Experimental analysis of energy conversion and noise suppression mechanism using polyvinylidene difluoride piezoelectric element

Abstract

Energy conversion and conservation techniques are well known for different natural sources but one of the common sources of energy that stays unutilized in the environment is sound energy. The noise around us is a form of unutilized energy. The acoustic energy can be utilized to produce electrical energy. In this article, the testing of the acoustic energy conversion technique is performed. Here the acoustic low amplitude pressure waves generated by the buzzer ringer have impinged over the surface of PVDF (Polyvinylidene Difluoride) Piezoelectric element which has the capability to vibrate after sound impingement. Here the experimental results are taken in the frequency range of 400 Hz to 1300 Hz, in which the highest response occurred at a frequency of 1000 Hz and 91.9 dB, which is +/−50 mV and the maximum Vrms (Voltage; root mean square) is 35.35 mV by the single piezoelectric unit. The performance of an array of resonating tube chambers as an enclosure to the noise source has been observed to accomplish noise reduction. Then the array pattern arrangement of this entire system has been discussed with the predictions of the output voltage.

Keywords

acoustic energy peizoselectric material energy conversion noise barrier electromechanical

Introduction

A sound wave is a vibration that propagates through a transmission medium such as a gas, liquid or solid. These waves hold energy in the form of pressure. Such sound waves or the form of energy stays unutilized in the environment. Acoustic waves are more mechanical waves that contain some energy and this energy can be easily found in noise and other sound sources. When the sound wave is undesired, it is referred to as noise. Common noise sources include airplanes, vehicles, high-speed trains, power plants, loudspeakers, machines, and expressways. Hence every noise source has been critically understood and the estimation of different noises has been studied such as, In-cabin noise levels during commercial aircraft flights, and the quantitative analysis of the aircraft noise is done for different aircraft.¹ Even under the flight path, and flyover zone of aircraft, large sound pressure levels have been observed.² Which are significantly high, and the variations of noise is the challenging part to overcome to reduce noise. But these specific points near the continuous in-use runways can be used. Also, Traffic noise analysis is done with dynamic and experimental observations of urban areas.³ And mechanical applications such as centrifugal pumps, simple noise, and low-frequency noise are estimated.^4,5 To point out this problem and give a solution to this noise generation, researchers have found many approaches to control noise.

There are many applications available in the daily environment becoming the sources of noise, and this noise cannot be avoided, so there is a need for noise absorption or suppression, just for a sake of human comfort. Hence many practices have been developed to lower the intensity of noise by absorption or attenuation of waves to make it quite comfortable for human hearing.^6-8

In the new era, the technology has increased its demand to extract unutilized energy from the environment and to convert it into a usable form such as electrical energy. This topic still has plenty of demands from researchers. Previously few successful energy harvesting methods have been established such as solar energy harvesting,^9,10 wind energy,^11,12 thermal energy,^13-15 mechanical vibrations^16,17 chemical energy,¹⁸ and radio frequencies.¹⁹ These are a few environmental sources of energy that has been investigated and produced methods to convert these energies into electrical form to drive small electronic components, sensors, and low power consuming devices.

Acoustic (sound) energy in the environment can be harvested into another form of energy, usually noise. This harvesting of acoustic energy would be described as the process of converting sound waveforms into electrical energy by incorporating acoustic transducers as converting devices.²⁰

Many researchers have been working on energy conversion methods and have found famous methods such as electromagnetic transduction,^21,22 electrostatic transduction,^23,24 and Piezoelectric generator^25,26 based energy conversion, which converts mechanical vibrations into electricity in many applications. Those are discriminated by their efficiency, structure, utility, and mechanism of energy conversion. Out of these methods, the Piezoelectric based generator is the best suitable mechanism to convert mechanical vibrations into electricity due to it is availability, simplicity to use with simple structure compared to other techniques.

Mechanical strain energy can be transformed into electrical energy by using piezoelectric material through the piezoelectric effect. There are commonly used piezoelectric materials available such as barium titanate (BaTiO3), polyvinylidene difluoride (PVDF), and lead zirconate titanate (PZT).

Experimentation analysis

In this experiment the two different functions over a single system are carried out, by looking at the necessity of noise reduction and output voltage generation, a single system is developed in order to overcome the difficulties of noise generation and energy requirement at the same time.

Voltage generation

In this study, the energy conversion is done through the direct piezoelectric effect, which gives electrical output over the mechanical input. Here the PVDF piezoelectric cantilever element with dimensions of a length of 30 mm, a width of 13 mm, and a thickness of around 1 mm is used as the primary module of this acoustic energy converter. This piezoelectric element is more sensitive to vibrations and produces an electrical signal as an output in terms of voltage. Here this piezoelectric element is placed under the acoustic source in such a way that the proper influence of wave over the surface of the piezoelectric plate to get possible maximum output. When a sound generator produces sound in terms of a pressure wave, this pressure wave targets the piezo from the acoustic energy conversion unit. Due to this pressure, there will be continuous deflection in the PVDF piezo cantilever that causes the generation of electrical potential at the output. This electrical signal or energy has been then measured using analog oscilloscope.

PVDF (Polyvinylidene fluoride or polyvinylidene difluoride) is a semi-crystalline, high purity thermoplastic fluoropolymer. With service temperatures up to 150°C, when appropriately manufactured (it is extended, an electric field applied during manufacture, and heated), gets to be a piezoelectric fabric, meaning that when the force is applied to it, it can generate some charge. Here, as the stretching happens, the particles head in the same direction, then from the electrical orientation standpoint, molecules align due to the electric field. PVDF holds some properties such as mechanical strength, high processability, chemical resistance, and piezoelectric and pyroelectric properties.

The PVDF piezoelectric sheets are available and are small sheets of the material with a layer of metal on each side and two crimped contacts (PVDF can be easily soldered by iron since it has a low melting point). It is about the size of a small normal stamp (Table 1).

Table 1.

PVDF piezoelectric transducer properties.

Elastic modulus	3	GPa
Density PVDF	1780	Kg/m3
Piezoelectric coupling	0.07	C/m2
Length	30	mm
Width	13	mm
Thickness	1	mm

Figure 1 compares the resonating frequency of piezo and clamped length of the PVDF piezoelectric element. In this experimentation, the piezo is clamped at 7 mm which makes this element become resonant at 1000 Hz.

Figure 1.

Resonant frequency versus clamped length.

PVDF piezoelectric element generates a charge when it is under mechanical stress. It turns out that to get the highest possible output from the piezoelectric element, the stress needs to be in a direction that depends on which axis of the element the film was expanded during manufacturing. The most suitable way to make use of this piezoelectric for general vibration or impulse detection is to perform tensile stress in that direction. The impingement of sound waves causes the force acting on the cantilever piezo, which can deflect the piezo sideways. The cantilever bending shown in Figure 2

Figure 2.

PVDF cantilever element.

When the sensor is stretched on one side while flexing, which is causing the generation of stress in the PVDF sensor on bending. However, the two opposite sides of the sensor need to be stretched to generate maximum output. So, this can be obtained by having PVDF film on some thick materials. Which will adjust the center of the structure as the force applied to it and that would lead to the generation of stress in the material.

As the external force is applied to the film, the thickness gets changed and it compresses the film, which generates some voltage at the electrodes. The electric field produced by mechanical stress is in relation to the piezoelectric coefficient $d_{m n}$ . Where suffix m = 1, 2, three and n = 1, 2, …, six are referred as electrical axis and mechanical axis respectively.²⁷ The third rank tensor $d_{m n}$ is ordinarily stated in terms of 3 x 6 matrix.²⁸ Whose matrix can be written as

d m n = [\begin{array}{c} 0 & 0 & 0 & 0 & d_{15} & 0 \\ 0 & 0 & 0 & d_{24} & 0 & 0 \\ d_{31} & d_{32} & d_{33} & 0 & 0 & 0 \end{array}]

The electrical flux density D of a PVDF element is defined as

D = \frac{Q}{A}

D = d_{3 n} X_{n}

Where the sensor develops the charge Q, the conductor electrode area is A, the piezoelectric coefficient for the axis of the applied stress is

d_{3 n}

and the stress applied in the relevant direction is Xn.⁷ In place of

d_{m n}

it is considered as

d_{3 n}

, since the electrical axis is associated with the electrode direction, which is situated at the top and the bottom of the film. In the case of a mechanical axis, stress can be applied through all sides, hence suffix n can be 1, 2, or 3.²⁹

Framework

When acoustic waves are implied on piezoelectric polymer film in the form of mechanical energy, it induces electrical charges between surfaces, which are then collected by electrode films. Hence using this technique, piezoelectric material serves as an electromechanical energy converter. Therefore, some conducting materials could be considered as electrodes for PVDF incorporated piezoelectric devices.

The current structure is a guide to store the energy from the continuous impingement of sound over time. This structure is able to gather millivolts but in continuous use at a certain frequency, the storage facilities such as capacitors and batteries will help to store voltage over time. This stored voltage can be utilized in form of energy. R-C circuit will facilitate the energy equation; since energy will be consumed to face resistance. In this way, less energy-consuming LED’s will be energized at small storage capacity and even with small input sound impulses. Considering, real-time applications in railway stations and subways, the sound pressure goes high at different frequencies and different points,³⁰ so in this case, more energy can be stored. Which might be later used to lit LED lamps. In parallel with just continuous impingement, the resonating frequency will obtain vibrations in a flexible PVDF piezo, which really helps create voltage. It’s not just impingement, the structural vibrations driven by the resonating characteristic of material have also been considered to utilize energy. Even for the accurate noise predictions of Aircraft, the research³¹ based on engine fan settings, the noise can be accurately predicted. If the predictions go accurate, according to the frequency and noise level, piezo material with respective resonating frequency can be kept at that point and the peak voltage can be observed as an output.

Also, this research has been performed over the sound of mobile phones, to show that even a small source can convert sound into an electric voltage with the help of sensitive PVDF material. In the case of large sound source areas, this structure would act as bifunctional, converts input sound into voltage as well as suppress sound in the environment.

Hence to observe the frequency of maximum output through PVDF piezoelectric element, the frequency of a sound wave is varies in-between the range of 400 Hz–1300 Hz.

The different units of the Acoustic Energy conversion system are represented in a flow as shown in Figure 3. The process flow of this system starts with the excitation of the sound source (Acoustic Source) which generates pressure waves and these waves are generated by a buzzer ringer in the input unit where acoustic waves are generated and then are arranged to impinge over the piezo inside Piezo Unit which is a part of the main unit where energy conversion function is accomplished, where the acoustic waves engender a generation of mechanical phenomena inside piezo material and then it is converted into electrical ac signal. This phenomenon occurs due to the smart properties of piezo material (PVDF) by the generation of mechanical strain inside the material. Then the generated output voltage signal is reflected by an analog oscilloscope.

Figure 3.

Different Units (Input Source, Main Unit and Signal Analysis) and their functions.

Here the piezo unit for the acoustic energy conversion system is placed at the rear end of the array of resonating tube chambers incorporated to perform noise reduction function. Here this piezo unit performs as a Helmholtz Resonator (HR) in which the piezo is supposed to experience the vibrations and intensified SPL (Sound Pressure Level) after the journey of sound through orifice throughout a single chamber in an array of a resonating tube of the sound suppression structure. Here the orifice is created inside one of the rectangular tube chambers as shown in Figure 4. At the extreme end of the rectangular tube chamber, at the orifice which is at the inlet of the Helmholtz resonator-like structure, the recorded sound pressure level is 110 dB at 1000 Hz frequency.

Figure 4.

Different perspectives of Acoustic energy harvesting unit and noise suppression a) Captured image of front side view, b) transparent view from front view which represents PVDF Piezoelectric element in yellowish rectangle, c) isometric view of unit and d) isometric transparent view of unit.

Noise suppression

Noise reduction is the process of abating the SPL in the environment. To make the proposed device multifunctional, the noise suppression capability of this system is investigated in parallel with the energy conversion process. In this investigation, the reduction in SPL is measured with respect to the variation of frequency. The noise reduction is accomplished at the main unit as shown in Figure 4 a, which is incorporated with energy conversion.

The variation of SPL (Sound Pressure Level) is observed over the frequency range of 300 Hz–1200 Hz. The proposed system of acoustic energy converter using an array of resonating tube chambers is supported as a partial enclosure to the source as shown in Figure 5 and the decibel m has been used to collect the SPL variation. Here each rectangular tube is acting as a resonator and the array of resonating tube chambers is proposed in this system for noise reduction capability. Wave distribution over the array of PVC (Polyvinyl chloride) tubes has occurred which splits the waves and varies the amplitude. A Series of sensitive PVDF materials will ultimately help cover the large spectrum through which energy can be stored in large quantities. The large size of PVDF piezo would not effectively work over a large range of frequencies, but it might give more efficiency only for a particular resonating frequency. Another way is to design PVDF according to the input frequency to which that structure would resonate. Hence, for more spectrum of frequency, the small-sized piezo will be effective, if placed in an array and in large quantity. Even in case of large sound pressure level, generated by the profound noise source, if this structure is employed, then over the time, as soon as batteries get charged, then it will be enough to charge mobile phones, even it can light battery-based LEDs, just like solar cells does. And in the case of large structures at constant noise generation places, it will definitely store enough energy. Considering, variations of sound frequencies, there will be an assessment required, before employing this structure at any place, one needs to observe the point-to-point SPL variation. So that, the proper resonating structure of piezo can be incorporated into the structure to get effective output voltage. Another limitation is that this structure can only be applied in selective places which are noise-prone areas. Other than this, anywhere, it would not generate energy but can only block even small noise.

Figure 5.

Proposed system supported as an enclosure to the source.

Results and Discussion

Here the results are also carried out for two functions, the first one is for voltage generation and the second one is for noise reduction.

Voltage generation

Voltage variations have been observed on an oscilloscope with respective to the input frequency and SPL,

Here the series of readings of the output voltage signal had been taken at different frequencies over an analog Oscilloscope as shown in Figure 6. The voltage signal Analysis is done in accordance with the increase in frequency by 50 Hz to the original one. Here the recorded peak value of voltage is +/− 50 mV at 1000 Hz frequency and at 1000 Hz the voltage signal is around about +/−40 mV. The reason for maximum output at 1000 Hz is that the resonating frequency of the piezoelectric element after clamping it at 7 mm is 1000 Hz. The dominant frequencies of the piezo-film are observed in the range of 500–1050 Hz. Also, the oscilloscope variation is modeled over MATLAB by the below wave condition function to get all the waves in a single display as in Figure 7 to discretize and observe the wave nature for different variable values.

{\begin{matrix} v (t) = v s i n (2 π f t) \\ v (t) = 0 \end{matrix} \begin{matrix} t \in (0,10000) \\ f = 0, t = 0 \end{matrix}

Figure 6.

Variation of Voltage signal from 500 Hz to 1050 Hz where the vertical axis represents voltage in mV and horizontal axis represents time in mSec.

Figure 7.

Modelled wave form for output voltage in time frame.

Here in the case of voltage, the output peak voltage obtained from this experiment is at the resonant frequency and 91.9 dB of sound level which is intensified through PVC rectangular tube and becomes 110 dB at the inlet of the orifice of Helmholtz resonator like structure produces a maximum voltage of ac 50 mV. Figure 8 stands for the variation of peak voltages in frequency range with the variation of sound level (dB). Then the root mean square values of output voltages (Vrms) is obtained at intervals of frequency which is represented in Figure 9, in which the peak value of Vrms has occurred as 35.35 mV.

Figure 8.

Variation of ac peak voltage and SPL (dB) with frequency change.

Figure 9.

Variation of rms Voltage and peak Voltage over the frequency change.

For an inline sound source, the pattern of a single unit of acoustic energy conversion could lead to harvesting an enormous amount of electrical energy by inscribing the unit inside two layers of sound protecting wall, becoming a thick wall as shown in Figure 5 and this wall will be constructed as an enclosure to the noise generation source. Since the single unit of the proposed design can convert 35.35 mV RMS for every, then for a large amount of time it can convert the tremendous amount of Voltage or energy which will be sufficient to drive electronic components. Considering the inline source and the output voltage connected in series will give direct multiple outputs. Here the sample construction gives a 0.848 V signal that is 24 units × (Vrms of single unit). Considering bridge rectifier unit, the dc output signal could be (0.848 V–0.7 V drop) = 0.148 for every millisecond. That is for every second it will give a 148 V signal. Hence it will be capable of driving 20 dc motors of 7V supply every second. So, this Acoustic energy conversion technique will perform dual applications such as energy conversion and noise control Figure 10.

Figure 10.

The enclosure wall with inscribed pattern of acoustic energy conversion unit.

Noise Reduction

Then the variation of sound pressure level over the frequency range of 300 Hz–1200 Hz is compared, where the comparison of SPL without employing the proposed conversion system as an enclosure and with employing this system as an enclosure to the noise source is represented in Figure 11 in which observed that the proposed Acoustic energy conversion system employing as an enclosure to the noise source can suppress the noise for the possible range. At the frequency of 1000 Hz proposed design as an enclosure reduces SPL from 91.9 dB to 74.5 dB. Also, the reduction in SPL is compared with the frequency variation as shown in Figure 12 where the maximum reduction occurred at 600 Hz of the input frequency.

Figure 11.

Variation of SPL with and without employing as an enclosure to the source in a frequency range.

Figure 12.

SPL reduction with respect to the variation of frequency.

Conclusion

Acoustic energy can be successfully converted into an electrical signal. In this study, a novel acoustic energy conversion mechanism using a PVDF Piezoelectric element and noise suppression through an array of resonating tube chambers is proposed. Here this piezoelectric element undertakes vibrations after the sound impingement from sound waves generated by a buzzer ringer. Here the piezoelectric element and incident SPL plays a significant role to convert energy. The output voltage results reflected from the oscilloscope observed in this mechanism are producing voltage at an exceedingly small instance of time. Hence gathering the amount of voltage over some time could produce enough energy to drive small electronic systems. In this study, the technique used to harvest sound wave energy is relevant and has immense potential in terms of converting free energy into useful energy. Here the generation of peak voltage +/− 50 mV (35.35 Vrms) occurs through a small vibrating PVDF piezo unit. Also, this system as an enclosure to the noise source promotes the SPL reduction from 91.9 dB to 74.5 dB at a frequency of 1000 Hz. The maximum reduction of SPL is occurred at 600 Hz. Hence the proposed design of acoustic energy converter is multifunctional, performing energy conversion and noise reduction.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Avadhut Tukaram Patil

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