Anechoic Tank Research Articles

GA-BP-Based Low-Noise FBG Hydroacoustic Monitoring System with Reference Sensor.

To address the issue of harsh marine background noise impacting the monitoring signal of fiber-optic hydrophones, we propose a low-noise fiber Bragg grating (FBG) hydroacoustic monitoring system with a reference sensor based on genetic algorithm backpropagation (GA-BP). Through theoretical analysis, we deduce the noise suppression steps of the GA-BP algorithm based on the reference sensor and construct train and test sets based on the data from the reference sensor and monitoring sensor at different times, optimizing the GA-BP algorithm to find the best fitting results and thereby obtaining the low-noise monitoring signal. Experimental results from the anechoic tank show that the proposed method can suppress background noise interference on effective signals and that the suppression effect improves as the background noise increases. The sound pressure sensitivity ranges from -173.76 dB to -171.33 dB at frequencies of 8 kHz to 12 kHz, with a response flatness of less than 2.43 dB. The noise suppression effect is obvious under the condition of poor signal-to-noise ratio (SNR), which can reach more than 18.3 dB. The advantages of the proposed algorithm in array signal processing are further demonstrated by the directivity experiment, which proves that the algorithm has a great potential for engineering applications in harsh marine environment.

A primary wideband calibration method for noise hydrophones based on three-transducer spherical wave reciprocity in the frequency domain.

A primary wideband calibration method for noise hydrophones is developed, which can be applied to obtain noise acoustic spectral density sensitivity in the free-field. In this method, a wideband three-transducer spherical wave reciprocity calibration method is proposed in the frequency domain. A lowpass spatial domain filter is utilized to eliminate the reflecting acoustic waves from boundaries and water surface, and the wideband frequency responses of the transducer pair are calculated. Wideband calibrations are performed in a laboratory anechoic water tank, and two hydrophones with different reference sensitivities are calibrated in the frequency range of 250 Hz to 20 kHz using noise signals. The decidecade band sensitivities are obtained by the primary wideband calibration, which is in agreement with the single frequency sensitivities. The expended uncertainty of the primary wideband calibration is analyzed and estimated to be 8.9% (k = 2).

Measurement method of the transient radiated noise in the reverberant pool

Transient radiated noise is the primary source of exposure for underwater targets due to its short duration and distinct spectral line characteristics. In order to accurately measure the sound source level of transient radiated noise, a reverberation method of radiated noise measurement based on power averaging processing in the frequency band is proposed. The power of the transient signal is obtained by using time-domain signal autocorrelation technology. The sound source level of the transient radiated noise in the frequency range of 500 kHz to 10 kHz is determined by the reverberant characteristics of the pool. The sound source level of the transient radiated noise in the frequency range of 1 kHz - 10 kHz is compared and measured in the anechoic tank. The deviation of the measurement results is less than 1.5 dB, which confirms the accuracy of the reverberant measurement method.

Cylindrical single-wall and double-wall structures with or without internal water subjected to underwater shock loading

A series of experimental studies were conducted for cylindrical structures subjected to underwater shock loading to understand their dynamic responses and failure characteristics. All tests were performed inside an anechoic water tank. The submerged test cylinders were freely suspended, and an underwater shock loading was generated by the Compressed Air Shock Pipe Underwater Release (CASPUR) system. Cylinders were made of two different materials. The first group of cylinders was fabricated from carbon fiber and resin using the filament winding technique. The winding angles were ± 45° resulting in the same properties along axial and hoop directions. The second group of cylinders was constructed using a 3-D printer with polylactic acid (PLA) material. The 3-D printed cylinders had an orthotropic material property with different values in the axial and hoop directions. Both single-wall and double-wall cylindrical structures were tested. The latter consisted of two concentric cylinders of different diameters with uniform spacing between them. In addition, within the single-wall cylinders and the annuli of double-wall cylinders, the water fill was varied at 0%, 50%, or 100%. Pressure and strain gages were used to measure the shock pressure and deformation of the cylinders. The number of cylinders such as single-wall or double-wall and the internal water resulted in significant effects on the measured dynamic response (i.e., strain gage response) as well as the failure loading and failure characteristics including major failure locations. Internal water reduced the strain on the cylinders and made them withstand greater shock loading for both single-wall and double-wall cylinders.

Experimental and numerical investigation on vibro-acoustic performance of a submerged stiffened cylindrical shell under multiple excitations

The underwater vibro-acoustic characteristics of a stiffened cylindrical shell have been investigated using experimental and numerical methods. A numerical analysis model is developed to capture the behavior of the cylindrical shell structure, and the validity of the model is verified based on experimental data. Experimental measurements are conducted in an anechoic tank to assess the vibration and acoustic radiation characteristics of the stiffened cylindrical shell under white noise excitations. Additionally, the disparities in underwater vibration noise characteristics of the stiffened cylindrical shell under both single excitation and multiple excitations are explored by numerical method. The experimental and numerical findings reveal that, within the frequency range of 10–2000 Hz, the stiffened cylindrical shell exhibits a rapid augmentation of low-frequency vibration noise followed by a gradual stabilization trend. This behavior can be attributed to the high density of low-frequency modes embedded within the structure. Furthermore, it is noteworthy that the underwater vibration noise of the cylindrical shell under multiple excitations arises not solely as a result of individual excitations being superimposed, but rather as a consequence of the intricate coupling effects induced by multiple excitations.

Research on low-frequency bender disk transducer driven by multiple relaxor ferroelectric single crystal disks

In this paper, we investigate the low-frequency bender disk transducer designed by the relaxor ferroelectric single crystal. First, we introduce the principle of the bender disk transducer, and use multiple single crystal disks as driving materials to excite the bending vibration mode of the metal plate, which reduces the operating frequency of the transducer. Then we establish the three-dimensional finite element model of the bender disk transducer driven by multiple single crystal disks, analyze the influence of the number and position distribution of single crystal disks on the transducer performance, and compare with that of the piezoelectric ceramic transducer with the same structure. Finally, we manufacture the proposed relaxor ferroelectric single crystal, piezoelectric ceramic transducers and the conventional piezoelectric ceramic bender disk transducer respectively, and carry out the experimental measurement in an anechoic water tank. The experimental results show that for the proposed single crystal transducer, its measured maximum transmitting voltage response (TVR) is 126.2 dB at 1.1 kHz, which is 13.4 dB higher than that of the proposed piezoelectric ceramic transducer, and its frequency corresponding to the maximum TVR is reduced by 700 Hz compared with the conventional piezoelectric ceramic bender disk transducer.

Individual Fish Echo Detection Method Based on Peak Delay Estimation and Instantaneous Frequency Characterization

In fisheries science research and farmed fish monitoring, acquiring individual fish echoes is the basis for the growth assessment, tracking, and target strength estimation of farmed fish. However, individual fish echo detection methods based on narrowband signal features cannot be applied well to high-density aquaculture scenarios. The broadband signaling system can improve the distance resolution of the detected target and can help to improve the performance of individual fish echo detection. In this study, for the broadband signal system and the characteristics of the underwater fish acoustic echoes, an individual fish echo detection method is proposed using the matched filter output envelope peak interval and instantaneous frequency characteristics of the echo as evaluation indices, and the simulation and experiments of the method are carried out in an anechoic water tank. The results show that the broadband signal system and the corresponding detection method perform better in detecting single target echoes than the narrowband signal system. Compared with the broadband single echo detection method that only relies on the peak interval of the matched filter envelope, the joint detection method that incorporates the instantaneous frequency characteristics of the echo signal has a better rejection capability for overlapping echoes, which can reduce the probability of misjudging the overlapping echoes. The combined detection methods may provide a better detection performance for individual fish echoes.

Design and realization of directivity adjustable ring transducer

In this paper, a directional segmented ring transmitting transducer, which can radiate sound waves in any horizontal region, has been developed. First, we introduce the structure of the segmented ring transducer and the characteristics of its radiation sound field, investigate the transmitting beam pattern control method based on the modal synthesis, and propose orthogonal beam pattern functions to adjust steering angles. Then, the three-dimensional finite element model of the segmented ring transducer is established, and the amplitude and phase of sound pressure excited by the first three modes, respectively, are calculated as a function of the frequency. We can combine each mode to obtain the desired beam patterns, and simulate the transmitting beam patterns steering to different directions. Finally, we manufacture the segmented ring transducer and measure its transmitting directivity in an anechoic water tank, and the first three modes are also calibrated according to the measured sound pressure. The experimental results show that the beam pattern synthesized by the first two modes has a 3 dB down beam width of about 156◦ at 15 kHz after calibration, and the maximum source level is approximately 24 dB higher than the minimum. The −3 dB beam width synthesized by the first three modes is about 84◦ at 17 kHz after calibration. In addition, we also carry out the experimental test on the proposed beam control method, and the driving-voltage weighting vectors are designed to obtain good transmitting beam patterns steering to different angles.

Approach to active control of scattered sound field from underwater targets.

Active control of scattered sound fields is of great significance for the acoustic stealth of underwater targets. In this paper, we propose an approach to control the target scattered field based on the measurement of a single holographic surface. Compared to existing methods, our approach significantly reduces the required number of hydrophones and only relies on the incident direction as prior information. First, we introduce a sound field separation method that uses the measurement of a single holographic surface to extract scattered field near the scatterer. Then two control strategies are presented to reduce redundant sound power outside the selected direction radiated by secondary sources in different situations. Finally, the proposed method is verified by the simulation based on finite element method and the experiment conducted in an anechoic tank. Experimental results in the tank show that the scattered sound pressure level in far-field is reduced by at least 10 dB at 2 kHz after activating the real-time control system.

Acoustic Target Strength of Jellyfish, Nemopilema nomurai, Measured at Multi-Frequency and Multi-Orientation

The jellyfish Nemopilema nomurai occupies an important position in the Northwest Pacific ecosystem, and monitoring its biomass is necessary to ensure the protection of fishery resources and the safety of offshore industrial production. The acoustic method has been proposed for jellyfish flux estimation and bloom warning, in which the target strength (TS) of the jellyfish is a crucial parameter. However, varied swimming orientations of jellyfish aggregation result in different backscatter strengths. The acoustic echo characteristics in horizontal swimming orientations and multi-frequency broadband signals are yet to be revealed. This study aims to obtain the TS of jellyfish in various orientations and to comprehensively investigate the jellyfish acoustic echo features at various frequency broadband sounds. In an anechoic tank, we used wide-band echosounder and 70 kHz, 120 kHz, and 200 kHz split-beam transducers to measure the TS of jellyfish swimming omnidirectionally. The results show a difference of approximately 4 dB in the jellyfish’s normalized TS at 70 kHz (frequency range: 45 kHz to 95 kHz) and 200 kHz (frequency range: 160 kHz to 260 kHz) center frequency. The normalized TS of jellyfish varies by around 8 dB between horizontal and vertical swimming orientations. For jellyfish swimming horizontally, the TS and bell diameter have the following least squares fits: T S D 70 k H z = 20 log D − 89.36 r 2 = 0.83 ; T S D 200 k H z = 20 log D − 93.85 r 2 = 0.83 . The swimming orientation has significant effects on TS estimation and model construction.

An Experiment of the Actual Vertical Resolution of the Sub-bottom Profiler in an Anechoic Tank

Sub-bottom profiler (SBP) is an acoustic instrument commonly used to survey underwater shallow geological structure and embedded objects whose most important performance parameter is the actual vertical resolution. This paper presented a methodology to measure and evaluate the actual vertical resolution of SBP based on an experiment in an anechoic tank, which was divided into three components: building of artificial geological model, measurement of acoustic parameters, and determination of actual vertical resolution of the acoustic profiles. First, the wedge-shaped geological model, whose thickness could be accurately controlled, was designed and built in an anechoic tank to try to directly measure the vertical resolution of SBP. Then, the acoustic pulse width of SBP was measured to calculate the theoretical general vertical resolution and extreme vertical resolution. Finally, based on the acoustic profiles obtained in the experiment, the method which was used to evaluate the actual vertical resolution by measuring the duration of reflection event was put forward. Due to comparing measurement data of different parameter settings of the SBP, the study has revealed that the SBP had the lowest resolution in the 4 kHz-500 μs setting, which was 226.5 μs, or 36.2 cm, and the highest resolution in the 15 kHz-67 μs setting, which was 72.7 μs, or 11.6 cm. The vertical resolution decreased with the increase of the pulse width. The results also showed that the actual resolution was close to the theoretical general resolution and far from the extreme resolution.

Experimental investigation of composite metamaterial for underwater sound absorption

We propose an underwater metamaterial combining aluminum foam and polyurea. Three samples with different aluminum foam scatterers were prepared and tested for sound absorption coefficient in the water-filled impedance and anechoic tank. The results show that the large and small conical scatterers significantly improve the sound absorption of the polyurea. This improvement in sound absorption comes from the change in the sound propagation path and waveform transformation. The design of the proposed underwater metamaterial and relevant experimental results presented in this paper have potential guiding values in developing underwater sound-absorbing functional materials.

Research on Information Transmission Technology Through the Water–Air Interface Combining a Sonar and a Radar

In order to realize the information transmission across the water-air interface, the radar has been used to detect the tiny surface perturbations caused by the underwater low-frequency sound source. Due to the differences of medium properties at the water-air interface, it is difficult for underwater targets to communicate with airborne platforms directly. In order to solve this problem, we propose the finite element method to establish the minute vibration model on the water surface caused by the underwater low-frequency sound source, and analyze the influence of sound source parameters on the tiny surface perturbations at the surface of water. Then we design the frame structure and synchronization signal of the single-carrier communication system, and propose a robust information transmission method across the water-air interface. Finally, we experimentally verify the information transmission method combining a low-frequency sonar and a millimeter-wave radar in an anechoic water tank. The experimental results show that the method we proposed can realize the information transmission across the water-air interface up to 500 bps.

Numerical analysis and experimental measurement on segmented ring transducer acoustic fields

In order to overcome the shortcomings of the two traditional methods of finite element method (FEM) and direct coupling of mechanical FEM and acoustic boundary element method (BEM) in the comprehensive simulation of acoustic characteristics of non-two-dimensional axisymmetrical transducers, the acoustic FEM and acoustic BEM coupling method is adopted in this study to model, calculate and analyze the three-dimensional (3D) model of the segmented ring transducer. Firstly, the basic theory of FEM, BEM and FEM-BEM coupling calculation of the transducer is introduced. Secondly, a 1/8 3D model of the segmented ring transducer is built to simulate the underwater electro-acoustic characteristics of the transducer. Finally, a transducer prototype is manufactured according to the numerical model, and the electro-acoustic performance of the transducer is measured in the anechoic tank. The numerical results of transmission voltage response (TVR), admittance and radiation directivity curves are in good agreement with the experimentally measured results, which indicate that the proposed method is correct and effective. Therefore, the method could be used to comprehensively calculate the acoustic characteristics of the segmented ring transducer. In addition, the method could be used not only to study the acoustic problems of other transducers or transducers arrays with complex structures that are non-two-dimensional axisymmetrical, but also to comprehensively analyze and predict the near-field and far-field acoustic characteristics of any transducers including two-dimensional (2D) axisymmetrical transducers.

Vision-Based Target Detection and Positioning Approach for Underwater Robots

The accurate target detection under different environmental conditions and the real-time target positioning are vital for the successful accomplishment of underwater missions of Remotely operated vehicles (ROVs). In this paper, we propose a vision-based underwater target detection and positioning approach to detect and estimate the position and attitude of artificial underwater targets. The proposed approach is composed of an underwater target detection algorithm YOLO-T and a target positioning algorithm. Firstly, we modify the structure of YOLOv5 algorithm using Ghost module and SE attention module to improve the calculation time of target detection. Secondly, a series of image processing operations are performed on the improved YOLOv5 detection results to increase the detection accuracy. Thirdly, a cooperative marker is designed as the artificial underwater target, and the corresponding positioning algorithm is presented to calculated the position and attitude of the target according to the geometric information of the designed marker. We validate our approach through experimental tests respectively in a water tank, an anechoic tank, and the sea trial in Huanghai Sea in China. The results demonstrate the accurate performance of the proposed detection and positioning method.

The application of a homemade boomer source in offshore seismic survey: From field data acquisition to post-processing

This work presents a case study about applying a homemade boomer source working with a well-designed field geometry in an offshore seismic survey. Some measurements were carried out to calibrate the boomer source acoustic characteristics in an anechoic tank. Then a catamaran, taking account of the boomer source signature, is designed to control the source depth and entirely use ghosts as part of the source wavelet. This way, the vertical resolution is sacrificed to signal-to-noise ratio (SNR), and the source wavelet meets the minimum phase characteristics. Furthermore, the detailed analytical results of the calibrated source wavelets also provide a reliable basis for determining post-processing parameters. Two surveys have been conducted on the same line using this source in combination with two different hydrophone streamers. Some valuable experiences in field data acquisition and post-processing are analyzed and summarized. The most important one is taking the ghost part of the source wavelet by controlling source depth. This work is a reliable reference to other research on improving the quality of seismic imaging of offshore shallow strata.

Research and Experimental Verification of a Flextensional Disk Transducer Based on Relaxor Ferroelectric Single Crystals

In this article, the electroacoustic properties of the flextensional disk transducer based on relaxor ferroelectric single crystals are studied. First, the transducer structure, consisting of relaxor ferroelectric single crystals, metal plates, and air cavity, and an operating principle are introduced. Then, we investigate, by finite element modeling, the influence of different air cavity sizes on the transducer behavior. Furthermore, we optimize the size of the air cavity and analyze the performance of flextensional disk transducers based on relaxor ferroelectric single crystals and piezoelectric ceramics, such as the resonant frequency and transmitting voltage response (TVR). Finally, transducers of the same size are manufactured using the two kinds of materials, and the experimental measurements are carried out in an anechoic water tank. The measured maximum TVR of the single-crystal transducer is 141.04 dB at 22.2 kHz. Compared with the piezoelectric ceramic transducer, the maximum TVR of the single-crystal transducer is increased by 12.51 dB, and the corresponding resonant frequency is reduced by 4.8 kHz, which extends the working frequency.

Semi-blind calibration for orbital angular momentum generating system using maximal likelihood estimation.

Generating a high quality vortex beam carrying orbital angular momentum (OAM) is of importance in the application of the OAM to target detections or communications. This paper presents a semi-blind method to calibrate the uniform circle array (UCA) OAM generating system by using a virtual receiving sensor array without knowing the precise position of this auxiliary array. A low computational complexity algorithm is proposed to solve the maximum likelihood estimate (MLE) for the amplitude and phase errors of the transducers of the UCA based on the separation of parameters to be estimated. Moreover, this MLE is proved to be related to the beamforming of the virtual array. Simulations indicate that this method can estimate the mismatch parameters of the UCA effectively, and experiments in an anechoic tank show that the OAM generating system can generate a desirable vortex beam after we compensate each transducer of the UCA.

Study of air-backed and water-backed carbon fiber composite plates subjected to underwater shock loading

A collection of experimental and numerical studies was conducted for laminated carbon fiber composite plates of different layup orientations to investigate their response and failure when subjected to underwater shock loading. The composite plates were clamped within a rigid fixture and submerged within an anechoic water tank while having either an air-back or water-back supporting condition. Plastic bottles partially filled with liquid nitrogen were used to generate shock pressures whose characteristics were closer to that resulting from the pulsation of a gas bubble rather than the detonation of an explosive. A multiphysics model based on fluid–structure interaction was considered for the numerical study. A multiscale approach was also used for the numerical study to predict failure based on the constituent material level (i.e., fiber and matrix material level). Both experimental and numerical results agreed well, indicating that the water-backed carbon fiber plate had a very localized failure while the air-backed composite plate showed global failure. The critical stand-off distance for the failure of the water-backed carbon fiber composite was approximately half of that for the air-backed composite plate of the same layup orientation. In other words, the water-backed carbon fiber composite plate had greater damage tolerance than the air-backed plates.

Underwater Carpet Cloak for Broadband and Wide-Angle Acoustic Camouflage Based on Three-Component Metafluid

Acoustic camouflage, which is realized by controlling wave propagation, has attracted great attention in recent years. Aided by a reflecting surface, the acoustic carpet cloak has become one of the most practically feasible invisibility devices and captured much experimental research enthusiasm. However, owing to the difficulty in the realization of ideal material parameters, the implemented underwater carpet cloaks could only work for a small incident angle. Here we design an underwater carpet cloak using a three-component metafluid composed of syntactic foam, steel, and water. The proposed syntactic foam, which is synthesized from epoxy resin and hollow glass microspheres, exhibits lower mass density and higher sound velocity relative to water. By periodically embedding the syntactic foam and steel rods in water, a carpet cloak is constructed and experimentally demonstrated in an anechoic water tank. Experimental results indicate that the designed carpet cloak works well under both normal and oblique incidences for broadband frequencies. The implementation of the designed underwater carpet cloak paves the way for future applications. Moreover, the introduction of syntactic foam in the design of the carpet cloak provides an extra degree of freedom for the acoustic parameter regulation of the metafluid in underwater acoustic devices.