Low-frequency Acoustic Energy Research Articles

Development of a low frequency acoustic energy harvesting system by using piezoelectric transducers mounted on a metallic plate placed inside the centre of a hollow cylinder

Abstract In this work, we are demonstrating an innovative acoustic energy harvesting system that utilizes a hollow polyvinyl dichloride cylinder with a metallic circular plate placed at the centre inside with five piezoelectric transducers mounted on it. A speaker, used as a source of acoustic waves, is fixed at the top of the cylinder by using a clamp. The cylinder, upon exposure to incident acoustic waves, induces resonance within it, generating an amplified stationary wave. The generated vibration drives the metallic plate mounted with piezoelectric transducers, which serves as a diaphragm. As a result, a pressure difference is developed across it and due to piezoelectric effect on the transducers, electricity is produced. Experimental results show that the system output voltage and power depends on various factors, including the elastic properties of the metallic plate, resonance frequency, and the distance of separation between the acoustic source and the plate. At an incident sound pressure level of 75 dB, the experimental results show that, at an acoustic resonance frequency of 310 Hz, the system yielded a maximum peak to peak voltage of 1000 mV and output power of 0.612 µW. Incorporation of a voltage doubler circuit with the harvester increases its power to 57.6 µW. The simplicity in design and cost-effectiveness of the proposed acoustic energy harvesting system renders it to be a promising avenue for energy harvesting implementations.

Flexible and ferroelectric electrospun PVDF- non-stoichiometric PZT nanogenerators (PENGs) membranes for low-frequency acoustic energy harvesting

Acoustic energy harvesting (AEH) at low-frequency has been exposed to serious problems such as low energy of input sound and lack of efficient materials to absorb the sound and convert it to electrical power. This study focused on the enhancement of the AEH performance of polyvinylidene fluoride (PVDF) membrane via the incorporation of non-stoichiometric PZT (nsPZT) nanoneedles as piezoelectric nanogenerators (PENG). The nanofibrous membranes with different amounts of PENG (x = 0, 2.5, 5, and 10 wt %) were synthesized by the electrospinning method. The nsPZT was provided via Nd3+ and Nb5+ co-doping as a novel approach. The microstructures revealed that the PENG was embedded well inside the fibers (x = 2.5), while the agglomerates were formed at higher amounts (x = 10). Also, the fiber diameters and porosity of membranes were changed. It was found that the maximum crystallization of the β-phase of PVDF has occurred at x = 2.5 (71 %). The dielectric of the PVDF membrane decreased with increasing the x factor due to the formation of further pores. In contrast, the ferromagnetic properties, piezoelectric sensitivity, and electrical conductivity were improved by adding the PENG. The maximum coercivity of 750 Oe, piezoelectric sensitivity of 1.959 V N−1, and minimum sheet resistance of 1.2 kΩ.sq−1 were achieved by x = 2.5. The noise reduction coefficient (NRC) of the PVDF membrane was enhanced drastically (53–163 %) by adding the PENG. The AEH performance was also enhanced by adding the PENG. The highest power density was achieved 0.0822 W g−1 (56.23 W m−2) by the PVDF-PENG membrane (x = 2.5) under 90 dB sound pressure at 2000 Hz frequency. These findings suggest the high potential of nanofibrous PVDF-PENG membrane for use in low-frequency AEH systems.

Enhancement of acoustic properties of carbon fibrous membrane (CFM) by in-situ crystallization of PZT nanogenerators (PENGs) inside the fibers for a low-frequency acoustic energy harvesting

Acoustic energy harvesting (AEH) by a nanostructured membrane has recently gained increasing attention. This study focused on a novel approach for in-situ crystallization of Pb(Zr0.58Ti0.42)O3 (PZT) as a piezoelectric nanogenerator (PENG) inside the carbon fibrous membranes (CFM). The electrospinning process and post-heat treatment were applied to synthesize the CFM-embedded PENG, which benefits from both the flexibility of carbon fibers and the piezoelectric properties of the crystallized PZT. The results indicated that the carbon fibers kept their microstructure over the heat treatment at 250 and 350 °C while the crystallization fraction of PZT increased. The XPS analysis revealed that the CFM embedded by in-situ crystallized PENG had a higher diversity of tetragonal and rhombohedral in comparison with the CFM embedded by the pre-crystallized PENGs. Furthermore, the electrical conductivity, dielectric constant, and ferromagnetic properties of CFM were improved with the incorporation of the in-situ crystallized PENGs. The acoustic studies in the range of 250–2000 Hz under sound pressure of 90 dB revealed that sound absorption performance of CFM was increased by about +35 %. Also, the ability of CFM in the AEH process was enhanced drastically by embedding the PENGs inside the carbon fibers. The maximum harvested electricity at 500 Hz was 5.9 and 13.9 W m−2 by CFM embedded by in-situ crystallized and pre-crystallized PENGs, respectively. The above results exhibit the great potential for the CFM-embedded PENGs to be used in low-frequency AEH applications.

Sensitivity to speech-relevant features in hallucination-prone individuals

As hallucinations occur in the absence of an external stimulus, they constitute an intriguing model for how percepts are generated and for how perception can fail. This study explores whether hallucination proneness is linked to an altered perception of speech-related acoustic features. Involving 320 healthy adults with varying predispositions for hallucinations, participants evaluated ambiguous sound textures for their speech-likeness. Psychophysical reverse correlation revealed that higher hallucination proneness was associated with reduced weighting of speech-typical low-frequency acoustic energy. Temporal modulation discrimination capabilities were unrelated to hallucination proneness in a subset of 41 participants. Confidence judgments in individual trials were influenced by both acoustic evidence and individual hallucination proneness and schizotypy scores. Overall, these findings suggest that hallucination-prone individuals exhibit qualitative and quantitative changes in their perception of speech-relevant modulations, supporting the notion of altered perceptual priors and differential weighting of sensory evidence.

Integrated acoustic metamaterial triboelectric nanogenerator for joint low-frequency acoustic insulation and energy harvesting

In this study, we present a new device designed to manipulate low-frequency sound waves. The device utilizes the principles of acoustic metamaterials to achieve high-performance sound insulation, while also possessing the capability to convert sound energy into electrical energy. Unlike previous studies on membrane acoustic metamaterials, this device incorporates a membrane component but eliminates the need for precise tension force. The structural parameters of the device are frequency-dependent, allowing for a lightweight design while maintaining satisfactory low-frequency sound insulation performance. At the sound insulation peak frequency, the IAM-TENG exhibits an impressive sound insulation capability of over 30 dB. Moreover, when subjected to bandlimited white noise excitation in the frequency range of 50 Hz-500 Hz, the IAM-TENG showcases a substantial enhancement of nearly 10 dB in sound insulation performance in comparison to a sample of the same weight and size. Additionally, the device achieves a uniform displacement distribution, with large amplitude displacement occurring around the sound transmission dip frequency. Exploiting this property, we have designed a triboelectric nanogenerator (TENG) that can convert incident acoustic energy into electrical energy. This nanogenerator structure utilizes multi-walled carbon nanotubes (MWCNTs) and FEP (Fluorinated Ethylene Propylene) membrane as the triboelectric layers. With an acoustic excitation level of 100 dB at the optimal frequency, the harvested electrical power reaches 0.93 mW. The device has a surface mass density of only 2kg/m2, yet it provides remarkable acoustic insulation performance and can power small-scale electronic devices. These innovative properties have the potential to accelerate the development of self-powered, smart, and multifunctional structures in the Internet of Things era.

Textile beeswax triboelectric nanogenerator as self-powered sound detectors and mechano-acoustic energy harvesters

The development of self-powered sensing technologies embedded in textiles is a cornerstone for enabling radically novel healthcare, well-being and defence solutions. The steppingstone for these advances is a new class of unconventional energy harvesters able to convert presently unused sources from our living environment such as sounds and vibrations into electrical energy. These generators would have to be flexible, mechanically robust, lightweight, resilient and they should produce a viable electric output. Here we present a novel textile-integrated triboelectric nanogenerator capable of sensing and harvesting low-frequency acoustic energy. These capabilities are enabled by a unique combination of sustainable materials such as beeswax as a triboelectric layer and graphene-based electrode, endowing multiple functionalities. Specifically, our device can sense ground-borne sounds in the 10–200 Hz frequency range, detect human voices, recognise human emotions, and harvest energy from environmental noise and vibrations. Additionally, the dynamic molecular interactions and the hydrophobic nature of the triboelectric layer provide intrinsic thermal healing, water-repellent, and self-cleaning features, ensuring device functionality in bending and humidity environments.

Ultra-broadband low-frequency high-efficiency acoustic energy harvesting with metamaterial-enhanced loudspeakers

Acoustic energy harvesters (AEHs) open up opportunities to recycle noise waste and generate electricity. They provide potential power solutions to a wide range of sensors. However, the practicality of AEHs has long been limited by their narrow bandwidths and low efficiencies. In this study, we present an ultra-broadband AEH and a highly efficient AEH that transforms sound energy into usable electrical power. Our broadband device comprises an electrodynamic loudspeaker driver and an optimized acoustic metamaterial matching layer and is capable of converting 7.6% to 15.1% of total incident sound energy from 50 to 228 Hz. Moreover, we demonstrate that by replacing the loudspeaker surround with a lower-loss material such as PDMS, the energy conversion rate can be significantly increased to 67%. The proposed broadband AEH has a fractional bandwidth eight times the state-of-the-art, while the proposed highly efficient AEH has a peak efficiency three times the state-of-the-art. The outstanding performance makes our designs cost-effective and scalable solutions for noise reduction and power generation.

Research on an Optimized Quarter-Wavelength Resonator-Based Triboelectric Nanogenerator for Efficient Low-Frequency Acoustic Energy Harvesting.

Sound wave is an extensively existing mechanical wave, especially in marine and industrial plants where low-frequency acoustic waves are ubiquitous. The effective collection and utilization of sound waves provide a fresh new approach to supply power for the distributed nodes of the rapidly developing Internet of Things technology. In this paper, a novel acoustic triboelectric nanogenerator (QWR-TENG) was proposed for efficient low-frequency acoustic energy harvesting. QWR-TENG consisted of a quarter-wavelength resonant tube, a uniformly perforated aluminum film, an FEP membrane, and a conductive carbon nanotube coating. Simulation and experimental studies showed that QWR-TENG has two resonance peaks in the low-frequency range, which effectively extends the response bandwidth of acoustic-electrical conversion. The structural optimized QWR-TENG has excellent electrical output performance, and the maximum output voltage, short-circuit current and transferred charge are 255 V, 67 μA, and 153 nC, respectively, under the acoustic frequency of 90 Hz and sound pressure level of 100 dB. On this basis, a conical energy concentrator was introduced to the entrance of the acoustic tube, and a composite quarter-wavelength resonator-based triboelectric nanogenerator (CQWR-TENG) was designed to further enhance the electrical output. Results showed that the maximum output power and the power density per unit pressure of CQWR-TENG reached 13.47 mW and 2.27 WPa-1m-2, respectively. Application demonstrations indicated that QWR/CQWR-TENG has good capacitor charging performance and is expected to realize power supply for distributed sensor nodes and other small electrical devices.

Acoustic Sensing Response in Human Tissues for Theranostics and Implants

This letter investigates the impacts of high- and low-frequency acoustic waves on various human tissues. It is observed that low-frequency (4.6 kHz) acoustic energy works as a better alternative for acoustic power distribution across the tissues. Using a 4.6 kHz piezoelectric diaphragm transmitter and receiver, finite-element simulation was performed, and then, an experimental examination was done to evaluate the energy distribution. The average electrical output voltage produced by the low-frequency transducers was 0.758 V in the male subject's palm, 0.187 V in the wrist, and 0.015 V close to the elbow region. In comparison, it was 1.27 V in the female subject's palm, 0.534 V in the wrist, and 0.146 V close to the elbow region. The low-frequency response of the sensors was found to be equal to or a maximum of 39.7% better than that of the high-frequency alternative. The obtained output voltage can be used to power medical implants and extract medical data at great depths with minimal attenuation.

Highly-efficiency low-frequency acoustic energy harvesting with PDMS-modified loudspeakers

Noise is everywhere in our lives. Low-frequency noise in particular has a conspicuous if not disturbing presence since thicker materials are required to absorb or block it. Meanwhile, the very ubiquitousness of noise in our environment makes it a good candidate as a potential power source for micro-devices. Here we present an acoustic energy harvester (AEH) capable of achieving 99% sound absorption coefficient and 67% of energy conversion ratio at 58 Hz, with a fractional bandwidth of more than 34 %. The peak power conversion efficiency is nearly three times the previous state of the art. The harvester is made from an electrodynamic loudspeaker driver retrofitted with a custom-made PDMS surround for lower mechanical loss on the diaphragm. The low cost (<$50) makes our AEH suitable for large-scale deployment.

Broadband Low-Frequency Acoustic Energy Harvesting Amplified by Sonic Crystal Metamaterial with Double Defects

Broadband Low-Frequency Acoustic Energy Harvesting Amplified by Sonic Crystal Metamaterial with Double Defects

The influence of projection angle on nappe stability at low energy heads over the weir crest

Abstract Under certain low head hydraulic conditions, weir discharge under gravity flow can produce an oscillating nappe characterized by horizontal waves and audible low-frequency acoustic energy, which in some cases can have a significant impact on the local environment. In an effort to better understand the mechanism behind nappe oscillation, an experimental investigation was conducted using a linear weir (1.83 m wide, 1.10 m in height, and 0.076 m in thickness) with four different crest shapes (rectangular, quarter-round upstream, half-round, and quarter-round downstream) at a wide range of unit discharge ranging from 8×10−3 m2/s to 36×10−3 m2/s. Using Particle Image Velocimetry (PIV) techniques, differential pressure measurements, and water surface profiles, the influence of Weber number, angle at which the nappe hits the downstream pool (angle of impact), and nappe projection angle on the nappe oscillation are reported. No significant relationship was found between the Weber number and nappe instability. The results suggest that the nappe is stable for unit discharge between 20×10−3 and 24×10−3m2/s regardless the shape of the weir crest. Also, the stable projection angle is −17° for rectangular and quarter-round upstream weirs and −35° for half-round and quarter-round downstream weirs for the specific tested weir.

Design of tunable low-frequency acoustic energy harvesting barrier for subway tunnel based on an optimized Helmholtz resonator and a PZT circular plate

This paper presents a tunable, low-frequency acoustic energy harvesting barrier, which contains the following four parts: the noise acquisition input module, the Helmholtz resonator optimization module, the resonant frequency tuning module, and the electricity generation module. The aim of this study is to make rational use of the huge noise energy inside the subway tunnel and convert it into electric energy to power the sensing and detecting equipment and lighting equipment along the railway. In this work, the optimal harvester unit and resonant frequency tuning part were pertinently designed to increase the output power of the unit. Experimental results show that one unit of the system can generate 317.5 mV instantaneous voltage and 100.8 μW instantaneous maximum power at 100 dB, and it verifies the efficiency and practicability of the proposed acoustic energy harvesting noise barrier. The device has a broad application prospect and can be adapted to different tunnel environments, including high-speed railway tunnels, highway tunnels, urban subway tunnels, etc., with excellent conversion efficiency. These results can provide valuable ideas for the design and practical application of new multi-functional noise barriers in the future.

Investigation of the ability of low-frequency acoustic energy for polishing of the CK60 steel using a hybrid FE/BE/DEM approach

Investigation of the ability of low-frequency acoustic energy for polishing of the CK60 steel using a hybrid FE/BE/DEM approach

Male Humpback Whale Chorusing in Hawai‘i and Its Relationship With Whale Abundance and Density

Passive acoustic monitoring (PAM) with autonomous bottom-moored recorders is widely used to study cetacean occurrence, distribution and behaviors, as it is less affected by factors that limit other observation methods (e.g., vessel, land and aerial-based surveys) such as inclement weather, sighting conditions, or remoteness of study sites. During the winter months in Hawai‘i, humpback whale male song chorusing becomes the predominant contributor to the local soundscape and previous studies showed a strong seasonal pattern, suggesting a correlation with relative whale abundance. However, the relationship between chorusing levels and abundance, including non-singing whales, is still poorly understood. To investigate how accurately acoustic monitoring of singing humpback whales tracks their abundance, and therefore is a viable tool for studying whale ecology and population trends, we collected long-term PAM data from three bottom-moored Ecological Acoustic Recorders off west Maui, Hawaii during the winter and spring months of 2016–2021. We calculated daily medians of root-mean-square sound pressure levels (RMS SPL) of the low frequency acoustic energy (0–1.5 kHz) as a measure of cumulative chorusing intensity. In addition, between December and April we conducted a total of 26 vessel-based line-transect surveys during the 2018/19 through 2020/21 seasons and weekly visual surveys (n= 74) from a land-based station between 2016 and 2020, in which the location of sighted whale pods was determined with a theodolite. Combining the visual and acoustic data, we found a strong positive second-order polynomial correlation between SPLs and abundance (land: 0.72 ≤ R2≤ 0.75, vessel: 0.81 ≤ R2≤ 0.85 for three different PAM locations; Generalized Linear Model:pland≪ 0.001,pvessel≪ 0.001) that was independent from recording location (pland= 0.23,pvessel= 0.9880). Our findings demonstrate that PAM is a relatively low-cost, robust complement and alternative for studying and monitoring humpback whales in their breeding grounds that is able to capture small-scale fluctuations during the season and can inform managers about population trends in a timely manner. It also has the potential to be adapted for use in other regions that have previously presented challenges due to their remoteness or other limitations for conducting traditional surveys.

Surface roughness study of polyamide in nano-metric polishing using low-frequency acoustic energy

Polymers have gained the attention of manufacturers due to their significant advantages such as low density, high corrosion resistance, and high humidity resistance. Producing high-precision polymeric components is one the most challenging issues especially in fabricating complex or micro-scale systems. Some of the machining techniques such as electro discharge machining (EDM) and electrochemical machining (ECM) cannot be employed for machining the non-conductive parts. Using abrasive particles is one of the best options for machining these types of materials. In this work, the capability of the acoustic energy for machining polyamide (PA) workpieces is studied. To this end, an experimental setup is installed and design of experiment (DoE) algorithm is employed to survey the effect of process parameters on surface roughness. Three parameters at three levels are considered as the effective factors of the process and the sensitivity of the surface roughness on the process factors is investigated. In the next step, a hybrid finite element/boundary element approach was used to discuss the relation of process parameters to the vibrational characteristics of the container, then the mechanism of the process was investigated employing the discrete element method. Finally, the surface topology of the optimal workpiece before and after the process was presented and compared. It was observed that acoustic energy can be considered as a vibration source of the container’s floor to provide kinetic energy for machining PA parts on the nano-metric scale. Moreover, it was found that the initial roughness of the workpiece and the chosen parameters play a crucial role in the machining process. Experimental results show that in this technique by selecting appropriate process factors the surface roughness can be reduced up to 50%.

High-Temperature and Low-Frequency Acoustic Energy Absorption by a Novel Porous Metamaterial Structure

In this paper, we propose a novel porous metamaterial structure with an improved acoustic energy absorption performance at high-temperature and in the low-frequency range. In the proposed novel porous metamaterial structure, a porous material matrix containing periodically perforated cylindrical holes arranged in a triangular lattice pattern is applied, and additional interlayers of another porous material are introduced around these perforations. The theoretical model is established by adopting the double porosity theory for the interlayer and the cylindrical hole which form an equivalent inclusion and then applying the homogenization method to the porous metamaterial structure formed by the equivalent inclusion and the porous matrix. The temperature-dependent air and material parameters are considered in the extended theoretical model, which is validated by the finite element results obtained by COMSOL Multiphysics. The acoustic or sound energy absorption performance can be improved remarkably at very low frequencies and high temperature. Furthermore, the underlying acoustic energy absorption mechanism inside the unit-cell is investigated by analyzing the distribution of the time-averaged acoustic power dissipation density and the energy dissipation ratio of each constituent porous material. The results reveal that regardless of the temperature, the acoustic energy is mostly dissipated in the porous material with a lower airflow resistivity, while the acoustic energy dissipated in the porous material with a higher airflow resistivity also becomes considerable in the high-frequency range. The novel porous metamaterial structure proposed in this paper can be efficiently utilized to improve the acoustic energy absorption performance at high temperature.

A 3D-printed acoustic triboelectric nanogenerator for quarter-wavelength acoustic energy harvesting and self-powered edge sensing

Abstract Low-frequency acoustic energy harvesting is of great significance in the academic field and industry. In this work, we propose a 3D-printed acoustic triboelectric nanogenerator (A-TENG) with the properties of structural controllability, one-time molding, easy fabrication, and low cost. A quarter-wavelength acoustic resonator system based on the A-TENG is demonstrated and systemically investigated for high-performance acoustic energy scavenging. The system is capable of generating a power output of 4.33 mW under 100 dB sound pressure level excitation. Up to 72 LEDs and a commercial calculator can be directly and continuously driven by the acoustic resonator system, indicating its application as a power source for electronic devices. Furthermore, we develop a self-powered edge sensing system consisting of the A-TENG, an AI speech recognition chip, and control circuits. The speech signals can be firstly converted into electrical signals through the A-TENG, and then recognized and processed by the AI chip with a built-in and pre-trained neural network to control the follow-up circuits. The self-powered edge sensing system is capable of real-time speech recognition without cloud computing, exhibiting great potential in the field of low-power and cost-effective intelligent Internet of Things.

Fluctuations in Hawaii’s humpback whale Megaptera novaeangliae population inferred from male song chorusing off Maui

Approximately half of the North Pacific humpback whaleMegaptera novaeangliaestock visits the shallow waters of the main Hawaiian Islands seasonally. Within this breeding area, mature males produce an elaborate acoustic display known as song, which becomes the dominant source of ambient underwater sound between December and April. Following reports of unusually low whale numbers that began in 2015/16, we examined song chorusing recorded through long-term passive acoustic monitoring at 6 sites off Maui as a proxy for relative whale abundance between 2014 and 2019. Daily root-mean-square sound pressure levels (RMS SPLs) were calculated to compare variations in low-frequency acoustic energy (0-1.5 kHz). After 2014/15, the overall RMS SPLs decreased between 5.6 and 9.7 dB re 1 µPa2during the peak of whale season (February and March), reducing ambient acoustic energy from chorusing by over 50%. This change in song levels co-occurred with a broad-scale oceanic heat wave in the northeast Pacific termed the ‘Blob,’ a major El Niño event in the North Pacific, and a warming period in the Pacific Decadal Oscillation cycle. Although it remains unclear whether our observations reflect a decrease in population size, a change in migration patterns, a shift in distribution to other areas, a change in the behavior of males, or some combination of these, our results indicate that continued monitoring and further studies of humpback whales throughout the North Pacific are warranted to better understand the fluctuations occurring in this recently recovered population and other populations that continue to be endangered or threatened.

A Novel Hybrid Piezoelectric-Electromagnetic Micro-Power Generator by Harvesting Low Frequency Acoustic Energy

In order to harvest low frequency acoustic energy, a novel hybrid piezoelectric-electromagnetic micro-power generator was developed with Helmholtz resonator, piezoelectric patch, coil and permanent magnet. When the sound waves enter Helmholtz resonator, piezoelectric patch and coil will be vibrated then the electric energy are generated from them. With Multisim software, the circuit of proposed micro-power generator was simulated and its output performance was analysed. Results show that there is always an optimal resistance to maximize the output power of the novel hybrid piezoelectric-electromagnetic micro-power generator. Especially when the sound waves at 82Hz, the output power of proposed hybrid generator can more than the sum of independent piezoelectric and electromagnetic micro-generator. It indicates the power generation efficiency is significant significantly improved by the proposed hybrid method.