Figure
1.
Structure and size of L-shaped harvester.
| Citation: |
Qingying Ren, Yuxuan Liu, and Debo Wang, “Research on low-frequency multi-directional piezoelectric energy harvester with combined cantilever beam,” Chinese Journal of Electronics, vol. 34, no. 1, pp. 156–164, 2025. DOI: 10.23919/cje.2023.00.351
|
Ren Qingying: Qingying Ren was born in China in 1987. She received the Ph.D. degree in microelectronics and solid state electronics from Southeast University, Nanjing, China, in 2017. She is currently a Lecturer at College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing, China. The discipline of her research focuses on energy harvesting technology, MEMS sensor, and temperature and humidity sensors, and LC passive wireless sensors.(Email: rqy@njupt.edu.cn)
Liu Yuxuan: Yuxuan Liu was born in China in 2005. He is currently pursuing the B.S. degree in communication engineering at Nanjing University of Posts and Telecommunications, Nanjing, China. His main research interest is piezoelectric energy harvesting technology. (Email: 2994369225@qq.com)
Wang Debo: Debo Wang was born in China in 1983. He received the B.S. degree in electronic science and technology from Hebei University of Science and technology, Shijiazhuang, China, in 2007, the M.S. degree in electronic science and engineering and the Ph.D. degree in microelectronics from Southeast University, Nanjing, China, in 2010 and 2012, respectively. He is now a Postdoctorsl Researcher in Nanjing University, Nanjing, China, and an Associate Professor of the Nanjing University of Posts and Telecommunication, Nanjing, China. The discipline of his research focuses on RF MEMS devices, particularly on microwave power sensor and its package. (Email: wdb@njupt.edu.cn)
In order to realize the collection and utilization of low-frequency vibration energy, a multi-directional piezoelectric energy harvester is proposed, which consists of a lower circular arc beam and an upper L-shaped beam. Both the lower and upper beams can achieve multi-directional energy harvesting, and the upper L-shaped beam can also act as a mass block to reduce the resonant frequency. The structure of this energy harvester is optimized. Four different structures are studied with varying combination angles between the upper and lower layers to acquire data related to resonant frequency, vibration shape, stress distribution, open-circuit voltage, and output power. Additionally, the performance of each structure is comprehensively prepared and measured to verify its effectiveness. The optimal structure achieved a resonant frequency of 11 Hz and an output power of 57.1 μW at the optimal load resistance of 201 kΩ. Consequently, this work provides valuable reference for the study of low-frequency vibration energy harvesting technology.
With the rapid development of integrated circuits, there is a trend toward miniaturization in electronic devices [1]-[4]. However, these devices face challenges in terms of power supply due to their complex application environments [5]-[6]. As a result, researchers have shifted their focus towards exploring the utilization of environmental energy. Among the various forms of environmental energy, vibration energy harvesting has garnered significant attention [7]-[10].
In 2003, Lu et al. [11] designed the first vibration energy harvester with a resonant frequency of 3 kHz and an output power of 15 μW. Building upon this work, Fang et al. [12] successfully reduced the resonant frequency to 600 Hz by improving the harvester’s geometry in 2006. In 2017, Guo et al. [13] proposed a bridge-type piezoelectric energy harvester, which could be employed in high-stress environments. To improve its output, finite element analysis was utilized by manipulating certain parameters. Despite these efforts, the energy harvester exhibited considerable size, limited vibration harvesting capability restricted to the z-direction, and a narrow frequency band, resulting in low harvesting efficiency. In the same year, another proposed energy harvester structure by Jing et al. [14] involved n cantilever beams and n mass blocks interconnected alternatively. When n = 2, the total length of the cantilever beam was 21 mm, with mass values of 1 mg and 1.7 mg for m1 and m2, respectively. The first two resonant frequencies were measured at 15.25 Hz and 23.08 Hz, respectively. However, this structure was relatively intricate and occupied a larger volume. In 2017, Wang et al. [15] proposed a bifurcated energy harvester. The results demonstrated that the energy harvester exhibited resonant frequencies of 21 Hz, 47 Hz, and 91 Hz, making it suitable for low-frequency vibration environments. However, its structure was limited to a right-angle cantilever beam configuration, allowing energy harvesting in only one direction. Furthermore, the significant deviation between the second and third order resonant frequencies indicated poor extension of the frequency band. In 2020, Asthana et al. [16] introduced a teeter-totter-shaped piezoelectric energy harvester with an operational bandwidth of 10 Hz. This configuration resembled two masses attached to a cantilever beam, did not overcome the limitation of single-direction energy harvesting. In the same year, Debnath et al. [17] proposed an axe-shaped piezoelectric energy harvester, incorporating a mass block. With the mass block, the energy harvester generated an output power of 7.81 nW and an output voltage of 88.41 mV. In contrast, the axe-shaped energy harvester without a mass block produced a lower output performance with an output voltage of 3.056 V and an output power of 9.34 μW. In 2022, Qin et al. [18] proposed a U-shaped structure with cross-connected beams. The U-shaped beam’s bi-directional load-bearing capability and the additional bending modes introduced by the cross-connected configuration enabled effective vibration energy capture from multiple directions. Experimental results revealed three resonant frequencies within the range of 2 Hz to 15 Hz, with an output voltage of up to 4 V. Nevertheless, this structure occupied a relatively large space, and the output voltage remained low.
The aforementioned harvesters exhibit certain drawbacks, such as intricate structures or limited capacity for harvesting vibration energy in a single direction, resulting in low efficient energy harvesting. To solve these drawbacks, a low-frequency multi-directional piezoelectric vibration energy harvester is proposed to improve the output performance and reduce the resonant frequency in this work, which consists of a combination of a lower circular arc beam and an upper L-shaped beam. Both the lower and upper beams can achieve multi-directional energy harvesting, and the upper L-shaped beam can also act as a mass block to reduce the resonant frequency. In Section II, four types of harvesters are optimized according to the combination angle. In Section III, the lumped parametric model is researched, and the multi-directional collection capability, frequency responses, and output characteristics of the structure are analyzed. In Section IV, the frequency response and output power of the four different structures are measured and the measured results are compared with the simulated results, and output characteristics of this harvester are compared with other structures. Finally, some conclusions are drawn in Section V.
As shown in Figure 1, the lower layer of the proposed low-frequency multi-directional piezoelectric vibration energy harvester is a circular arc beam structure, transforming the cantilever beam into a quarter-arc configuration. The upper surface of the cantilever beam is affixed with a quarter-arc piezoelectric material. The fixed end of the harvester is determined at one end of the cantilever beam, while the other end serves as the free end. In order to reduce the resonant frequency of the harvester, a mass block is positioned at the free end. The specific structural parameters are provided in Table 1.
| Name | Length/width (mm) | Outer diameter (mm) | Inner diameter (mm) | Thickness (mm) |
| Cantilever beam | – | 30 | 20 | 0.5 |
| Piezoelectric layer | – | 30 | 20 | 0.5 |
| Mass block | 10 × 10 | – | – | 10 |
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The upper layer of the vibration energy harvester adopts an L-shaped beam design, as shown in Figure 2. It consists of several distinct regions, namely the fixed end, the moment-shaped areas P1 and P2 where piezoelectric materials are positioned, the L-shaped corner area C and the mass block M, as shown in Figure 2(a). The cantilever beam base and the mass block portion are constructed using metallic copper in order to ensure structural stability. Moreover, the cantilever beam serves a dual role, acting as both the driving component of the entire system and facilitating the connection of lead wires for the subsequent measurement. Both the copper metal beam and the piezoelectric material have a thickness of 0.5 mm, while the mass block has a thickness of 10 mm, as shown in Figure 2(b).
The specific material parameters of the circular arc and L-shaped structures are shown in Table 2. The structure of the low-frequency multi-directional harvester, which encompasses a combination of the lower circular arc configuration and the upper L-shaped design, as shown in Figure 3.
| Structure | Material | Density (kg/m3) | Young’s modulus (GPa) |
Poisson’s ratio |
Relative dielectric constant |
Piezoelectric constant (C/m2) |
| Beam substrate/mass block | Copper | 120 | 0.34 | – | – | |
| Piezoelectric material | PZT-5H | 76 | 0.34 | −6.6 |
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To analyze the performance of various combinations between the upper and lower layers, four distinct structures are designed as shown in Figure 4. In these structures, the lower cantilever beam structure has a fixed end while the upper cantilever beam structure is rotated 90o clockwise around the mass block of the lower structure. This arrangement generates four different combinations. These combinations are labeled as “1”−“4” based on their respective relative positions, and are denoted as L1, L2, L3, and L4 to correspond with their specific structures.
This piezoelectric energy harvester can be equivalent to the spring-mass-damper model, as shown in Figure 5. When the base of the system is subjected to a harmonic excitation, this piezoelectric energy harvester vibrates periodically, the relative displacement are represented as x1 and x2, respectively.
The dynamic model of this system with the base excitation force of F is given as
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M(¨x1¨x2)+η(˙x1˙x2)+K(x1x2)=(F0) |
(1) |
where
|
M=(m100m2) |
(2) |
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η=(η1+η2−η2−η2η2) |
(3) |
|
K=(k1+k2−k2−k2k2) |
(4) |
where the parameters m1 and m2 represent the lumped mass values of two segmented beams, respectively. The spring stiffness values k1 and k2 denote the equivalent stiffness of two segmented beams, respectively. Furthermore, the mechanical damping of two segmented beams is expressed by η1 and η2, respectively.
Taking the first segment beam as a representative case, the expressions of the lumped mass m and the equivalent stiffness k of the spring are derived as follows:
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m1=ma+βM(mb+mp) |
(5) |
|
k1=cYI |
(6) |
where the masses of the mass block, substrate copper sheet, and piezoelectric layer are denoted as ma, mb, and mp respectively, βM is the Rayleigh constant, YI denotes the bending stiffness of piezoelectric beam, and the value of c is related to the specific structure.
Let F = F0ejωt, the corresponding relative displacement x(t) is as follows:
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x(t)=mω2k−mω2+jωηF0ejωt |
(7) |
In the undamped case, the resonant frequency is as follows:
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ω2=[(k1+k2)m2+k2m1]−√[(k1+k2)m2+k2m1]2−4m1m2k1k22m1m2 |
(8) |
In the structure designed in this work, each segmented beam has a piezoelectric layer. Considering that each piezoelectric layer can be equivalent to a parallel connection of a current source and an internal capacitor, these three piezoelectric layers (as shown in Figure 4) are connected in parallel, as shown in Figure 6.
The output of the circuit is connected to the load resistance RL. According to Kirchhoff’s current law, the following equation can be obtained:
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I∗(t)−Cp˙V=V/VRLRL |
(9) |
where
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I∗(t)=3∑n=1θn˙xn |
(10) |
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Cp=3∑n=1Cpn |
(11) |
where I∗(t) is the equivalent current, V is load resistance voltage. θn (n = 1, 2, 3) is the electromechanical coupling coefficient of the segmented beam, Cpn (n = 1, 2, 3) is their equivalent internal capacitance. The expression of capacitance is Cpn= εS/d, where ε is the dielectric constant, S and d are the area and thickness of the piezoelectric layer, respectively.
Under the source sine wave excitation F(t), the wave forms of relative displacement xn(t) and the equivalent current I∗(t) can be described as
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xn(t)=xnsin(ωt+τn+φn) |
(12) |
|
I∗(t)=I∗cos(ωt−φ) |
(13) |
where φn is the phase difference of between the relative displacement xn(t) and the excitation force F(t).
According to (12) and (13), equation (10) can be expressed as
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I∗=3∑n=1xnθnωej(φ+φn+τn) |
(14) |
Based on the equation provided above, the expressions for load resistance voltage V and the harvested average power P are as follows:
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V=I∗(t)CpRL|RL+1/1jωCpjωCp| |
(15) |
|
P=V22RL |
(16) |
The finite element analysis on the single circular arc and L-shaped harvesters are finished. The acceleration excitation of 0.1g (g is gravity) is applied in different directions to obtain the stress distribution of the circular arc harvester as shown in Figure 7 and the stress distribution of the L-shaped harvester as shown in Figure 8. It can be seen that for a particular direction of excitation, the stresses generated on the cantilever beam are more concentrated in the fixed end for both the circular arc harvester and the L harvester, and the stress values are gradually decaying with the extension toward the free end of the cantilever beam. In the case of the circular arc harvester, stress values induced by excitation in the Y and Z directions are greater compared with those in the outer diameter. Conversely, stresses associated with excitation in the X-direction diffuse from the outer to the inner diameter. In contrast, the stress value generated by the Z-direction excitation is the largest, and the stress distribution under the Y-direction excitation is more uniform. In addition to the Z-direction excitation, there is a more uniform stress distribution under the X and Y directions of excitation, which can better achieve the multi-directional vibration energy harvesting.
For the L-shaped harvester, the stress is mainly distributed at the moment region P1 near the fixed end and the edge part of the region under the X-direction excitation, and the stress is also generated in the corner region C. But the stress value in the P2 region is smaller; there is a higher stress value in the corner region C under the Y-direction excitation. There is a higher stress value in the region P1 under the Z-direction, and the stress in the region is also spread outward from the fixed end. But the difference is that the maximum stress value is generated at the corner region C. At the same time, the stresses in this area spread to areas P1 and P2, resulting in a more uniform distribution of stresses in area P1 as a whole, rather than a significant decay in the other directions of excitation. In summary, the harvester experiences certain stress distribution under the excitation in the X and Y directions in addition to the Z-direction.
The open-circuit voltages of circular arc-type and L-type under multi-directional excitation are shown in Table 3. The results show that the open-circuit voltage in the Z-direction is the highest for both circular arc-type and L-type harvesters, while the open-circuit voltage in the X and Y directions is also higher for circular harvesters.
| Excitation direction | Circular harvester (V) | L-shaped harvester (V) |
| X | 1.52 | 0.97 |
| Y | 1.82 | 1.76 |
| Z | 2.75 | 3.45 |
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An excitation in the Z-direction is applied with an acceleration of 0.1g and the distribution and modes are studied with finite element simulation. It can be obtained the first-order resonant frequency and second-order resonant frequency of harvesters L1–L4 as shown in Table 4. The results demonstrate that all four low-frequency multi-directional piezoelectric vibration energy harvesters can achieve the harvesting of more low-frequency environmental vibration energy. The harvester L2 has the lowest resonant frequency of 15 Hz because it has the lowest stiffness, which is most suitable for the low-frequency environment.
| Harvester | First-order (Hz) | Second-order (Hz) |
| L1 | 23 | 39 |
| L2 | 15 | 31 |
| L3 | 22 | 40 |
| L4 | 35 | 65 |
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The stress distribution of the harvester L1–L4 is obtained as shown in Figure 9. It can be seen that the stresses generated by the lower arc-shaped beams are diffused from the inner and outer directions, from the fixed end to the joint of the two harvesters, while the stresses generated by the upper L-shaped beams are mainly distributed in the area P1 and the corner area C. Under the same excitation conditions, the stresses in the lower beam of harvester L1 are uniformly distributed, and the high-stress area exceeds half of the arc, while the stresses in the upper L-shaped beam have more concentrated stresses in corner area C, except for area P1. The stresses in the upper L-beam have more concentrated stresses in the corner region C except for the region P1; the lower layers of harvester L2 have high stresses, while the fixed end region F and the corner region C of the upper L-beam also have very high stresses. As the stresses in region C spread to both sides, region P1 has high and uniformly distributed stresses and region P2 also has certain stresses. Harvester L3 only produces large stress in the lower arc beam and is more concentrated in the part near the inner diameter; while harvester L4 only produces a considerable stress value in the upper L beam.
The modal analysis of the harvester L1–L4 is obtained as shown in Figure 10. It can be seen that L2 has the largest vibrational displacement, while L4 has the smallest vibrational displacement and L1 and L3 are closer. According to the theory of positive piezoelectric effect, the larger the deformation of the piezoelectric layer, the larger the potential difference between the upper and lower surfaces perpendicular to the polarization direction, so L2 has the largest output voltage.
The frequency response is a crucial parameter for evaluating the performance of a piezoelectric vibration energy harvester. Figure 11 presents the frequency response of harvesters L1–L4. It demonstrates that the open-circuit voltage reaches its peak when the externally applied excitation drives the harvester into resonance. Consequently, as the externally applied excitation frequency deviates from the resonant frequency, the open-circuit voltage experiences a significant decline. Among several harvesters, L1 has an open-circuit voltage of 4.3 V at 23 Hz excitation; harvester L2 has the lowest resonant frequency of 15 Hz, but can produce the highest open-circuit voltage of 8.46 V; harvester L3 has an open-circuit voltage of 4.85 V at 22 Hz; harvester L4 has an open-circuit voltage of only 2.06 V at 35 Hz resonant frequency due to the pronounced interaction between the upper and lower cantilever beam girders. Overall, harvester L2 has the lowest resonant frequency and the highest open-circuit voltage after applying the same acceleration excitation. Remarkably, its open-circuit voltage surpasses that of harvester L4 by a factor of 4.1.
This work studies the fabrication and experimental evaluation of a low-frequency multi-directional piezoelectric vibration energy harvester. The harvester design comprises a circular arc beam harvester in the lower layer and an L-shaped beam harvester in the upper layer. Laser cutting is employed to prepare the copper beams and mass blocks for each layer, followed by welding the mass blocks to the respective free ends. The fixed end of the upper L-shaped beam is welded to the mass block of the lower circular-arc beam to form a complete harvester; the PZT-5H piezoelectric ceramic is prepared by the solid-phase sintering method. The photos of this piezoelectric vibration energy harvester is shown in Figure 12.
The four harvesters are measured by the test platform, which mainly consists of a signal generator, a power amplifier, a shaker and an oscilloscope, as shown in Figure 13. The harvesters L1–L4 are fixed on the shaker in turn, a sine excitation of 0.1g acceleration is applied, and the harvester wire is connected to the pins of the oscilloscope. At the same time, the vibration amplitude of the harvester on the shaker is observed and the oscilloscope is used to display the resonant frequency. The open-circuit voltage is recorded at different frequencies of each harvester, then the measured voltages and the simulated voltages are compared as shown in Figure 14.
For harvester L1, the open-circuit voltage obtained in the simulation is 4.30 V, while the open-circuit voltage obtained in the measurement is 3.40 V. The resonant frequency obtained in the simulation is 23 Hz, while the resonant frequency obtained in the measurement is 18 Hz.
For harvester L2, the open-circuit voltage obtained in the simulation is 8.46 V, while the open-circuit voltage obtained in the measurement is 7.28 V. The resonant frequency obtained in the simulation is 15 Hz, while the resonant frequency obtained in the measurement is 11 Hz.
For harvester L3, the open-circuit voltage obtained in the simulation is 4.85 V, while the open-circuit voltage obtained in the measurement is 4.21 V. The resonant frequency obtained in the simulation is 22 Hz, while the resonant frequency obtained in the measurement is 16 Hz.
For harvester L4, the open-circuit voltage obtained in the simulation is 2.06 V, while the open-circuit voltage obtained in the measurement is 1.52 V. The resonant frequency obtained in the simulation is 35 Hz, while the resonant frequency obtained in the measurement is 31 Hz.
As can be seen from Figure 14, the measured results basically agree with the simulated results, but there are still errors. The measured harvesters all have low resonant frequency and open-circuit voltage. The resonant frequency is low because the lumped mass of the structure is increased when the structure is fixed. According to (8), an increase in lumped mass decreases the resonant frequency. The open-circuit voltage of the measurement is less than the open-circuit voltage of the simulation, this is because the simulation is a relatively ideal condition, while there is the effect of damping in the measurement, which will reduce the open-circuit voltage. In addition, some losses will produce in the circuit rectification.
The relationship of the output power with the load resistance of each harvester in different directions is measured, and the corresponding results are shown in Figure 15. Harvester L1 is measured to have maximum output power at a load resistance of about 124 kΩ. The output power in the X-direction, Y-direction, and Z-direction are 18.2 μW, 18.0 μW, and 20.1 μW, respectively. Harvester L2 is measured to have maximum output power at a load resistance of about 201 kΩ. The output power in the X-direction, Y-direction, and Z-direction are 34.8 μW, 34.2 μW, and 57.1 μW, respectively. Harvester L3 is measured to have maximum output power at a load resistance of about 177 kΩ. The output power in the X-direction, Y-direction, and Z-direction are 24.1 μW, 21.8 μW, and 33.4 μW, respectively. Harvester L4 is measured to have maximum output power at a load resistance of about 68 kΩ. The output power in the X-direction, Y-direction, and Z-direction are 6.78 μW, 6.49 μW, and 8.49 μW, respectively. Therefore, this piezoelectric energy harvester can achieve multi-directional energy harvesting. Among these harvesters, harvester L2 exhibited the highest output power, approximately 6.73 times higher than that of harvester L4. As can be seen from Figure 9, harvester L2 has a greater stress distribution for the same vibration excitation. The higher the stress in the structure, the more polarization and charge will be generated. Therefore, harvester L2 has optimal output performance.
Comparing harvester L2 with other harvesters, the results are shown in Table 5. The normalized power density (NPD) [19] is a criterion for evaluating the power conversion efficiency of the harvester, and the NPD is calculated as the normalized power divided by the volume of the structure, the unit is μW∙cm−3∙m−2∙s4. NPD can be expressed as
| Structure | Acceleration (m/s2) | Resonant frequency (Hz) | Power (μW) | Volume (cm3) | NPD (μW∙cm−3∙m−2∙s4) |
| Nonlinear spring[20] | 1.372 | 520.0–591.0 | 0.083 | 0.027 | 1.630 |
| Multimodal electret-based[21] | 0.900 | 590.0–641.0 | 14.800 | 58.940 | 0.310 |
| AIN wideband[22] | 19.600 | 859.9 | 82.240 | 0.112 | 1.910 |
| Wide-band shaped[23] | 4.900 | 615.0 | 0.288 | 0.005 | 2.240 |
| Asymmetric-gap [24] | 0.980 | 35.4–126.4 | 1.100 | 52.06 | 0.022 |
| Flared-U shaped[17] | 0.686 | 28.0 | 0.011 | 0.002 | 12.050 |
| This work | 0.980 | 11.0 | 57.100 | 3.420 | 17.380 |
| Note: AIN, aluminium nitride. | |||||
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NPD = PVsa2 |
(17) |
where P is the output power, Vs is the structure volume, and a is the excitation acceleration. By considering this metric, our proposed energy harvester, which combines rectangular and spiral structure, demonstrates notable advantages in terms of high-power density and low resonant frequencies. In summary, the harvester can balance the volume and output performance, effectively increasing the energy harvesting efficiency per unit volume.
In this paper, a low-frequency multi-directional piezoelectric vibration energy harvester is innovatively proposed based on the conventional harvester by combining the lower circular beam and the upper L-shaped beam with each other. By studying a total of four harvesters with different combinations of the upper and lower beams, the optimal harvester configuration is identified. And the proposed harvesters are experimentally verified to successfully reduce the resonant frequency and realize the multi-directional harvesting capability of low-frequency vibration energy. The resonant frequency of the optimal harvester L2 is as low as 11 Hz, while the output power reaches 57.1 μW at the optimal load resistance of 201 kΩ. This structure of piezoelectric energy harvester can be effectively applied in wireless sensors and microelectronic devices.
This work was supported in part by the National Natural Science Foundation of China (Grant No.
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Figures(15) / Tables(5)
CJE QR CODE
CIE QR CODE
Copyright © Editorial Office of Chinese Journal of Electronics |
京ICP备12041980号-1 |
| Name | Length/width (mm) | Outer diameter (mm) | Inner diameter (mm) | Thickness (mm) |
| Cantilever beam | – | 30 | 20 | 0.5 |
| Piezoelectric layer | – | 30 | 20 | 0.5 |
| Mass block | 10 × 10 | – | – | 10 |
DownLoad:
CSV
| Structure | Material | Density (kg/m3) | Young’s modulus (GPa) |
Poisson’s ratio |
Relative dielectric constant |
Piezoelectric constant (C/m2) |
| Beam substrate/mass block | Copper | 120 | 0.34 | – | – | |
| Piezoelectric material | PZT-5H | 76 | 0.34 | −6.6 |
DownLoad:
CSV
| Excitation direction | Circular harvester (V) | L-shaped harvester (V) |
| X | 1.52 | 0.97 |
| Y | 1.82 | 1.76 |
| Z | 2.75 | 3.45 |
DownLoad:
CSV
| Harvester | First-order (Hz) | Second-order (Hz) |
| L1 | 23 | 39 |
| L2 | 15 | 31 |
| L3 | 22 | 40 |
| L4 | 35 | 65 |
DownLoad:
CSV
| Structure | Acceleration (m/s2) | Resonant frequency (Hz) | Power (μW) | Volume (cm3) | NPD (μW∙cm−3∙m−2∙s4) |
| Nonlinear spring[20] | 1.372 | 520.0–591.0 | 0.083 | 0.027 | 1.630 |
| Multimodal electret-based[21] | 0.900 | 590.0–641.0 | 14.800 | 58.940 | 0.310 |
| AIN wideband[22] | 19.600 | 859.9 | 82.240 | 0.112 | 1.910 |
| Wide-band shaped[23] | 4.900 | 615.0 | 0.288 | 0.005 | 2.240 |
| Asymmetric-gap [24] | 0.980 | 35.4–126.4 | 1.100 | 52.06 | 0.022 |
| Flared-U shaped[17] | 0.686 | 28.0 | 0.011 | 0.002 | 12.050 |
| This work | 0.980 | 11.0 | 57.100 | 3.420 | 17.380 |
| Note: AIN, aluminium nitride. | |||||
DownLoad:
CSV
