Omnidirectional Response Research Articles

All-optical platform for ultrasound transmission matrix measurements

Piezoelectric ultrasound transducers are constrained by size, bandwidth, and angular response, limiting their ability to fully characterize the acoustic properties of objects. In this study, we introduce a novel modular all-optical platform for ultrasound generation and detection to overcome these limitations, demonstrating wideband operation (>50 MHz), omnidirectional response, and high signal fidelity. Ultrasound generation is performed via the optoacoustic effect by illuminating an optically absorbing coating with spatially modulated pulsed light, and ultrasound detection is carried out using a silicon-photonic acoustic detector. By illuminating patterns that span a basis and scanning the detector, the full transmission matrix is measured, consisting of the acoustic waveforms for all the transmitter–receiver pairs in the measurement geometry. Our method is experimentally demonstrated in transmission mode for beam steering, beam focusing, and imaging, achieving excellent agreement with the theory.

Intelligent bending photothermal converter based on light-driven PDMS bimorph soft actuator

The angle of the light receiving panel in traditional optical equipment is fixed. When the light does not shine vertically onto the panel, there will be a problem of energy density loss due to oblique incidence. In this article, we prepared an intelligent responsive light-driven bimorph soft actuator using the thermal expansion properties of polydimethylsiloxane. Intelligent bending photothermal converter can be achieved by placing a photothermal converter at the top of the device. The device adopts a hollow cylindrical structure to achieve omni-directional response to light, and has the advantages of fast response speed, high tracking accuracy, and strong repeatability. When light is irradiated from a 40 ° direction, the device can be bent in the direction of illumination, aligning the top photothermal converter with the direction of illumination, proving that the light-driven soft actuator we have prepared can effectively recover energy density loss of oblique incidence.

Evaluating the directivity of compact underwater acoustic recording devices

Commercially available underwater acoustic recorders have become commonplace tools in ocean-acoustics research, due to their ease of deployment, compact size, and relatively low cost. The size of these systems typically results in a configuration with the hydrophone in close proximity to the electronics housing, flotation devices, and other equipment that can degrade the generally assumed omnidirectional response of the hydrophone. The mid-frequency acoustic regime (0.5–10 kHz) is particularly affected due to the similarity between the length-scales of these structures and the corresponding acoustic wavelengths. Calibration measurements made at an open-water test facility with a calibrated source/receiver pair characterized the frequency-dependent receiver directivity for three underwater recording devices with different housings and hydrophones: the PVC air-filled Loggerhead Snap, PVC oil-filled SoundTrap ST300, and titanium air-filled SoundTrap ST600. Furthermore, directivity measurements of a TOSSIT mooring [Zitterbart et al., HardwareX (2022)] were taken with the SoundTrap ST300 and SoundTrap ST600. Results suggest the frequency-dependent acoustic directivity of the recorders should not be neglected. In particular, the Loggerhead Snap had variations in receive level of over 20 dB as a function of receiver orientation angle and frequency, introducing a bias that obscures spectral levels of the in situ environment.

Engineering Helmholtz resonators for directional low-frequency sound attenuation

Controlling the absorption and diffusion of sound in the audible range constitutes an exciting field of research. Acoustic absorbers and diffusers perform extraordinarily well at high frequencies with sizes comparable to the wavelength of the working frequency. On the other hand, efficient low-frequency attenuators demand large volumes leading to unpractical sizes and high manufacturing costs. However, can the size of the resonator be reduced while also decreasing the working frequency? The answer is, counterintuitively, yes. This work investigates this phenomenon by studying a series of 3-D-printed monoatomic metamaterials based on membrane-coupled Helmholtz resonators. The results reveal that these systems, apart from significantly decreasing the bandgap frequency by around 36%, produce a cardioid directional response that contrasts with the omnidirectional response from traditional Helmholtz resonators. The obtained results suggest that by following this approach, low-frequency attenuation can be achieved via miniaturised devices while additionally providing them with a sense of directionality. This combination of features makes these sensors the perfect candidates for the next generation of acoustic hearing devices and acoustic attenuators, since the freedom that 3-D-printing provides allows for fine control of wave manipulation.

A Quantitative Evaluation of the Performance of the Low-Cost AudioMoth Acoustic Recording Unit.

The AudioMoth is a popular autonomous recording unit (ARU) that is widely used to record vocalizing species in the field. Despite its growing use, there have been few quantitative tests on the performance of this recorder. Such information is needed to design effective field surveys and to appropriately analyze recordings made by this device. Here, we report the results of two tests designed to evaluate the performance characteristics of the AudioMoth recorder. First, we performed indoor and outdoor pink noise playback experiments to evaluate how different device settings, orientations, mounting conditions, and housing options affect frequency response patterns. We found little variation in acoustic performance between devices and relatively little effect of placing recorders in a plastic bag for weather protection. The AudioMoth has a mostly flat on-axis response with a boost above 3 kHz, with a generally omnidirectional response that suffers from attenuation behind the recorder, an effect that is accentuated when it is mounted on a tree. Second, we performed battery life tests under a variety of recording frequencies, gain settings, environmental temperatures, and battery types. We found that standard alkaline batteries last for an average of 189 h at room temperature using a 32 kHz sample rate, and that lithium batteries can last for twice as long at freezing temperatures compared to alkaline batteries. This information will aid researchers in both collecting and analyzing recordings generated by the AudioMoth recorder.

Ultrasound sensing at thermomechanical limits with optomechanical buckled-dome microcavities.

We describe the use of monolithic, buckled-dome cavities as ultrasound sensors. Patterned delamination within a compressively stressed thin film stack produces high-finesse plano-concave optical resonators with sealed and empty cavity regions. The buckled mirror also functions as a flexible membrane, highly responsive to changes in external pressure. Owing to their efficient opto-acousto-mechanical coupling, thermal-displacement-noise limited sensitivity is achieved at low optical interrogation powers and for modest optical (Q ∼ 103) and mechanical (Q ∼ 102) quality factors. We predict and verify broadband (up to ∼ 5 MHz), air-coupled ultrasound detection with noise-equivalent pressure (NEP) as low as ∼ 30-100 µPa/Hz1/2. This corresponds to an ultrasonic force sensitivity ∼ 2 × 10-13 N/Hz1/2 and enables the detection of MHz-range signals propagated over distances as large as ∼ 20 cm in air. In water, thermal-noise-limited sensitivity is demonstrated over a wide frequency range (up to ∼ 30 MHz), with NEP as low as ∼ 100-800 µPa/Hz1/2. These cavities exhibit a nearly omnidirectional response, while being ∼ 3-4 orders of magnitude more sensitive than piezoelectric devices of similar size. Easily realized as large arrays and naturally suited to direct coupling by free-space beams or optical fibers, they offer significant practical advantages over competing optical devices, and thus could be of interest for several emerging applications in medical and industrial ultrasound imaging.

Analysis and design of 2-dimensional acoustic particle velocity horns

A two-dimensional acoustic horn packaged with a 2-D acoustic particle velocity sensor (APVS) is presented. The particle velocity amplification factor of the 2-D acoustic horn is analyzed theoretically and experimentally. The experimental results show a good agreement with the theoretical expectations. Compared with the previous horn model, this model considers the gain attenuation at low frequency and the influence of chip assembly deviation. As the particle velocity distribution changes greatly in the boundary layer, the assembly deviation of the chip has a great influence on the package gain. Furthermore, a microphone and the 2-D particle velocity sensor was packaged in the acoustic horn to detect the acoustic pressure and 2-D particle velocity at one point. The acoustic horn has an omnidirectional response for acoustic pressure. By using the mode domain beamforming, the direction of the incident wave was estimated with this two-dimensional sound intensity probe.

A Personalized Acoustic Interface for Wearable Human–Machine Interaction

AbstractCommunication and interaction with machines are changing our ways of life. However, developing an acoustic interface that simultaneously features waterproofness, wearability, high fidelity, and high accuracy for human–machine interaction remains a grand challenge. Herein, a waterproof acoustic sensor (WAS) as a wearable translation interface to communicate with machines is reported. Owing to the sound‐response ability of internal microparticles, the WAS holds a significantly broad frequency response range of 0.1–20 kHz, covering almost the entire human audible range. The WAS is stable against human perspiration, shows omnidirectional response, and displays an excellent frequency detection resolution of 0.0001 kHz. With a collection of compelling features, the WAS can serve as a wearable acoustic human–machine interface and a high‐fidelity auditory platform for music recording. Moreover, the WAS‐based acoustic interface holds a remarkable 98% accuracy for speech recognition with the assistance of an artificial intelligence algorithm. Finally, the WAS‐based acoustic interface demonstrates speaker verification and identification for implementation in highly secure biometric authentication systems and wireless control of an intelligent car using speech recognition. Such a WAS‐based acoustic interface represents the advancement of high‐fidelity translation platforms for human–machine interactions toward practical applications, including the Internet of Things, assistive technology, and intelligent recognition systems.

Perceptual roughness of spatially assigned sparse noise for rendering reverberation.

Multichannel auralizations based on spatial room impulse responses often employ sample-wise assignment of an omnidirectional response to form loudspeaker responses. This leads to sparse impulse responses in each reproduction loudspeaker and the auralization of transient signals can sound rough. Based on this observation, we conducted a listening test to examine the general phenomenon of roughness due to spatial assignment. First, participants assessed the roughness of both Gaussian noise and velvet noise, assigned sample-wise to up to 36 loudspeakers by two algorithms. The first algorithm assigns channels merely by selecting random indices, while the second one constrains the time between two peaks on each channel. The results show that roughness already occurs when few channels are used and that the assignment algorithm influences it. In a second experiment, virtualizations of the test were used to examine the factors contributing to increased roughness. We systematically show the effect of spatial assignment on noise and conclude that besides time-differences, level-differences caused by head-shadowing are the principal cause for the perceived roughness. The results have significance in spatial room impulse response rendering and spatial reverberator design.

An optical beam deflection based MEMS biomimetic microphone for wide-range sound source localization

Miniature microphones inspired by the ears of the parasitic fly Ormia Ochracea are an important means to overcome the fundamental size constraint of microphone arrays for sound source localization (SSL). However, the signal demodulation performance and the SSL range of the biomimetic microphones need to be improved. This study aims to develop a miniature biomimetic microphone with good signal demodulation capability and wide SSL range. Specifically, in this paper, an optical beam deflection (OBD) demodulation system was designed and combined with a micro-electro-mechanical system omimetic diaphragm to form a miniature microphone for SSL. The OBD-based biomimetic microphone shows many merits in signal demodulation, including low noise floor (⩽−110 dB), high repeatability of sound pressure measurement, and linear temperature response. The theoretical and experimental results demonstrated that the two wings of the biomimetic microphone have the bi-directional polar patterns with different sizes of lobes at the rocking-mode eigenfrequency, and have omnidirectional response at the bending-mode eigenfrequency. Furthermore, a new SSL method was proposed, which effectively extends the SSL range of the biomimetic microphone at the rocking-mode eigenfrequency to ±90° by combining the interaural intensity difference between the two wings. The proposed OBD-based biomimetic microphone not only has the advantages of small volume and low power consumption, but also has the stable and wide-range SSL capability, which shows that it has a great prospect in the practical applications of SSL, such as wearable devices and rescue robots, etc.

Beam‐sweeping antenna with beamwidth reconfigurable response

In this work, a simple and compact reconfigurable antenna, whose radiation direction and beamwidth can be manipulated, is proposed based on active frequency-selective surfaces (AFSSs). This design is completed by using a monopole antenna and six reflective AFSS screens, while its reconfigurable behavior is obtained by employing semiconductor switches. By applying different biasing voltages on the diode switches, the operating state of the AFSS screens can be configurated; therefore, the radiation direction and beamwidth of the monopole antenna can be controlled. From 1.8 to 2 GHz, the omnidirectional scanning response can be achieved at xoy-plane with the step of 60° and the maximum radiant efficiency is 96%. Furthermore, the 3 dB beamwidth can also be adjusted from 176° to 120° with the measured gains of 4.58–6.53 dB. The diameter of this structure is only 50 mm, which is a relatively small structural size for an omnidirectional beam-sweeping antenna. Finally, the proposed design is validated by numerical simulations and experimental measurements.

Spherical-Omnidirectional Piezoelectric Composite Transducer for High- Frequency Underwater Acoustics.

Spherical-omnidirectional acoustic source has become a powerful tool to provide a near-ideal omnidirectional beam pattern for acoustic tests and communications. Current spherical-omnidirectional acoustic sources do not combine an omnidirectional beam pattern with a high transmitting voltage response (TVR) in the frequency range of above 200 kHz. This work presents the design, fabrication, and measurements of a high-frequency spherical-omnidirectional transducer that can provide a near-ideal omnidirectional beam pattern and a high TVR. The active element of the transducer consists of six identical square coupons with spherical curvature 1-3 piezoelectric composites operating in thickness mode. Electroacoustic responses of the fabricated transducer in water were measured. The measured resonance frequency of the transducer was 280 kHz. The maximum TVR was 161.3 dB re 1 μ Pa/V@1 m. The horizontal and vertical beamwidths of the transducer were 360° and 346°, respectively. Measurements show that the spherical piezoelectric composite transducer has a favorable spherical-omnidirectional behavior and a high TVR at high frequency. These results demonstrate that the spherical piezoelectric composite transducer is potentially a strong candidate for a high-frequency underwater acoustic source that requires an omnidirectional response.

Occupant pre-crash kinematics in rotated seat arrangements

The introduction of automated L5 driving technologies will revolutionise the design of vehicle interiors and seating configurations, improving occupant comfort and experience. It is foreseen that pre-crash emergency braking and swerving manoeuvres will affect occupant posture, which could lead to an interaction with a deploying airbag. This research addresses the urgent safety need of defining the occupant’s kinematics envelope during that pre-crash phase, considering rotated seat arrangements and different seatbelt configurations. The research used two different sets of volunteer tests experiencing L5 vehicle manoeuvres, based in the first instance on 22 50th percentile fit males wearing a lap-belt (OM4IS), while the other dataset is based on 87 volunteers with a BMI range of 19 to 67 kg/m2 wearing a 3-point belt (UMTRI). Unique biomechanics kinematics corridors were then defined, as a function of belt configuration and vehicle manoeuvre, to calibrate an Active Human Model (AHM) using a multi-objective optimisation coupled with a Correlation and Analysis (CORA) rating. The research improved the AHM omnidirectional kinematics response over current state of the art in a generic lap-belted environment. The AHM was then tested in a rotated seating arrangement under extreme braking, highlighting that maximum lateral and frontal motions are comparable, independent of the belt system, while the asymmetry of the 3-point belt increased the occupant’s motion towards the seatbelt buckle. It was observed that the frontal occupant kinematics decrease by 200 mm compared to a lap-belted configuration. This improved omnidirectional AHM is the first step towards designing safer future L5 vehicle interiors.

Fiber-Optic Hot-Wire Anemometer With Directional Response Based on Symmetry-Breaking Induced Heat Transfer Mechanism

Optical fiber hot-wire anemometer is an emerging technique for wind speed measurement. In many practical applications, both the magnitude and the direction of the flow need to be obtained. However, most of previously reported fiber optic anemometers show omnidirectional response, rendering difficulty in recognizing the flow direction. Here, we propose a simple and effective fiber-optic flow sensor with directional response based on an asymmetric coating of single-wall carbon nanotubes (SWCNTs) on the surface of tilted fiber Bragg grating (TFBG), paving the way for the vector analysis of wind flow field. FLUENT software was employed to analyze the wind speed field, temperature distribution field, and vortex street. The temperature field shows asymmetrical distribution directly related to the wind directions, causing directional response of the sensor to the wind flow. In the experiments, with optimized 12˚ TFBG with 1.3 μm thick asymmetric SWCNTs coating, three wind directions at different angle (0˚, 180˚, 270˚) were clearly distinguished. Moreover, high sensitivity and efficiency were also achieved by investigating and optimizing the system parameters like pumping power, coating thickness. In addition to directional response, the proposed sensor exhibits many advantages such as simple structure, low power consumption, small size, and high sensitivity, which has good potential application for infield static wind field and slowly varying wind (SVW) measurement.

Quasiordered, subwavelength TiO2 hole arrays with tunable, omnidirectional color response

Fabrication, optical characterization, and simulation of subwavelength TiO2 hole arrays exhibiting geometry-tunable, omnidirectional color response across the visible spectrum is described. Partially suspended TiO2 grating “membranes” (hole arrays supported by a high void-fraction, low-index underlayer) with quasiperiodic hexagonal order were created on an Si substrate using colloidal lithography, metal mask, plasma-based pattern transfer, and XeF2 etching. Optical measurements under specular and diffuse reflection conditions, along with finite-difference time-domain simulations, indicated that the omnidirectional color response of the hole arrays emerges from a broad distribution of Fano resonance states formed by coupling between guided and internal Fabry–Pérot (FP) cavity modes of the TiO2 layer. Higher-order FP resonances from the external cavity formed between the TiO2 layer and substrate control the apparent color when devices are viewed in direct light. The simulated modal behavior of arrays was found to be very sensitive to the degree of Si removal from the underlayer, in agreement with experimental observations. The fabrication methodology presented herein is substrate-agnostic and can be employed to fabricate suspended, subwavelength hole arrays in many material systems, with potential application to optical filters and reflectors, photocatalytic electrodes, photovoltaics, and sensors.

Polarization-insensitive, omnidirectional fiber-optic ultrasonic sensor with quadrature demodulation.

We report an ultrasonic sensor system based on a low-finesse Fabry-Perot interferometer (FPI) formed by two weak chirped fiber Bragg gratings (CFBGs) on a coiled single-mode fiber. The sensor system has several desirable features for practical applications in detecting ultrasound on a solid surface. By controlling the birefringence of the fiber coil during the sensor fabrication, the sensor is made insensitive to the polarization variations of the laser source. The circular symmetric structure of the fiber coil also renders the omnidirectional response of the sensor to ultrasound. While the fiber coil is bonded directly to the structure, the CFBGs are suspended from the structure and free from large background strains with little reduction to the sensitivity of the sensor. The low-finesse FPI features a sinusoidal reflection spectrum. Like the conventional phase-generated carried technique, a phase modulator is utilized to implement quadrature demodulation. Therefore, the sensing system is adaptive to large background perturbations experienced by the fiber coil.

A wide band porous silicon omnidirectional mirror for the near infrared range

We report the design, fabrication, and characterization of a porous silicon-based omnidirectional mirror for the near infrared range. The structure consists of 300 porous silicon chirped dielectric layers, optimized to have omnidirectional reflectivity response from 1000 to 2000 nm wavelength range. Measurements of reflectivity spectra are presented for non-polarized light at several incident angles (range 8°–65°) with a reflectivity >95% covering a 1μm band-width. Transfer matrix method calculations were carried out to show the complete angular range for both TM and TE polarizations including a simple model to illustrate the interface scattering effects.

Design, fabrication and characterization of piezoelectric cantilever MEMS for underwater application

This work shows a preliminary microfabrication route for a novel directional hydrophone based on a cross-shaped design of piezoelectric cantilevers. A thin layer of aluminum nitride (AlN) using Molybdenum (Mo) thin film as electrodes will be exploited as piezoelectric functional layer for the microfabrication of a cantilever-based ultrasonic micro electro mechanical system (MEMS) hydrophone. A parameterized simulation based on length of these cantilevers between 100 and 1000 μm allowed to set the first resonant mode between 20 kHz and 200 kHz, the desired underwater ultrasonic acoustic range. The microsystem was designed with cantilevers facing each other in a cross configuration in order to have novel MEMS hydrophone with an omnidirectional response. In order to investigate the first resonance frequency mode and displacement measurements, a Laser Doppler Vibrometer was used and good agreement between simulations and experimental results was achieved. Responsivity and directionality measurements of the piezoelectric MEMS cantilevers were performed in water. Maximum sensitivity up to −153 dB with omnidirectional directivity pattern was achieved by fabricated MEMS sensor. • We show the microfabrication of a novel directional hydrophone based on a cross-shaped design of piezoelectric cantilevers. • Cantilevers facing each other in a cross configuration allow to have an omnidirectional response to underwater ultrasounds. • Aluminum nitride (AlN) using Molybdenum (Mo) thin film as electrodes is exploited as piezoelectric functional layer. • Maximum sensitivity underwater up to -153 dB with omnidirectional directivity pattern was achieved.

Cooperative localization for multiple AUVs based on the rough estimation of the measurements

The accuracy of cooperative localization for multiple autonomous underwater vehicles (AUVs) equipped with low precise proprioceptive localization sensors can be improved by using relative location information between individuals and Bayesian filtering. However, when the relative location measurement errors are high, its accuracy will be reduced. Two measurement for rough estimation algorithms under the constraint environment of the cooperative structure are developed in this paper: the first algorithm is based on the underwater acoustic isotropic transmission. And the second algorithm is based on the common observation environment. In the first algorithm, it builds under the assumption that the distance errors calculated from the simultaneous omnidirectional response signals from the same transmitting source have correlated. Similarly, in the second algorithm, the assumption that “common observation environment” is correlated is made. First, the correlation between the errors is used roughly to estimate the measurement of information. Then, a suitable filter is applied to fuse the rough estimation measurement with dead-reckoning estimation that improves the location estimation accuracy. The final simulation, by changing the AUV formation navigation paths and the sensor observation noises, shows the proposed processing methods have effectiveness and consistency compared to the traditional algorithm.

Barrier Lyapunov function and disturbance observer based omnidirectional robust gust response stabilization for multi-control-effectors aircraft

Purpose The purpose of this paper is to find an omnidirectional robust gust response stabilization (GRS) scheme with anti-disturbance and state-limited features. Design/methodology/approach Disturbance observer and barrier Lyapunov techniques, which can, respectively, estimate the lumped disturbances of the dynamic system in real-time and ensure the middle states within some prescribed ranges according to some flight safety indexes. Findings In the existing literature, almost all of the GRS controllers are either only for the longitudinal dynamics or only for the latitudinal dynamics. Few studies have considered the gust response alleviation problem with omnidirectional wind disturbance and full aircraft model. Originality/value This paper proposes a fresh scheme to deal with a more holistic GRS problem; the disturbance observer based (DOB) barrier Lyapunov backstepping longitudinal controller has been put forward; DOB nonlinear dynamic inversion to handle the multi-input-multi-output lateral dynamics; and to closely connect the two loops of the latitudinal dynamics, a manipulating variable conversion method is proposed.