called the quarter-wavelength tube resonator,Another has commonly also been used acousticto amplify resonator, incident which sound is pressure. called the When quarter-wavelength the tube’s length tube is resonator, equal to thehas quarter also been wavelength used to amplify of the incident incident sound sound wave, pressure. the reactance When the part tube’s of lengththe acoustic is equal system to the becomes quarter zerowavelength and acoustic of the resonance incident sound will occur. wave, the reactance part of the acoustic system becomes zero and acousticFor resonanceinstance, willfor occur.a 0.6 m long quarter-wavelength tube resonator, at resonance, the correspondingFor instance, acoustic for a 0.6wavelength m long quarter-wavelength equals to 2.4 m, which tube corresponds resonator, at to resonance, 143 Hz resonant the corresponding frequency. SPLacoustic distribution wavelength and equals particle-velocity to 2.4 m, which distribution corresponds inside to the 143 tube Hz resonant under this frequency. condition SPL are distribution shown in Figureand particle-velocity 4. distribution inside the tube under this condition are shown in Figure4.

Figure 4. Sound-pressure level (SPL) distribution and particle-velocity distribution distribution of of a a 0.6 m long quarter-wavelength tube resonator at 143 Hz.

These figuresfigures provide provide important important information information on acoustic on acoustic resonators, resonators, so that theso energy-conversionthat the energy- conversionparts can be parts appropriately can be appropriately located to achieve located effective to achieve AEH effective design. AEH design. Aside fromfrom the the aforementioned aforementioned classical classical approaches, approaches, rapidly developing rapidly acousticdeveloping metamaterials, acoustic metamaterials,which are artificial which structures, are artificial exhibit extraordinarystructures, exhibit abilities extraordinary for sound-wave abilities manipulation. for sound-wave Acoustic manipulation.metamaterials Acoustic are powerful metamaterials devices forare powerful sound-wave devices manipulation for sound-wave and their manipulation properties, and which their properties,include sound-pressure which include amplification, sound-pressure local amplification, resonance, subwavelength local resonance, scaling, subwavelength and energy scaling, focusing, and energyare favorable focusing, for are developing favorable AEH for developing devices. Several AEH de relevantvices. Several review relevant papers review have been papers published have been on publishedacoustic metamaterials on acoustic metamaterials and provide readersand provide with areaders quick andwith cleara quick overview and clear of thisoverview topic [24of –this28]. topicSpecifically, [24–28]. energy Specifically, harvesting energy using harvesting metamaterials using is elaboratedmetamaterials on in is References elaborated [19 on,28 ]in and References provides [19,28]a solid and foundation provides for a thissolid field. foundation for this field.

3.2. Energy ConversionConversion When acoustic waveswaves areare impinging impinging on on the the AEH AEH device, device, acoustic acoustic energy energy is usually is usually converted converted into intoelastic-strain elastic-strain energy energy through through a ﬂuid–structure a fluid–struct interactionure interaction process process [29–31 ].[29–31]. Subsequently, Subsequently, an energy an energyconverter converter is essential is essential to transform to transform this kind ofthis energy kind intoof energy electrical into energy. electrical For AEHenergy. applications, For AEH applications,piezoelectric piezoelectric and electromagnetic and electromagnetic materials arematerials the most are the common most common approaches approaches described described in the inAEH the literature.AEH literature.

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3.2.1. Piezoelectric Conversion Piezoelectric material can transform the mechanical strain energy into electrical energy through the piezoelectric effect. Commonly used piezoelectric materials, which include polyvinylidene diﬂuoride (PVDF), lead zirconate titanate (PZT), and barium titanate (BaTiO3), are summarized in Table2[32].

Table 2. Properties of commonly used piezoelectric materials. PVDF–Polyvinylidene diﬂuoride,

BaTiO3–Barium titanate, PZT–Lead zirconate titanate.

Property Units PVDF BaTiO3 PZT Relative dielectric constant – 12 600–1200 1000–4000 Piezo charge constant d31 pC/N 20 −60–−30 −600–−100 Electromechanical coupling factor % 11 21 30–75 Young’s modulus 1010N/m2 0.3 11–12 6–9 Density kg/m3 1780 5300–5700 7500–770

Piezoelectric coefficient d31 significantly influences energy-conversion efficiency because the achieved electromechanical coupling factor is proportional to piezoelectric coefficient d31. In addition, the single-crystal piezoelectric material has attracted the attention of researchers [33]. The single crystal piezoelectric material achieves much higher electromechanical coupling than the commonly used PZT-5A and PZT-5H materials, thereby increasing the energy harvesting performance significantly. The highly oriented PZT film also exhibits a strong piezoelectric effect [34,35], which generates sufficient strain under low-frequency conditions and weak excitations. These materials could be applied to AEH, but more investigations are required.

3.2.2. Electromagnetic Conversion Following Faraday’s law of electromagnetic induction, when a permanent magnet is moved relative to a coil, an electrical potential is created at the coil’s terminals. Induced voltage V is proportional to the change in the rate of magnetic ﬂux density B and the turns of coil n:

V = −n · (dB/dt) (8)

Because the magnetic ﬁeld is created by a permanent magnet, the magnetic ﬁeld has to have a high value of remanence permeability Br. Nowadays, the most commonly used type of magnetic material is neodymium iron boron (NdFeB), which is the strongest magnetic material available for commercial use. NdFeB is usually graded using numbers and a larger grade number indicates a higher Br value. For instance, for the N30 grade, the Br value is 1.08 ∼ 1.15 T and, for the N52 grade, the Br value is 1.42 ∼ 1.47 T.

4. Established AEH Approaches Many studies have investigated AEH mechanisms. In this section, based on the type of acoustic-wave manipulation, the mechanisms are classified into Helmholtz resonator-based approaches, quarter-wavelength resonator-based approaches, and acoustic metamaterial based-approaches. A review of existing studies using these mechanisms is presented.

4.1. Helmholtz Resonator-Based Approaches This intuitive concept is based on the pioneering AEH research by Horowitz et al. [36]. As shown in Figure5, a coupled resonant system is established to achieve AEH at the MEMS scale. Although the reported energy-harvesting power is low, this approach demonstrates the feasibility of AEH. Micromachines 2019, 10, x FOR PEER REVIEW 7 of 21 Micromachines 2019, 10, 48 7 of 21

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Figure 5. Schematic diagram of a Helmholtz resonator with a compliant bottom; a piezoelectric ring serves as the converter. This represents a MEMS-scale device. FigureFigureFigure 5. 5. SchematicSchematic 5. Schematic diagram diagram diagram of of ofa a Helmholtza Helmholtz Helmholtz resonator resonator wi withthth aa acompliant compliant compliant bottom; bottom; bottom; a piezoelectric a a piezoelectric piezoelectric ring ring ring Noh et al. [37] fabricated a PVDF cantilever beam, which was placed inside the cavity of the servesservesserves as as the the as converter. converter.the converter. This This This represents represents represents a a aMEMS-scale MEMS-scale MEMS-scale device. device. Helmholtz resonator to achieve AEH. The schematic diagram of the proposed device is shown in FigureNohNoh 6. Noh et etUsing al. al. et [37] [al. 37this ] [37]fabricated fabricated approach, fabricated a a 0.1PVDFa PVDF PVDF µW cantilever cantileverpowercantilever was beam,beam, beam, harvested. whichwhich which was Thewas was placed veryplaced placed smallinside inside inside amountthe thecavity the cavity cavityof of harvested the of of the the Helmholtz resonator to achieve AEH. The schematic diagram of the proposed device is shown in HelmholtzHelmholtzenergy was resonator resonator attributed to to to achieve achieve two reasons, AE AEH.H. Thei.e., The theschematic schematic low piezoelectric diagram diagram of ofcoefficient the the proposed proposed of the device devicePVDF ismaterial is shown shown andin in Figurethe acousticFigure6. Using 6. resonant Using this this approach, frequency; approach, 0.1 internal0.1µ µWW powerpower sound-pr wasessure harvested. harvested. distribution The The very very in small the small Helmholtzamount amount of harvested resonator of harvested was Figureenergy 6. Using was attributedthis approach, to two 0.1reasons, µW poweri.e., the lowwas pi harvested.ezoelectric coefficientThe very ofsmall the PVDF amount material of harvested and energyuniform was (Figure attributed 3), indicating to two reasons, that the i.e.,net theexcitati lowon piezoelectric strength of coefﬁcientthe cantilever of the beam PVDF was material weak. and energythe was acoustic attributed resonant to frequency; two reasons, internal i.e., sound-prthe low essurepiezoelectric distribution coefficient in the Helmholtz of the PVDF resonator material was and the acoustic resonant frequency; internal sound-pressure distribution in the Helmholtz resonator was the acousticuniform resonant (Figure 3), frequency; indicating internalthat the net sound-pr excitationessure strength distribution of the cantilever in the Helmholtz beam was weak. resonator was uniformuniform (Figure (Figure 3),3), indicating that the net excitationexcitation strength of the cantilever beambeam waswas weak.weak.

FigureFigureFigure 6.6. Polyvinylidene 6. Polyvinylidene diﬂuoridedifluoride difluoride (PVDF)(PVDF) (PVDF) cantilevercantilever beam beambeam inside insideinside the thethe Helmholtz Helmholtz resonator resonatorresonator to achieve toto achieveachieve AEH.AEH.AEH. ReproducedReproduced Reproduced withwith with permissionpermission permission fromfr fromom [ [37];37]; publishedpublishedpublished by byby Springer, Springer,Springer, 2013. 2013. 2013. Figure 6. Polyvinylidene difluoride (PVDF) cantilever beam inside the Helmholtz resonator to achieve AA permanentpermanentA permanent magnetmagnet magnet cancan can bebe be placedplaced placed atat thethe bottom bottom of ofof the thethe Helmholtz HelmholtzHelmholtz resonator, resonator,resonator, resulting resultingresulting in an inin anan AEH. Reproduced with permission from [37]; published by Springer, 2013. electromagneticelectromagneticelectromagnetic AEH AEH AEH device. device. device. As As shownAs shown shown in Figurein Figure7, a 7, coil7, a acoil is coil placed is isplaced placed on theon on ﬂexiblethe theflexible flexible bottom bottom andbottom and a magnet a and a is placedmagnet in ais holder;placed in the a holder; coil and the magnet coil and aremagnet separated are separated by an airby passagean air passage [38]. [38]. magnetA permanent is placed inmagnet a holder; can the be coilplaced and at ma thegnet bottom are separated of the Helmholtz by an air passageresonator, [38]. resulting in an electromagnetic AEH device. As shown in Figure 7, a coil is placed on the flexible bottom and a magnet is placed in a holder; the coil and magnet are separated by an air passage [38].

Figure 7. Schematic diagram of the electromagnetism-based AEH device; a coil is placed at the

resonator’s bottom and a permanent magnet is placed inside a holder. Reproduced with permission FigureFigurefrom 7.7. [38]; Schematic publishe d diagram by Springer, of the 2016. electromagnetism-basedelectromagnetism-bas ed AEH device; a coil is placedplaced atat thethe resonator’sresonator’s bottombottom andand aa permanentpermanent magnetmagnet isis placedplaced insideinside aa holder.holder. ReproducedReproduced withwith permissionpermission fromfrom [[38];38]; publishedpublished byby Springer,Springer, 2016.2016. Figure 7. Schematic diagram of the electromagnetism-based AEH device; a coil is placed at the resonator’s bottom and a permanent magnet is placed inside a holder. Reproduced with permission from [38]; published by Springer, 2016.

MicromachinesMicromachines2019 2019,,10 10,, 48 x FOR PEER REVIEW 88 of 21 21 Micromachines 2019, 10, x FOR PEER REVIEW 8 of 21 To improve the Helmholtz resonator-based AEH performance, several improved approaches To improve the Helmholtz resonator-based AEH performance, several improved approaches were wereTo subsequently improve the developed. Helmholtz resonator-based AEH performance, several improved approaches subsequently developed. wereA subsequently hybrid AEH developed. device, which integrates the piezoelectric and electromagnetic conversion A hybrid AEH device, which integrates the piezoelectric and electromagnetic conversion approaches,A hybrid was AEH proposed device, by whichKhan and integrates Izhar [39]. the As pi shownezoelectric in Figure and 8,electromagnetic the air passage inconversion the AEH approaches, was proposed by Khan and Izhar [39]. As shown in Figure8, the air passage in the approaches,device was specifically was proposed designed by Khan to andminimize Izhar [39].the air As damping shown in influence. Figure 8, theThe air harvested passage inpower the AEH was AEH device was speciﬁcally designed to minimize the air damping inﬂuence. The harvested power device49 µW was(from specifically the piezoelectric designed part) to minimize and 3.16 theµW air (from damping the electromagnetic influence. The part)harvested at 2100 power Hz, wasand was 49 µW (from the piezoelectric part) and 3.16 µW (from the electromagnetic part) at 2100 Hz, 49excitation µW (from strength the piezoelectric was 130 dB. part) and 3.16 µW (from the electromagnetic part) at 2100 Hz, and andexcitation excitation strength strength was was130 130dB. dB.

Figure 8. Schematic diagram of the hybrid AEH device, which is based on the Helmholtz resonator Figure 8. Schematic diagram of the hybrid AEH device, which is based on the Helmholtz resonator Figureand integrates 8. Schematic piezoelectric diagram anof dthe electromagnetic hybrid AEH device, conversion. which (isa) basedCross-section on the Helmholtz view; (b) resonatorexploded and integrates piezoelectric and electromagnetic conversion. (a) Cross-section view; (b) exploded view. andview. integrates Reproduced piezoelectric with permission and electromagnetic from [39]; published conversion. by American (a) Cross-section Institute of view; Physics, (b) 2016.exploded Reproducedview. Reproduced with permission with permission from [ 39from]; published [39]; published by American by American Institute Inst ofitute Physics, of Physics, 2016. 2016.

KhanKhan andand IzharIzhar [[40]40] alsoalso proposedproposed aa conicalconical HelmholtzHelmholtz resonatorresonator forfor AEH.AEH. UnlikeUnlike thethe classicalclassical HelmholtzKhan and resonator, Izhar [40] the also conical proposed configuration a conical ensures Helmholtz that resonatora higher sound-pressurefor AEH. Unlike amplification the classical HelmholtzHelmholtz resonator,resonator, thethe conicalconical configurationconfiguration ensuresensures thatthat aa higherhigher sound-pressuresound-pressure amplificationamplification ratioratio cancan bebe achieved.achieved. As As the the pressure pressure difference difference of of the the flexible flexible bottom bottom increases, increases, AEH AEH performance performance is ratiois improved. can be achieved. The schematic As the diagram pressure of difference the proposed of the device flexible is shown bottom in increases, Figure 9. AEH performance improved.is improved. The The schematic schematic diagram diagram of of the the proposed proposed device device is shownis shown in Figurein Figure9. 9.

Figure 9. Schematic diagram of the proposed AEH, which is equipped with a conical Helmholtz Figure 9. Schematic diagram of the proposed AEH, which is equipped with a conical Helmholtz resonator and a piezoelectric conversion device. Reproduced with permission from [40]; published by Figureresonator 9. Schematicand a piezoelectric diagram conversionof the proposed device. AEH, Reproduced which is with equipped permission with afrom conical [40]; Helmholtz published American Institute of Physics, 2016. resonatorby American and Institut a piezoelectrice of Physics, conversion 2016. device. Reproduced with permission from [40]; published by American Institute of Physics, 2016. A conical Helmholtz resonator with an electromagnetic conversion device was proposed by ReferenceA conical [41]; theHelmholtz schematic resonator diagram with of the an device electrom is shownagnetic in conversion Figure 10. device was proposed by ReferenceA conical [41]; Helmholtzthe schematic resonator diagram with of the an device electrom is shownagnetic in conversion Figure 10. device was proposed by Reference [41]; the schematic diagram of the device is shown in Figure 10.

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Figure 10. Schematic diagram of the proposed AEH, which is equipped with a conical Helmholtz FigureFigure 10. 10. SchematicSchematic diagram diagram of of the the proposed proposed AEH, AEH, which which is is equipped equipped with with a a conical conical Helmholtz Helmholtz resonator and an electromagnetic conversion device. Reproduced with permission from [41]; resonatorresonator andand anan electromagnetic electromagnetic conversion conversion device. device. Reproduced Reproduced with permissionwith permission from [41 ];from published [41]; published by Emerald Insight, 2018. publishedby Emerald by Insight,Emerald 2018. Insight, 2018.

AEHAEHAEH performanceperformance performance cancan can alsoalso also bebe improvedimproved be improved byby resoreso bynantnant resonant coupling.coupling. coupling. ForFor instance,instance, For aa instance, dualdual HelmholtzHelmholtz a dual structure,structure,Helmholtz consistingconsisting structure, consistingofof neck–cavity–neck–cavneck–cavity–neck–cav of neck–cavity–neck–cavityityity components,components, components, waswas proposedproposed was proposed toto enhanceenhance to enhance thethe acoustoelectricacoustoelectricthe acoustoelectric couplingcoupling coupling [42].[42]. AA [42 modifiedmodified]. A modiﬁed versionversion version ofof thethe devicedevice of the waswas device usedused was toto usedextendextend to thethe extend energy-energy- the harvestingharvestingenergy-harvesting bandwidthbandwidth bandwidth [43].[43]. [43]. IfIfIf thethe the resonantresonant resonant frequencyfrequency frequency ofof of thethe the mechanicalmechanical mechanical portionportion portion isis is tunedtuned tuned toto to thethe the samesame same frequencyfrequency frequency asas as thethe the acousticalacousticalacoustical resonantresonant resonant frequency,frequency, frequency, thethe the harvestingharvesting harvesting efficiencyefficiency efﬁciency cancan can bebe be imprimpr improvedovedoved substantially.substantially. substantially. ForFor For instance,instance, instance, YangYangYang etet et al.al. al. [44][44] [44 utilizedutilized] utilized twotwo cant twocantileverilever cantilever beamsbeams beams andand aa HelmholtzHelmholtz and a Helmholtz resonaresonatortor resonator toto achieveachieve strongtostrong achieve acoustical–acoustical– strong mechanicalmechanicalacoustical–mechanical coupling.coupling. TheThe coupling. system’ssystem’s The schematicschematic system’s schematicdiagdiagramram isis diagram shownshown isinin shown FigureFigure in 11;11; Figure experimentexperiment 11; experiment resultsresults demonstrateddemonstratedresults demonstrated thatthat aa maximummaximum that a maximum ofof 1.431.43 of mWmW 1.43 powerpower mW power couldcould could bebe extractedextracted be extracted forfor 100100 for dB 100dB SPLSPL dB SPL excitationexcitation excitation atat 170–206170–206at 170–206 Hz.Hz. Hz. TheThe The cantilevercantilever cantilever beambeam beam cancan can alsoalso also bebe be combcomb combinedinedined withwith with aa aHelmholtzHelmholtz Helmholtz resonatorresonator resonator withwith with aaa taperedtapered tapered cavity,cavity,cavity, whichwhich isis favorablefavorable toto extendextend the the bandwidth bandwidth of of the the AEH AEH system system [45]. [[45].45].

FigureFigureFigure 11.11. 11. SchematicSchematicSchematic diagramdiagram diagram ofof of thethe the AEHAEH AEH systemsystem system withwith with strongstrong strong coupling.coupling. coupling. ReproducedReproduced Reproduced withwith with permissionpermission permission fromfromfrom [44];[44]; [44]; publishedpublished published byby by AmericAmeric Americananan InstituteInstitute Institute of of Physics, Physics, 2014. 2014.

AAA tunabletunable tunable neckneck neck isis is anan an alternativealternative alternative approachapproach approach toto to achieveachieve achieve resonantresonant resonant coupling.coupling. coupling. AsAs As shownshown shown inin in FigureFigure Figure 12,12, 12 , neckneckneck lengthlength length isis is variable,variable, variable, whichwhich which changeschanges changes thethe the acousticacoustic acoustic resonanceresonance resonance frequency.frequency. frequency. Specifically,Specifically, Speciﬁcally, thethe the acousticacoustic acoustic resonanceresonanceresonance cancan can bebe be tunedtuned tuned toto to matchmatch match thethe the bottom’sbottom’s mechmech mechanicalanicalanical resonance,resonance, resonance, therebythereby thereby increasingincreasing increasing thethe the AEHAEH AEH performanceperformanceperformance substantiallysubstantially substantially [46].[46]. [46].

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FigureFigure 12. 12. PhotographPhotograph of of the the tunable tunable neck neck to to increase increase AEH AEH efficiency. efﬁciency.efficiency. ReproducedReproduced withwith permissionpermission fromfromfrom [46]; [[46];46]; publishe publishedpublishedd by by ELSEVIER, ELSEVIER, 2017.2017.

TheThe traditional traditional Helmholtz Helmholtz resonator resonator is is bulky bulky for for low-frequency low-frequency applications. applications. ToTo overcome overcome this this drawback,drawback, YuanYuan Yuan etet et al.al. al. [47][47] [47 ]proposedproposed proposed aa aplanarplanar planar HelmholtzHelmholtz Helmholtz AEHAEH AEH devicedevice device forfor for aa low-frequencylow-frequency a low-frequency range.range. range. AsAs shownAsshown shown inin FigureFigure in Figure 13,13, 13 anan, anembeddedembedded embedded coiledcoiled coiled neckneck neck isis usus ised useded toto tomakemake make thethe the resonatorresonator resonator compact.compact. compact. InIn Inaddition,addition, addition, aa taperedatapered tapered neckneck neck isis is usedused used toto to reducereduce reduce thethe the viscousviscous viscous lossloss loss alal alongongong thethe the neck.neck. TheThe resultsresults demonstrated demonstrated that that this this configurationconﬁgurationconfiguration waswas favorablefavorablefavorable to toto improve improveimprove the thethe resonator’s resonaresonator’stor’s performance performanceperformance and andand generated generatedgenerated 27.2 27.227.2µ W µWµW power powerpower at at217at 217217 Hz HzHz and andand 100 100100 dB dBdB SPL SPLSPL excitation. excitation.excitation.

Figure 13. Photograph of the components of the tapered AEH device [47]. Figure 13. Photograph of the components of the tapered AEH devicedevice [[47].47].

AA multifunctionalmultifunctional noisenoise noise barrier,barrier, barrier, whichwhich which simultaneouslysimultaneously simultaneously achievesachieves achieves soundsound sound insulationinsulation insulation andand AEH, andAEH, AEH, waswas designed,wasdesigned, designed, fabricated,fabricated, fabricated, andand evaluatedevaluated and evaluated byby WangWang by et Wanget al.al. [4[4 et8].8]. al. AsAs [ 48shownshown]. As inin shown FigureFigure in 14,14, Figure aa PVDFPVDF 14 ,filmfilm a PVDF waswas usedused ﬁlm forwasfor energyenergy used for conversionconversion energy conversion inin thisthis inapplicationapplication this application becausebecause because thethe PVDF thePVDF PVDF materialmaterial material hashas has aa alowlow low piezoelectric piezoelectric coefficientcoefﬁcientcoefficient and, and, inin aa a singlesingle single unit,unit, unit, thethe the harvestedharvested harvested powepowe powerrr waswas was onlyonly only 0.380.38 0.38 µWµWµW underunder under 100100 100 dBdB dB ofof ofexcitation.excitation. excitation. IfIf multipleIfmultiple multiple unitsunits units werewere were used,used, used, aa anoisenoise noise barrierbarrier barrier couldcould could bebe be createdcreated created andand and thethe the harvestedharvested power would would substantiallysubstantially increase. increase.

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Figure 14. Components of the AEH unit and the array form to create a noise barrier. Reproduced with Figure 14. Components of the AEH unit and the array form to create a noise barrier. Reproduced with permission from [48]; published by ELSEVIER, 2018. permission fromfrom [[48];48]; publishedpublished byby ELSEVIER,ELSEVIER, 2018.2018. Yang et al. [49] proposed a triboelectric nanogenerator (TENG) for AEH. As shown in Figure 15, Yang etet al.al. [[49]49] proposed a triboelectrictriboelectric nanogenerator (TENG) for AEH. As shown in Figure 15,15, the nanogenerator was placed on a Helmholtz resonator, which can be excited strongly at the the nanogeneratornanogenerator was was placed placed on on a Helmholtz a Helmholtz resonator, resonator, which which can be can excited be excited strongly strongly at the resonant at the resonant frequency. The tribolectric charges result in increases in voltage. frequency.resonant frequency. The tribolectric The tribolectric charges result charges in increases result in inincreases voltage. in voltage.

Figure 15. Schematic diagram of the nanogenerator-based AEH device. (a) AEH Structure; Figure 15. Schematic diagram of the nanogenerator-based AEH device. (a) AEH Structure; (b) cross- (b) cross-sectional view of the triboelectric nanogenerator (TENG); (c) SEM image of the pores on the sectional view of the triboelectric nanogenerator (TENG); (c) SEM image of the pores on the electrode; electrode; (d) SEM image of the nanowires. Reproduced with permission from [49]; published by ACS (d) SEM image of the nanowires. Reproduced with permission from [49]; published by ACS NANO, NANO, 2014. 2014. Subsequently, Cui et al. [50] demonstrated an improved version of the TENG with good wideband Subsequently, Cui et al. [50] demonstrated an improved version of the TENG with good AEHSubsequently, performance and Cui durability. et al. [50] demonstrated an improved version of the TENG with good wideband AEH performance and durability. 4.2. Quarter-Wavelength Resonator-Based Approaches 4.2. Quarter-Wavelength Resonator-Based Approaches Li et al. [51] placed multiple PVDF beams in a quarter-wavelength tube resonator and achieved Li et al. [51] placed multiple PVDF beams in a quarter-wavelength tube resonator and achieved low-frequencyLi et al. [51] AEH. placed It wasmultiple found PVDF that thebeams beam in placeda quarter-wavelength at the inlet generated tube resonator the maximum and achieved power low-frequency AEH. It was found that the beam placed at the inlet generated the maximum power andlow-frequency the zigzag beamAEH. placementIt was found resulted that the in betterbeam placed performance at the thaninlet thegenerated aligned the conﬁguration. maximum power Later, and the zigzag beam placement resulted in better performance than the aligned configuration. Later, Liand et the al. [zigzag52] used beam PZT placement beams to replaceresulted the in PVDFbetter beams,performance and experimental than the aligned tests configuration. showed a signiﬁcant Later, Li et al. [52] used PZT beams to replace the PVDF beams, and experimental tests showed a significant improvementLi et al. [52] used in PZT the AEHbeams performance. to replace the ThePVDF schematic beams, and diagram experimental of the quarter-wavelengthtests showed a significant tube improvement in the AEH performance. The schematic diagram of the quarter-wavelength tube resonatorimprovement and installedin the AEH PZT performance. beams is shown The in Figureschema 16tic. diagram of the quarter-wavelength tube resonator and installed PZT beams is shown in Figure 16.

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Figure 16. Schematic diagram of quarter-wavelength resonator tube and the installed PZT beams. FigureFigure 16. 16. SchematicSchematic diagram diagram of of quarter-wavelength quarter-wavelength re resonatorsonator tube tube and and the the installed installed PZT PZT beams. beams. Subsequently, Li and You [53] designed a self-powered synchronized switch harvesting on inductorSubsequently,Subsequently, (SSHI) circuitLi Li and and and You You integrated [53] [53] designed designed it into a a selfquarter-wavelength self-powered-powered synchronized synchronized resonator-based switch switch harvesting harvestingAEH device. on on It inductorinductorwas demonstrated (SSHI) (SSHI) circuit that andand the integratedintegrated self-powered it it into into a S-SSHIa quarter-wavelength quarter-wavelength circuit achieved resonator-based resona the tor-basedbest performance AEH AEH device. device. and It was Itthe wasdemonstratedharvested demonstrated power that was thethat self-powered 0.796the self-powered mW under S-SSHI 112 S-SSHI circuitdB SPL circuit achieved excitation achieved the at best 196 the performanceHz. best performance and the harvestedand the power was 0.796 mW under 112 dB SPL excitation at 196 Hz. harvested power was 0.796 mW under 112 dB SPL excitation at 196 Hz. 4.3. Acoustic Metamaterial Based-Approaches 4.3. Acoustic Metamaterial Based-Approaches 4.3. AcousticAcoustic Metamaterial metamaterials Based-Approaches provide many valuable approaches to develop AEH structures. One of Acoustic metamaterials provide many valuable approaches to develop AEH structures. One of the theAcoustic important metamaterials applications provide of acoustic many valuablemetamateri approachesals is to toobtain develop sound AEH pressure structures. focusing One of or important applications of acoustic metamaterials is to obtain sound pressure focusing or ampliﬁcation. theamplification. important applications This mechanism of acoustic has inspired metamateri severalals AEH is studies.to obtain sound pressure focusing or This mechanism has inspired several AEH studies. amplification.Wu et.al This [54] mechanism used sonic has crystals inspired with several a point AEH defect studies. to confine acoustic energy at the center; a Wu et al. [54] used sonic crystals with a point defect to conﬁne acoustic energy at the center; PVDFWu patchet.al [54] was used placed sonic at thecrystals defect with to conver a pointt acoustic defect to energy confine into acoustic electrical energy energy. at the center; a a PVDF patch was placed at the defect to convert acoustic energy into electrical energy. PVDF patchLater, was Yang placed et al. at[55] the designed defect to an conver improvedt acoustic performance energy into prototype electrical base energy.d on the sonic crystal Later, Yang et al. [55] designed an improved performance prototype based on the sonic crystal resonatorLater, Yang (SCR). et Asal. [55]shown designed in Figure an improved17, a delic ateperformance electromechanical prototype Helmholtz based on resonatorthe sonic crystal(EMHR) resonator (SCR). As shown in Figure 17, a delicate electromechanical Helmholtz resonator (EMHR) resonatorwas then (SCR). placed As at shown the center, in Figure which 17, resulted a delic inate strong electromechanical acoustical–mechanical Helmholtz coupling. resonator The (EMHR) coupled was then placed at the center, which resulted in strong acoustical–mechanical coupling. The coupled wassystem then placedachieved at the 23 center, and 262which times resulted higher in strongperformance acoustical–mechanical than the EMHR coupling. and SCR The structures,coupled system achieved 23 and 262 times higher performance than the EMHR and SCR structures, respectively. systemrespectively. achieved 23 and 262 times higher performance than the EMHR and SCR structures, respectively.

Figure 17. Coupled AEH system consisting of a sonic crystal resonator (SCR) and electromechanical Figure 17. Coupled AEH system consisting of a sonic crystal resonator (SCR) and electromechanical Helmholtz resonator (EMHR). Reproduced with permission from [55]; published by the Japan Society Helmholtz resonator (EMHR). Reproduced with permission from [55]; published by the Japan Society Figureof Applied 17. Coupled Physics, AEH 2016. system consisting of a sonic crystal resonator (SCR) and electromechanical Helmholtzof Applied resonator Physics, (EMHR). 2016. Reproduced with permission from [55]; published by the Japan Society ofTo Applied reduce Physics, harvesting 2016. frequency, Qi et al. [56] proposed a planar acoustic metamaterial-based To reduce harvesting frequency, Qi et al. [56] proposed a planar acoustic metamaterial-based approach for AEH, which is shown in Figure 18. The large strain energy is converted into approach for AEH, which is shown in Figure 18. The large strain energy is converted into electrical electricalTo reduce energy harvesting using a bondedfrequency, piezo Qi patch.et al. [56] The proposed proposed a structureplanar acoustic also resulted metamaterial-based in satisfactory energy using a bonded piezo patch. The proposed structure also resulted in satisfactory sound- approachsound-insulation for AEH, performance. which is shown in Figure 18. The large strain energy is converted into electrical energyinsulation using performance. a bonded piezo patch. The proposed structure also resulted in satisfactory sound- insulation performance.

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FigureFigure 18.18. SchematicSchematic diagram diagram of of the the planarplanar acousticacoustic metamaterial-basedmetamaterial-based AEH.AEH. ReproducedReproduced withwith permissionpermission from from [[56]; 56[56];]; published published byby by AmericanAmerican American InstituteInstitute Institute ofof of Physics,Physics, Physics, 2016.2016. 2016.

Later,Later, a a subwavelength subwavelength defect defect was was created created by by Oudich Oudich and and LiLi [57] [[57]57] using using an an acousticacoustic metamaterialmetamaterial plateplate toto conﬁne confineconfine the thethe plate’s plate’splate’s elastic elasticelastic energy energyenergy to toto the thethe defect. dedefect.fect. The TheThe simulation simulationsimulation results resultsresults showed showed showed that, that,that, at ataat sizea a sizesize of 2 × 2 µ 3ofof 3 3 3× × cm33 cm cm, 182, ,18 18 WµW µW electrical electrical electrical power power power was was was harvested harvested harvested under unde unde 520rr 520 520 Hz Hz Hz and and and 100 100 100 dB dB dB acoustic acoustic acoustic excitation. excitation. excitation. QiQi and and Assouar Assouar [58] [58] [58 ]utilized utilized utilized an an an acoustic acoustic acoustic multilateral multilateral multilateral metasurface metasurface metasurface to to achieve toachieve achieve sound sound sound focusing. focusing. focusing. As As Asshownshown shown in in Figure Figure in Figure 19, 19, a a 19piezoelectric piezoelectric, a piezoelectric bimorph bimorph bimorph was was placed placed was placedat at the the confined confined at the conﬁned sound sound energy energy sound point, point, energy resulting resulting point, resultinginin strong strong inexcitation excitation strong excitation and and AEH. AEH. and AEH.

Figure 19. (a) Schematic diagram of the metasurface; the piezo beam is placed at the sound-focusing FigureFigure 19. 19. (a). (a). Schematic Schematic diagram diagram of of the the metasurface; metasurface; the the pi piezoezo beam beam is is placed placed at at the the sound-focusing sound-focusing location. (b) Detailed piezo beam description. Reproduced with permission from [58]; published by location.location. (b). (b). Detailed Detailed piezo piezo beam beam description. description. Repr Reproducedoduced with with permission permission from from [58]; [58]; published published by by American Institute of Physics, 2017. AmericanAmerican Institute Institute of of Physics, Physics, 2017. 2017. Sun et al. [59] proposed a double coiled-up acoustic cavity for sound pressure enhancement. SunSun etet al.al. [59][59] proposedproposed aa doubledouble coiled-upcoiled-up acacousticoustic cavitycavity forfor soundsound pressurepressure enhancement.enhancement. Inside the cavity, a piezoelectric bimorph plate is used for AEH. The schematic diagram of the device InsideInside the the cavity, cavity, a a piezoelectric piezoelectric bimorph bimorph plate plate is is used used for for AEH. AEH. The The schematic schematic diagram diagram of of the the device device is shown in Figure 20. isis shown shown in in Figure Figure 20. 20. Another advantage of acoustic metamaterials is that they produce a deep subwavelength effect in acoustic devices, which is favorable to develop small-scale AEH devices for low frequencies. For instance, Yuan et al. [60] proposed a helix-type AEH structure that represents one type of bionic acoustic metamaterials [61]. The helix structure changes the sound propagation path and achieves low-frequency AEH in a compact design. The components of the helix-type AEH structure are shown in Figure 21. This device can harvest 7.3 µW of electrical power at 175 Hz under 100 dB SPL excitation.

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Figure 20. Configuration of the proposed sound energy-harvesting system.

Another advantage of acoustic metamaterials is that they produce a deep subwavelength effect in acoustic devices, which is favorable to develop small-scale AEH devices for low frequencies. For instance, Yuan et al. [60] proposed a helix-type AEH structure that represents one type of bionic acoustic metamaterials [61]. The helix structure changes the sound propagation path and achieves low-frequency AEH in a compact design. The components of the helix-type AEH structure are shown in Figure 21. ThisFigureFigure device 20.20. can ConﬁgurationConfiguration harvest 7.3 ofµWof thethe of proposedproposed electrical soundsound power energy-harvestingenergy-harvesting at 175 Hz under system.system. 100 dB SPL excitation.

Another advantage of acoustic metamaterials is that they produce a deep subwavelength effect in acoustic devices, which is favorable to develop small-scale AEH devices for low frequencies. For instance, Yuan et al. [60] proposed a helix-type AEH structure that represents one type of bionic acoustic metamaterials [61]. The helix structure changes the sound propagation path and achieves low-frequency AEH in a compact design. The components of the helix-type AEH structure are shown in Figure 21. This device can harvest 7.3 µW of electrical power at 175 Hz under 100 dB SPL excitation.

Figure 21. Schematic diagram of the helix structurestructure AEHAEH prototype.prototype. Reproduced with permission from [[60];60]; published by AmericanAmerican Institute of Physics, 2018.2018.

Acoustic metamaterials also exhibit local resonance characteristics. Accordingly, Accordingly, thin structures can exhibitexhibit extraordinaryextraordinary sound-insulation sound-insulation performance performance in thein low-frequencythe low-frequency range. range. In addition, In addition, when whenlocal resonance local resonance occurs, substantialoccurs, substantial strain energy strain is generatedenergy is from generated the metacell, from therebythe metacell, enabling thereby AEH. enablingFor instance,AEH. Ma et al. [62] developed an acoustic metasurface with a hybrid resonance that not onlyFor resultedFigure instance, 21. in Schematic perfect Ma et absorption al.diagram [62] developed of but the also helix AEH.an structure acoustic A permanent AE metasurfaceH prototype. magnet with Reproduced was a hybrid used towith resonance efficiently permission that convert not onlyacoustic fromresulted energy [60]; publishedin into perfect electrical by absorption Americ energy.an Institute but also of AEPhysics,H. A 2018. permanent magnet was used to efficiently convertLater, acoustic Li et energy al. [63] into proposed electrical a membrane energy. acoustic metamaterial for AEH. The experimental resultsLater,Acoustic showed Li etmetamaterials a al. close [63] relationship proposed also exhibit betweena membrane local energy-conversion resonance acoustic characteristics.metamaterial efficiency forAccordingly, and AEH. tensile The stress. thin experimental structures However, resultsbecausecan exhibit showed the applicationextraordinary a close relationship of precisesound-insulation tension between stress energy-c performance is complexonversion in and the efficiency the low-frequency thin film and is tensile ductile, range. stress. a moreIn However,addition, robust becauseconfigurationwhen local the applicationresonance has to be developed.ofoccurs, precise substantial tension stress strain is complexenergy isand generated the thin film from is ductile,the metacell, a more thereby robust configurationenablingHence, AEH. Yuan has to et be al. developed. [64] utilized a metallic substrate and a proof mass to form a local resonant acousticHence,For metamaterial;instance, Yuan etMa al. et this [64] al. approach[62] utilized developed dida metallic not an require acoustic substrate tension. metasurface and A a piezoelectricproof with mass a hybrid to patch form resonance was a local placed resonant that inside not acoustictheonly proof resulted metamaterial; mass in and perfect metallic this absorption approach substrate, didbut converting not also requir AEH. thee tension. A structural permanent A piezoelectric strain magnet energy patchwas into used was electrical placedto efficiently energy. inside Thetheconvert proof schematic acoustic mass diagram and energy metallic is into shown substrate, electrical in Figure energy.converting 22. A simulation the structural analysis strain showed energy that into the electromechanicalelectrical energy. couplingLater, strength Li et al. was [63] greatly proposed increased a membrane after structural acoustic optimization.metamaterial for In theAEH. experimental The experimental study, 200resultsµW showed of electrical a close power relationship was harvested between at 155 energy-c Hz withonversion 114 dB SPLefficiency excitation. and tensile Hence, stress. if five However, units are usedbecause simultaneously, the application harvest of precise power tension can be stress up to is 1 complex mW. and the thin film is ductile, a more robust configuration has to be developed. Hence, Yuan et al. [64] utilized a metallic substrate and a proof mass to form a local resonant acoustic metamaterial; this approach did not require tension. A piezoelectric patch was placed inside the proof mass and metallic substrate, converting the structural strain energy into electrical energy.

Micromachines 2019, 10, x FOR PEER REVIEW 15 of 21 Micromachines 2019, 10, x FOR PEER REVIEW 15 of 21 Micromachines 2019, 10, x FOR PEER REVIEW 15 of 21 The schematic diagram is shown in Figure 22. A simulation analysis showed that the The schematic diagram is shown in Figure 22. A simulation analysis showed that the electromechanical coupling strength was greatly increased after structural optimization. In the Theelectromechanical schematic diagram coupling is strengthshown inwas Figure greatly 22 increased. A simulation after structural analysis optimization.showed that In the the experimental study, 200 µW of electrical power was harvested at 155 Hz with 114 dB SPL excitation. electromechanicalexperimental study, coupling 200 µW strength of electrical was power greatly wa increaseds harvested after at 155 structural Hz with optimization.114 dB SPL excitation. In the Hence, if five units are used simultaneously, harvest power can be up to 1 mW. experimentalHence, if five study, units 200are usedµW of simultaneously, electrical power harvest was harvested power can at 155be up Hz to with 1 mW. 114 dB SPL excitation. Hence,Micromachines if five2019 units, 10 ,are 48 used simultaneously, harvest power can be up to 1 mW. 15 of 21

Figure 22. Schematic diagram of the proposed metamaterial structure for AEH [64]. Figure 22. Schematic diagram of the proposed metamaterial structure for AEH [64]. FigureFigure 22. 22. SchematicSchematic diagram diagram of ofthe the proposed proposed metamaterial metamaterial structure structure for for AEH AEH [64]. [64 ]. Ahmed and Banerjee [65] and Ahmed et al. [66] proposed sub-wavelength scale acoustoelastic Ahmed and Banerjee [65] and Ahmed et al. [66] proposed sub-wavelength scale acoustoelastic sonicAhmed crystals and (AESC) Banerjee to achieve [65] and energy Ahmed harvesting et al. [66 ]in proposed the low-frequency sub-wavelength range. scaleAs shown acoustoelastic in Figure sonicAhmed crystals and (AESC) Banerjee to achieve[65] and energy Ahmed harvesting et al. [66] in proposed the low-frequency sub-wavelength range. scale As shown acoustoelastic in Figure sonic23, a crystalsPZT wafer (AESC) is placed to achieve inside energy the unit harvesting to conv inert the the low-frequency strain energy range. into electrical As shown energy in Figure under 23, sonic23, a crystals PZT wafer (AESC) is placed to achieve inside energy the unit harvesting to conv ertin thethe low-frequency strain energy intorange. electrical As shown energy in Figure under amultiple PZT wafer excitation is placed frequencies. inside the unit The to versatile convert unit the straincan be energy adapted into to electrical different energy frequency under ranges, multiple and 23,multiple a PZT excitationwafer is placed frequencies. inside Thethe versatileunit to conv unitert can the be strain adapted energy to different into electrical frequency energy ranges, under and excitationthe unit is frequencies. also capable The of simultaneously versatile unit can filtering be adapted acoustic to different waves. frequency ranges, and the unit is multiplethe unit excitationis also capable frequencies. of simultaneously The versatile filtering unit can acoustic be adapted waves. to different frequency ranges, and thealso unit capable is also of capable simultaneously of simultaneously ﬁltering acousticfiltering waves.acoustic waves.

Figure 23. Proposed sonic-crystal structure for noise filtering and energy harvesting. Reproduced Figure 23. ProposedProposed sonic-crystalsonic-crystal structure structure for for noise noise ﬁltering filtering and and energy energy harvesting. harvesting. Reproduced Reproduced with with permission from [66]; published by SAGE journals, 2017. Figurewithpermission permission 23. Proposed from from [66 sonic-crystal]; published[66]; published bystructure SAGE by SAGE journals, for noisejournals, 2017. filtering 2017. and energy harvesting. Reproduced with permission from [66]; published by SAGE journals, 2017. Later, as shown in Figure 24, multiple AESC units were assembled into an acoustoelastic Later, asas shown shown in in Figure Figure 24, multiple24, multiple AESC AESC units wereunits assembledwere assembled into an acoustoelasticinto an acoustoelastic MetaWall MetaWall for simultaneous noise blocking and energy harvesting [67]. The simulation results MetaWallforLater, simultaneous asfor shown simultaneous noise in blockingFigure noise 24, and multipleblocking energy AESC harvestingand energy units [ 67wereharvesting]. The assembled simulation [67]. intoThe results ansimulation acoustoelastic indicated results that indicated that at 460 Hz, the harvested power was 2 mW, when PVDF material was used. Overall MetaWallindicatedat 460 Hz, forthat the simultaneous harvestedat 460 Hz, powerthe noise harvested was blocking 2 mW, power whenand wa energy PVDFs 2 mW, materialharvesting when wasPVDF [67]. used. mate The Overallrial simulation was noise-reduction used. resultsOverall noise-reduction performance was also in good condition. indicatednoise-reductionperformance that wasat 460performance also Hz, in the good harvested was condition. also inpower good wa condition.s 2 mW, when PVDF material was used. Overall noise-reduction performance was also in good condition.

FigureFigure 24. 24.MetaWall MetaWall structure structure for for noise noise blocking blocking and and energy energy harvesting. harvesting. Reproduced Reproduced with with permission permission Figure 24. MetaWall structure for noise blocking and energy harvesting. Reproduced with permission fromfrom [ 67[67];]; published published by by ELSEVIER, ELSEVIER, 2018. 2018. Figurefrom [67];24. MetaWall publishe dstructure by ELSEVIER, for noise 2018. blocking and energy harvesting. Reproduced with permission fromZhang [67]; et publishe al. [68]d proposed by ELSEVIER, another 2018. type of acoustic metamaterial for simultaneous noise insulation and energy harvesting. The proposed structure is composed of a Helmholtz resonator and a built-in decorated membrane; the schematic diagram is shown in Figure 25. The added harvested power was 3.22 µW at 353 and 460 Hz, and two noise-insulation bands were created. MicromachinesMicromachines 20192019,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 16 16 of of 21 21

ZhangZhang etet al.al. [68][68] proposedproposed anotheranother typetype ofof acousticacoustic metamaterialmetamaterial forfor simultaneoussimultaneous noisenoise insulationinsulation andand energyenergy harvesting.harvesting. TheThe proposedproposed structurestructure isis composedcomposed ofof aa HelmholtzHelmholtz resonatorresonator andand aMicromachinesa built-inbuilt-in decorateddecorated2019, 10, 48 membrane;membrane; thethe schematicschematic diagramdiagram isis shownshown inin FigureFigure 25.25. TheThe addedadded harvestedharvested16 of 21 powerpower waswas 3.223.22 µWµW atat 353353 andand 460460 Hz,Hz, andand twotwo noise-insulationnoise-insulation bandsbands werewere created.created.

FigureFigure 25. 25. SchematicSchematic diagram diagram of of the the proposed proposed acoustic acoustic metamaterial.metamaterial.

LiuLiu etetet al. al.al. [69 [69][69]] proposed proposedproposed a broadband aa broadbandbroadband AEH metasurface,AEHAEH metasurface,metasurface, that was thatthat composed waswas composedcomposed of coupled ofof Helmholtz coupledcoupled HelmholtzresonatorsHelmholtz resonators andresonators a PZT andand wafer. aa PZTPZT A wafer. push–pullwafer. AA push–pullpush–pull effect was effecteffect achieved waswas achievedachieved with different withwith differentdifferent neck lengths neckneck lengthslengths of the ofHelmholtzof thethe HelmholtzHelmholtz resonator resonatorresonator and resulted andand resultedresulted in good in excitationin goodgood exex ofcitationcitation the PZT ofof patch thethe PZTPZT ina patchpatch wide range.inin aa widewide The range.range. maximum TheThe maximumharvestedmaximum power harvestedharvested was powerpower 90 mW waswas at 620 9090 HzmWmW with atat 620620 160 HzHz dB withwith SPL 160160 excitation. dBdB SPLSPL The excitation.excitation. schematic TheThe diagram schematicschematic of thediagramdiagram AEH ofmetasurfaceof thethe AEHAEH metasurfacemetasurface is shown in is Figureis shownshown 26 .inin FigureFigure 26.26.

FigureFigure 26.26. SchematicSchematicSchematic diagramdiagram diagram ofof of thethe AEHAEH metasurfacemetasurface ReproducedReproduced withwith permissionpermission fromfrom [69]; [[69];69]; publishedpublished byby AmericanAmerican InstituteInstInstituteitute ofof Physics,Physics, 2018.2018.

BasedBased on on the the extraordinary extraordinary acoustic acoustic transmission transmission phenomenon,phenomenon, CuiCui etet al.al. [70] [[70]70] proposed proposed acoustic acoustic gratinggrating for for AEH. AEH. The The acoustic acoustic field ﬁeldfield received received a a strong strong boost boost at at thethe resonanceresonance stage,stage, and and aa PZTPZT patchpatch waswas usedused toto convertconvert thethe strainstrain energyenergy intointointo electricalelectricalelectrical energy.energy.energy.

4.4.4.4. Other Other Approaches Approaches OtherOther AEHAEH approachesapproaches approaches thatthat that dodo do notnot not fallfall fall intointo into thethe the aforementionedaforementioned aforementioned threethree three categoriescategories categories alsoalso also exist.exist. exist. ForFor instance,Forinstance, instance, ZhouZhou Zhou etet al.al. et proposedproposed al. proposed aa bistablebistable a bistable AEHAEH [71][71] AEH thatthat [71 generatedgenerated] that generated coherencecoherence coherence resonanceresonance resonance underunder acousticacoustic under excitation.acousticexcitation. excitation. UnderUnder thisthis Under state,state, this maximummaximum state, maximum powerpower was powerwas harvested.harvested. was harvested. TheThe open-loopopen-loop The open-loop voltagevoltage voltage reachedreached reached 0.40.4 VV under0.4under V under 105105 dBdB 105 SPLSPL dB excitation.excitation. SPL excitation. However,However, However, thethe powerpower the power leverlever leverwaswas notnot was reported.reported. not reported. InIn another another interesting interesting studystudy byby JiangJiang etet al.al. [72], [[72],72], an an acoustic acoustic bubble bubble that that induced induced a a synthetic synthetic jet jet to to powerpower thethe rotorrotor andand aa piezoelectricpiezoelectric beambeam onon thethe ro rotorrotortor werewere usedused forfor AEH.AEH. TheThe schematic schematic diagram diagram of of thethe proposed proposed devicedevice isis shownshown inin FigureFigure 2727.27..

Micromachines 2019, 10, 48 17 of 21 Micromachines 2019, 10, x FOR PEER REVIEW 17 of 21

Figure 27.27.Components Components of of the the proposed proposed AEH AEH device. device. Reproduced Reproduced with permission with permission from [72 ];from published [72]; publishedby ELSEVIER, by ELSEVIER, 2018. 2018.

5. Performance Comparison In this this section, section, the the reported reported AEH AEH performances performances of ofthe the experimental experimental studies studies are aresummarized. summarized. As Asshown shown in Equation in Equation (5), (5), the the incident incident acoustic acoustic energy energy is isin in quadratic quadratic proportion proportion to to the the sound sound pressure; pressure; therefore, aa metric metric is usedis used to compare to compare the performance. the performance. This metric This is metric the harvested is the powerharvested normalized power normalizedby the square by ofthe the square sound of pressure the sound and pressure volume. and Because volume. the Because required the information required information for the metric for thecalculation metric calculation was not provided was not in provided some studies, in some only studies, the available only the results available are presented.results are presented. The detailed AEH performance values are summarized in Table3 3..

TableTable 3. Acoustic Energy Harvesting (AEH) performance comparison.

Incident Sound Volume of Harvested Metric Frequency Sound Volume of Reference IncidentSPL (dB) Pressure (Pa) Harvester (cm3) HarvestedPower (µW) µW/(PaMetric2·cm3 ) Frequency(Hz) Pressure Harvester 2 3 Reference SPL (dB) Power (µW)−6 µW/(Pa .cm−12 ) (Hz) 149(Pa) 563.7 (cm 2.4453) 6 × 10 7.7228 × 10 13570 [36] 145.5 376 55.5 0.094 × −8 834 [37] 149 563.7 2.445 6 × 10−6 1.1987.7228 10× 10−12 13570 [36] 100 2 13.57 789.65 14.536 319 [38] −8 145.5130 376 63.2 55.5 21.2 0.094 49 0.00057871.198 × 10 2100834 [39[37]] 100100 2 213.57 735 789.65 7.5 0.002614.536 1324319 [43[38]] 130100 63.2 221.2 3970 49 1430 0.0005787 0.09 1702100 [44[39]] 100130 2 63.2735 13.12 214.237.5 0.00410.0026 15011324 [45[43]] 100 2 160 3.49 0.0054 332 [46] 100100 2 23970 200 1430 27.2 0.0340.09 217170 [47[44]] 130113 63.2 913.12 1160 214.23 2.2 0.00002340.0041 1461501 [51[45]] 100113 2 9160 840 3.49 12700 0.1870.0054 199332 [52[46]] 100100 2 2200 19.44 27.2 8.8 0.11320.034 2257.5217 [56[47]] 110 6.3 3027.6 429 0.0036 5545 [55] 113 9 1160 2.2 0.0000234 146 [51] 100 2 251 7.3 0.0072 183 [60] 113100 9 2840 676 12700 0.345 0.00012760.187 600199 [59[52]] 100114 2 1019.44 11.14 8.8 210 0.18850.1132 1552257.5 [64[56]] 11094 6.3 13027.6 154.96 429 3.22 0.0210.0036 ~360,4505545 [68[55]] 100 2 251 7.3 0.0072 183 [60] In100 should be noted2 that although676 energy performance0.345 can be0.0001276 used to judge the600 overall performance[59] 114 10 11.14 210 0.1885 155 [64] of an AEH device, more focus should be placed on applicable frequency and size. 94 1 154.96 3.22 0.021 ~360,450 [68] In general, harvesting low-frequency acoustic energy usually requires a larger device than harvesting mid–high frequency acoustic energy. However, deep subwavelength AEH devices can be In should be noted that although energy performance can be used to judge the overall designed using acoustic metamaterials, which are more suitable to handle these conditions. In addition, performance of an AEH device, more focus should be placed on applicable frequency and size. the local resonant property can effectively trap elastic energy, and it can be converted into electric In general, harvesting low-frequency acoustic energy usually requires a larger device than energy via the piezoelectric effect, generating an effective method for energy harvesting. harvesting mid–high frequency acoustic energy. However, deep subwavelength AEH devices can be For instance, although quarter-wavelength tube resonators achieve high power output at a low designed using acoustic metamaterials, which are more suitable to handle these conditions. In frequency, tube size may hinder its wide application. An alternative approach is the local-resonant addition, the local resonant property can effectively trap elastic energy, and it can be converted into acoustic metamaterial-based AEH [64], which achieves similar performance to the quarter-wavelength electric energy via the piezoelectric effect, generating an effective method for energy harvesting. tube resonator-based approach but requires less space. For instance, although quarter-wavelength tube resonators achieve high power output at a low frequency, tube size may hinder its wide application. An alternative approach is the local-resonant

Micromachines 2019, 10, 48 18 of 21

It has also been shown that resonant coupling is favorable for boosting energy-harvesting performance but it requires a delicate tuning process. In addition, AEH performance markedly differs for piezoelectric materials versus electromagnetic mechanisms, indicating that this topic requires further research.

6. Conclusions In AEH, acoustic energy is converted into electrical energy, and this method has potential applications for the Internet of Things. An AEH device has low requirements in terms of installation and operation, and can be integrated into other engineering structures to obtain smart structures. In this review, the mechanism and potential applications of AEH have been described. The literature survey provides details on the different approaches used in this ﬁeld that can be categorized into the Helmholtz-based approach, the quarter-wavelength-based approach, and the acoustic metamaterial- based approach. A quantized metric was also provided to evaluate the performance of different AEH devices. We have witnessed and participated in the fast development of this exciting technique. In our view, the development of acoustic metamaterials provides fruitful approaches to achieve efﬁcient AEH with compact devices. In addition, the development of novel materials will improve the harvesting performance. Although many effective prototypes have been developed, a large gap remains between research and actual applications. Multidisciplinary studies and the development of power management and storage techniques are required to close this gap. Moreover, for large-scale deployment, system-level optimization is required, which includes unit-design optimization, circuit optimization for power management and storage, cost optimization, integration with other optimized energy-harvesting techniques, and optimization of the power-consumption device.

Funding: This work was funded by the National Natural Science Foundation of China (grant numbers 61701250, 61372044), the Natural Science Foundation of Jiangsu Province (grant number BK20160895), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (grant number 14KJA510002). Acknowledgments: The authors sincerely thank the reviewers for their valuable comments and suggestions, which have led to the present improved version. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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