Detecting low-frequency underwater acoustic signals can be a challenge for marine applications. Inspired by the notably strong response of the auditory organs of pectis jellyfish to ultralow frequencies, a kind of otolith-inspired vector hydrophone (OVH) is developed, enabled by hollow buoyant spheres atop cilia. Full parametric analysis is performed to optimize the cilium structure in order to balance the resonance frequency and sensitivity. After the structural parameters of the OVH are determined, the stress distributions of various vector hydrophones are simulated and analyzed. The shock resistance of the OVH is also investigated. Finally, the OVH is fabricated and calibrated. The receiving sensitivity of the OVH is measured to be as high as −202.1 dB@100 Hz (0 dB@1 V/μPa), and the average equivalent pressure sensitivity over the frequency range of interest of the OVH reaches −173.8 dB when the frequency ranges from 20 to 200 Hz. The 3 dB polar width of the directivity pattern for the OVH is measured as 87°. Moreover, the OVH is demonstrated to operate under 10 MPa hydrostatic pressure. These results show that the OVH is promising in low-frequency underwater acoustic detection. Jellyfish sensory organs offer the inspiration for a new sensor design that can sensitively detect low-frequency sounds underwater. These capabilities are particularly for the long-range detection of submarines, which produce faint sound signatures in the 5–200 Hertz range. Researchers have struggled to develop compact ‘hydrophone’ devices that can sensitively capture such frequencies underwater, but Wendong Zhang of the North University of China and colleagues have identified a promising solution. Modeled on the pectis jellyfish auditory organ, this system features an otolith-like structure mounted on a cross-beam that produces a piezoresisitive signal in response to vibrations. Based on this concept, the researchers fabricated a device that effectively detects very faint sounds in the 20–200 Hertz range at depths of at least 1000 meters, highlighting the potential of such ‘otolith-inspired MEMS vector hydrophone’ devices.

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Design and implementation of a jellyfish otolith-inspired MEMS vector hydrophone for low-frequency detection.

This paper presents a comprehensive review of hydrophones, their design considerations, physical aspects, and structures by reviewing many publications that have been published over the last two decades. The investigation of the fundamental aspects and essential considerations of hydrophones in the literature are distributed in different papers and have not been brought together in a conclusive manner. This paper has collected and classified all the information about hydrophones including the parameters that affect their performance as well as their features. We have categorized the hydrophones according to their mechanism of operation, structure design, frequency response, and application. The literature has been summarized in a way that enables the readers' easy referral to appropriate designs for their desired application scenarios.

Hydrophones, fundamental features, design considerations, and various structures: A review

The two-dimensional direction-of-arrival (2D-DOA) estimation for the L-shaped array is usually divided into two one-dimensional DOA (1D-DOA) estimation for the two linear subarrays, and then the elevation and azimuth angles are obtained from the two correctly matched 1D-DOAs. This letter presents a novel method for pair-matching of the two 1D-DOAs. The key points of the proposed method are: 1) construct two signal covariance matrices corresponding to the two estimated 1D-DOAs; 2) introduce an objective function, defined over the two signal source covariance matrices, to find the matched 1D-DOAs; 3) by introducing a permutation matrix for the signal covariance matrices, the optimal pair-matching is obtained with an efficient method. Simulations under different scenarios, with low signal-to-noise ratio (SNR), snapshot deficiency, and uncorrelated or coherent signal settings, show the robustness of the proposed method.

Pair-Matching Method by Signal Covariance Matrices for 2D-DOA Estimation

This paper discusses the problem of two-dimensional (2D) direction of arrival (DOA) estimation for acoustic vector-sensor array, and derives a successive multiple signal classification (MUSIC) algorithm therein. The proposed algorithm obtains initial estimations of the azimuth and elevation angles obtained from the signal subspace, and uses successively one-dimensional local searches to achieve the joint estimation of 2D-DOA. The proposed algorithm, which requires the one-dimension local searches, can avoid the high computational cost within 2D-MUSIC algorithm. The proposed algorithm can obtain automatically-paired 2D-DOA estimation for acoustic vector-sensor array, and it has better DOA estimation performance than propagator method, estimation of signal parameters via rotational invariance technique algorithm and trilinear decomposition algorithm. Meanwhile, it has very close angle estimation to 2D-MUSIC algorithm. Furthermore, it is suitable for non-uniform linear arrays, works well for the sources with the same azimuth angle, and imposes less constraint on the sensor spacing, which does not have to be restricted within half-wavelength. We have also derived the mean-square error of DOA estimation of the proposed algorithm and the Cramer-Rao bound of DOA estimation. Simulation results verify the usefulness of the proposed algorithm.

Two-dimensional DOA estimation for acoustic vector-sensor array using a successive MUSIC

In this paper, micro-electromechanical systems (MEMS) technology and the bionic principle are used to develop a low-frequency high-sensitivity three-dimensional omni-vector hydrophone that can obtain vector information of an underwater sound field by imitating the auditory principle of a fish's lateral line organ. The key features are smaller size, better consistency, better low-frequency characteristic, higher sensitivity, and rigid mounting, which thus allow a spatial acoustic source to be detected directionally by a single hydrophone. The bionic MEMS microstructure was designed and fabricated and consists of two components: the vertical detection unit including a four-beam-cilium structure and a level detection unit including a double T-shaped beam structure. On the basis of theoretical analysis, the structure size and layout location of the piezoresistors are determined by simulation analysis and the double cilia type microstructure is fabricated integrally by MEMS manufacturing technology; after which the acoustic package of the microstructure is complete and the prototype is produced. Finally, this paper presents the experimental characterization of the microdevice, validating the concept and the analytical models used. The test results show that the three-dimensional vector hydrophone has a flat frequency response curve, exhibits a sensitivity of ?185 dB (X, Y) and ?181 dB (Z) (1 kHz, 0 dB reference 1 V/uPa) and shows a good directivity pattern in the form of an "8" shaped. More importantly, the depth of the concave point reaches 47.7 dB, and the asymmetry is only 0.5 dB, indicating that the three-dimensional vector hydrophone has great advantages in spatial orientation, which is suitable for applications in sonar systems.

Design of a monolithic integrated three-dimensional MEMS bionic vector hydrophone

Two-dimensional DOA estimation of coherent signals using acoustic vector sensor array

In this paper, a two-dimensional (2-D) direction-of-arrival (DOA) angle estimation method for coherent wideband signals using L-shaped array is introduced. This method estimates the elevation and azimuth angles using ESPRIT technique.The incident wideband signals are assumed to be coherent and hence some decorrelation techniques are employed to decorrelate them. Then the cross-correlation matrix is found. The proposed method reconstructs the signal subspace using cross-correlation matrix and then ESPRIT algorithm is employed to estimate the azimuth and elevation angles. This approach enables the 2-D DOA estimation without the use of peak search and is robust to noise. Simulation results are presented to verify the effectiveness of the proposed algorithm.

P. M. Swetha

Papers

Two-dimensional DOA estimation of coherent signals using acoustic vector sensor array

2-D DOA Estimation of Coherent Wideband Signals Using L-Shaped Sensor Array