Search Results (23)

EMFIT, also referred to as EMFi, is a ferroelectret film related to polyvinylidene fluoride (PVDF) sensors. It is an electroactive polymer (EAP) based on a polyolefin structure and consists of three layers of polyester film. Its application in geophysical monitoring has not been reported in the literature. At present, EMFIT is mainly employed in ballistocardiography and medical sleep monitoring, as developed by the manufacturer Emfit Ltd. (Vaajakoski, Finland). Within the multidisciplinary monitoring network of the National Institute for Earth Physics (NIEP), EMFIT is used as a pressure sensor in combination with infrasound transducers and microphones deployed in seismic areas. The primary aim of this study is to evaluate its suitability for detecting seismic noise that precedes earthquakes, generated by rock fracturing associated with crustal deformation. Although similar studies have been reported, they have not involved the use of EMFIT sensors. The novelty of this approach lies in the large surface area and mechanical flexibility of the material. Beyond seismic forecasting, the research also examines whether this type of sensor can contribute to seismic monitoring as a complement to conventional instruments such as accelerometers, seismometers, and microbarometers. Data analysis relies primarily on spectral time-series methods and incorporates measurements from other acoustic sensors (microphones and microbarometers) as well as a weather station. The working hypothesis is the potential correlation between the recorded data and the presence of enhanced noise prior to the detection of seismic waves by standard seismic sensors. The target area for this investigation is Vrancea, specifically the Vrâncioaia seismic station, where multidisciplinary monitoring includes infrasound, radon, thoron, soil temperature, and atmospheric electrical discharges. Preliminary tests suggest that the EMFIT sensor may function as a highly sensitive device, effectively serving as an “ear” for detecting ground noise. Full article

In the context of autonomous environmental monitoring, this study investigates a surface acoustic wave (SAW) sensor designed for selective carbon dioxide (CO₂) detection. The sensor is based on a LiTaO₃ piezoelectric substrate with copper interdigital transducers and a polyetherimide (PEI) layer, chosen for its high electromechanical coupling and strong CO₂ affinity. Finite element simulations were conducted to analyze the resonance frequency response under varying gas concentrations, film thicknesses, pressures, and temperatures. Results demonstrate a linear and sensitive frequency shift, with detection capability starting from 10 ppm. The sensor’s autonomy is ensured by a piezoelectric energy harvester composed of a cantilever beam structure with an attached seismic mass, where mechanical vibrations induce stress in a piezoelectric layer (PZT-5H or PVDF), generating electrical energy via the direct piezoelectric effect. Analytical and numerical analyses were performed to evaluate the influence of excitation frequency, material properties, and optimal load on power output. This integrated configuration offers a compact and energy-independent solution for real-time CO₂ monitoring in low-power or inaccessible environments. Full article

An innovative design of a hydrophone based on a piezoelectric composite film of $AlN/S{c}_{0.2}A{l}_{0.8}N$ is presented. By designing a non-uniform composite sensitive layer, the dielectric loss and defect density are significantly reduced, while the high-voltage electrical characteristics of scandium-doped aluminum nitride are retained. X-ray diffraction analysis shows that the sensitive films have excellent crystal quality (FWHM is 0.34°). According to the standard underwater acoustic calibration test, the device exhibits full directivity with a minimum deviation of ±0.5 dB at 1 kHz frequency, sound pressure sensitivity of −162.9 dB (re: 1 V/$\mathsf{\mu }$Pa) and equivalent noise density of 46.1 dB (re: 1 $\mathsf{\mu }$Pa/√Hz). The experimental results show that the comprehensive performance of the piezoelectric heterostructure hydrophone meets the standard of commercial high-end hydrophones while maintaining mechanical stability, and provides a new solution for underwater acoustic sensing. Full article

MEMS acoustic sensors are a type of physical quantity sensor based on MEMS manufacturing technology for detecting sound waves. They utilize various sensitive structures such as thin films, cantilever beams, or cilia to collect acoustic energy, and use certain transduction principles to read out the generated strain, thereby obtaining the targeted acoustic signal’s information, such as its intensity, direction, and distribution. Due to their advantages in miniaturization, low power consumption, high precision, high consistency, high repeatability, high reliability, and ease of integration, MEMS acoustic sensors are widely applied in many areas, such as consumer electronics, industrial perception, military equipment, and health monitoring. Through different sensing mechanisms, they can be used to detect sound energy density, acoustic pressure distribution, and sound wave direction. This article focuses on piezoelectric, piezoresistive, capacitive, and optical MEMS acoustic sensors, showcasing their development in recent years, as well as innovations in their structure, process, and design methods. Then, this review compares the performance of devices with similar working principles. MEMS acoustic sensors have been increasingly widely applied in various fields, including traditional advantage areas such as microphones, stethoscopes, hydrophones, and ultrasound imaging, and cutting-edge fields such as biomedical wearable and implantable devices. Full article

Ultrasound is widely applied in diverse domains, such as medical imaging, non-destructive evaluation, and acoustic communication. Piezoelectric micromachined ultrasonic transducers (PMUTs) capable of generating and receiving ultrasonic signals at the micrometer level have become a prominent technology in the field of ultrasound. It is important to enrich the models of the PMUTs to meet the varied applications. In this study, a series of PMUT devices featured with various top electrode configurations, square, circular, and doughnut, were designed to assess the influence of shape on the emission efficacy. It was demonstrated that the PMUTs with a circular top electrode were outperformed, which was calculated from the external acoustic pressure produced by the PMUTs operating in the fundamental resonant mode at a specified distance. Furthermore, the superior performance of PMUT arrays were exhibited through computational simulations for the circular top electrode geometries. Conventional microelectromechanical systems (MEMS) techniques were used to fabricate an array of PMUTs based on aluminum nitride (AlN) films. These findings make great contributions for enhancing the signal transmission sensitivity and bandwidth of PMUTs, which have significant potential in non-destructive testing and medical imaging applications. Full article

In order to investigate the factors affecting the acoustic performance of the extrinsic Fabry–Perot interferometer (EFPI) fiber-optic acoustic pressure sensor and to effectively improve its detection capability, this paper enhances the sensor’s detection sensitivity by adding more sensitized rings to its acoustic pressure-sensitive film. Furthermore, a novel real-time coupled acoustic test method is proposed to simultaneously monitor the changes in the spectral and acoustic metrics of the sensor to characterize its overall performance. Finally, an EFPI-type fiber-optic acoustic pressure sensor was developed based on the Micro-Optical Electro-Mechanical System (MOEMS). The acoustic tests indicate that the optimized fiber-optic acoustic pressure sensor has a sensitivity as high as 2253.2 mV/Pa, and the acoustic overload point (AOP) and signal-to-noise ratios (SNRs) can reach 108.85 dB SPL and 79.22 dB, respectively. These results show that the sensor produced through performance characterization experiments and subsequent optimization has a very high acoustic performance index, which provides a scientific theoretical basis for improving the overall performance of the sensor and will have broad application prospects in the field of acoustic detection. Full article

In order to improve the response characteristics of the surface acoustic wave (SAW) sensor to trace gases, a SAW CO gas sensor based on a Pd–Pt/SnO₂/Al₂O₃ film with a high-frequency response performance is proposed in this paper. The gas sensitivity and humidity sensitivity of trace CO gas are tested and analyzed under normal temperatures and pressures. The research results show that, compared with the frequency response of the Pd–Pt/SnO₂ film, the CO gas sensor based on a Pd–Pt/SnO₂/Al₂O₃ film has a higher frequency response performance, and the sensor has high-frequency response characteristics to CO gas with a concentration in the range of 10–100 ppm. The average response recovery time of 90% ranges from 33.4 s to 37.2 s, respectively. When the CO gas with a concentration of 30 ppm is tested repeatedly, its frequency fluctuation is less than 5%, indicating that the sensor has good stability. In the range of relative humidity (RH) from 25% to 75%, it also has high-frequency response characteristics for CO gas with a 20 ppm concentration. Full article

Properties of the Langmuir-Blodgett (LB) films of arachidic and stearic acids, versus the amount of the films’ monolayers were studied and applied for chloroform vapor detection with acoustoelectric high-frequency SAW sensors, based on an AT quartz two-port Rayleigh type SAW resonator (414 MHz) and ST-X quartz SAW delay line (157.5 MHz). Using both devices, it was confirmed that the film with 17 monolayers of stearic acid deposited on the surface of the SAW delay line at a surface pressure of 30 mN/m in the solid phase has the best sensitivity towards chloroform vapors, compared with the same films with other numbers of monolayers. For the SAW resonator sensing using slightly longer arachidic acid molecules, the optimum performance was reached with 17 LB film layers due to a sharper decrease in the Q-factor with mass loading. To understand the background of the result, Atomic Force Microscopy (AFM) in intermittent contact mode was used to study the morphology of the films, depending on the number of monolayers. The presence of the advanced morphology of the film surface with a maximal average roughness (9.3 nm) and surface area (29.7 µm²) was found only for 17-monolayer film. The effects of the chloroform vapors on the amplitude and the phase of the acoustic signal for both SAW devices at 20 °C were measured and compared with those for toluene and ethanol vapors; the largest responses were detected for chloroform vapor. For the film with an optimal number of monolayers, the largest amplitude response was measured for the resonator-based device. Conversely, the largest change in the acoustic phase produced by chloroform adsorption was measured for delay-line configuration. Finally, it was established that the gas responses for both devices coated with the LB films are completely restored 60 s after chamber cleaning with dry air. Full article

In this study we report the specific interaction of various gases on the modified surface of acoustic wave devices for gas sensor applications, using the piezoelectric ceramic material BaSrTiO₃ (BST), with different concentrations of Sr. For enhancing the sensitivity of the sensor, the conductive polymer polyethylenimine (PEI) was deposited on top of BST thin films. Thin films of BST were deposited by pulsed laser deposition (PLD) technique and integrated into a test heterostructure with PEI thin films deposited by matrix assisted pulsed laser evaporation (MAPLE) and interdigital Au electrodes (IDT). Further on, the layered heterostructures were incorporated into surface acoustic wave (SAW) devices, in order to measure the frequency response to various gases (N₂, CO₂ and O₂). The frequency responses of the sensors based on thin films of the piezoelectric material deposited at different pressures were compared with layered structures of PEI/BST, in order to observe differences in the frequency shifts between sensors. The SAW tests performed at room temperature revealed different results based on deposition condition (pressure of oxygen and the percent of strontium in BatiO₃ structure). Frequency shift responses were obtained for all the tested sensors in the case of a concentration of Sr x = 0.75, for all the analysed gases. The best frequency shifts among all sensors studied was obtained in the case of BST50 polymer sensor for CO₂ detection. Full article

Pd/SnO₂ bilayers for surface acoustic wave (SAW) sensors were obtained using pulsed laser deposition (PLD). Bilayers were made at several deposition pressures in order to observe the influence of the morphology of the sensitive films on the response of the sensors. The morphological properties were analyzed by scanning electron microscopy (SEM). The SnO₂ monolayers were initially deposited on quartz substrates at 100, 400 and 700 mTorr, to observe their morphology at these pressures. The Pd/SnO₂ bilayer depositions were made at 100 and 700 mTorr. The sensors realized with these sensitive films were tested at different hydrogen concentrations, in the range of 0.2–2%, at room temperature. In order to establish selectivity, tests for hydrogen, nitrogen, oxygen and carbon dioxide were carried out with SnO₂-700, Pd-100/SnO₂-700 and Pd-700/SnO₂-700 sensors. The sensor with the most porous sensitive film (both films deposited at 700 mTorr) had the best results: a sensitivity of 0.21 Hz/ppm and a limit of detection (LOD) of 142 ppm. The morphology of the SnO₂ film is the one that has the major influence on the sensor results, to the detriment of the Pd morphology. The use of Pd as a catalyst for hydrogen improved the sensitivity of the film considerably and the selectivity of the sensors for hydrogen. Full article

A wind tunnel experiment is an important way and effective method to research the generation mechanism of aerodynamic noise and verify aerodynamic noise reduction technology. Acoustic measurement is an important part of wind tunnel experiments, and the microphone is the core device in an aerodynamic acoustic measurement system. Aiming at the problem of low sound pressure (several Pa) and the small measuring surface of an experimental model in a wind tunnel experiment, a microphone sensor head with high sensitivity and small volume, based on film bulk acoustic resonator (FBAR), is presented and optimized in this work. The FBARs used as a transducer are located at the edge of a diaphragm for sound pressure level detection. A multi-scale and multi-physical field coupling analysis model of the microphone is established. To improve the performance of the microphone, the structural design parameters of the FBAR and the diaphragm are optimized by simulation. The research results show that the microphone has a small size, good sensitivity, and linearity. The sensor head size is less than 1 mm × 1 mm, the sensitivity is about 400 Hz/Pa when the sensor worked at the first-order resonance frequency, and the linearity is better than 1%. Full article

Rayleigh surface acoustic wave (RSAW)-based resonant sensors, functionalized with single and multiple monomolecular layers of Langmuir–Blodgett (LB) films, were thickness and density optimized for the detection of volatile organic compounds (VOC), which could impose a serious threat on the environment and human health. Single layers of a phospholipid (SLP), hexane dissolved arachidic acid (HDAA), and chloroform dissolved arachidic acid (CDAA) were used for the LB film preparation. Several layers of these compounds were deposited on top of each other onto the active surface of high-Q 434 MHz two-port RSAW resonators in a LB trough to prepare a highly sensitive vapor detection quartz surface microbalance (QSM). Frequency shift was measured with a vector network analyzer (VNA). These devices were probed with saturated vapors of hexane, chloroform, methanol, acetone, ethanol, and water after each deposited layer to test the behavior of the QSM’s insertion loss, loaded Q, vapor sensitivity, and to find the optimum trade-off between these parameters for the best real-life sensor performance. With 2200 ppm and 3700 ppm sensitivity to chloroform, HDAA and CDAA coated QSM devices reached the optimum sensor performance at 15 and 11–15 monolayers, respectively. Surface pressure optimized single monolayers of phospholipid LB films were found to provide up to 530 ppm sensitivity to chloroform vapors with a negligible reduction in loss and loaded Q. This vapor sensitivity is higher than the mass of the sensing layer itself, making SLP films an excellent choice for QSM functionalization. Full article

We propose a new type of fiber hydrophone composed of an etched fiber Bragg grating and a special packaging structure for detecting acoustic waves in the low-frequency band under water. The operating mechanism is based on the mechanical vibration of the fiber Bragg grating from the induced vibrating stress of acoustic pressure. The induced pressure of acoustic waves pushes the silicone rubber thin film, causing its vibration and then stretching the fiber Bragg grating, thus resulting in the grating wavelength shift which is overlapped with a tunable laser. The variation in the overlapped light intensity is transferred to an electrical signal by using a photodetector. From the experimental results, we can determine that the smaller the fiber diameter, the higher the sensitivity and frequency response. In order to confirm that this FBG hydrophone has the ability to work in high-frequency acoustic waves, this fiber grating hydrophone and a standard piezoelectric hydrophone are experimentally compared to in the same test conditions in the frequency range from 4 to 10 kHz. According to the experimental results, the fiber grating hydrophone has better responsivity than that of the conventional hydrophone. Due to the unique sensing structure design, this wide-band fiber hydrophone can be useful in long-term continuous monitoring of acoustic waves. Full article

In our previous work, we have demonstrated that dielectric elastic grating can support Fabry–Perot modes and provide embedded optical interferometry to measure ultrasonic pressure. The Fabry–Perot modes inside the grating provide an enhancement in sensitivity and figure of merit compared to thin film-based Fabry–Perot structures. Here, in this paper, we propose a theoretical framework to explain that the elastic grating also supports dielectric waveguide grating mode, in which optical grating parameters control the excitation of the two modes. The optical properties of the two modes, including coupling conditions and loss mechanisms, are discussed. The proposed grating has the grating period in micron scale, which is shorter than the wavelength of the incident ultrasound leading to an ultrasonic scattering. The gap regions in the grating allow the elastic grating thickness to be compressed by the incident ultrasound and coupled to a surface acoustic wave mode. The thickness compression can be measured using an embedded interferometer through one of the optical guided modes. The dielectric waveguide grating is a narrow bandpass optical filter enabling an ultrasensitive mode to sense changes in optical displacement. This enhancement in mechanical and optical properties gives rise to a broader detectable pressure range and figure of merit in ultrasonic detection; the detectable pressure range and figure of merit can be enhanced by 2.7 times and 23 times, respectively, compared to conventional Fabry–Perot structures. Full article

This article presents an extrinsic fiber-optic acoustic sensor applied for partial discharge (PD) detection in air. A Fabry–Perot (F-P) cavity consisting of a single-mode fiber (SMF) and a graphene oxide (GO) film, whose thickness and effective vibration diameter are approximately 500 nm and 4.377 mm, respectively, is used as this sensing core, and the manufacturing process of GO diaphragms and this sensing probe is illustrated to be simple and controllable. Performance tests indicate that this proposed sensor maintains a linear acoustic-pressure response and a flat frequency response in the range of 200 Hz to 20 kHz, while being an omnidirectional sensor and having high working stability during a ten-day test period. Additionally, PD detection results show that the minimum PD size detected by this proposed sensor in air was approximately 100 pC, which demonstrates that this proposed sensor can achieve high-sensitivity PD detection in air. Full article