Shie Yang

The classification of underwater acoustic target signals based on wave structure and support vector machine

The sound of propeller is a remarkable feature of ship-radiated noise, the loudness and timbre of which are usually applied to identify types of ships. Since the information of loudness and timbre is indicated in the wave structure of time series, the feature of wave structure can be extracted to classify types of various underwater acoustic targets. In this paper, the method of feature vector extraction of underwater acoustic signals based on wave structure is studied. The nine-dimension features are constructed via signal statistical characteristics of zero-crossing wavelength, peek-to-peek amplitude, zero-crossing wavelength difference, and wave train areas. And then, the support vector machine (SVM) is applied as a classifier for two kinds of underwater acoustic target signals. The kernel function is set radial basis function (RBF). By properly setting the penalty factor and parameter of RBF, the recognition rate reaches over 89.5%, respectively. The sea-test data shows the validity of target recognit...

This Submission is for Special Issue on Underwater Acoustics: Perfectly Matched Layer Technique for Parabolic Equation Models in Ocean Acoustics

In ocean waveguides, the ocean bottom is usually approximated as a half-space. Thus, there exist no reflection waves at the half-space bottom and condition of radiation at infinity should be satisfied. In numerical solutions like parabolic equation methods, the depth domain has to be truncated, which can generate reflection waves from the truncated ocean bottom. To reduce the effect of reflection waves and to simulate an unbounded ocean bottom accurately, an artificial absorbing layer (ABL) was used. As was demonstrated, an ABL meets well the demand of accuracy in sound field calculation. However, both the sea-bottom layer and the artificial absorbing layer are needed to be set quite thick by using an ABL technique. Fortunately, a PML with several wavelengths can keep similar calculation accuracy with an ABL with dozens of wavelengths. In this paper, perfectly matched layer (PML) techniques for three parabolic equation (PE) models RAM, RAMS and a three-dimensional PE model in underwater acoustics are presented. A key technique of PML “complex coordinate stretching” is used to truncate unbounded domains and to simulate infinity radiation conditions instead of the ABL in those models. The numerical results illustrate that the PML technique is of higher efficiency than the ABL technique at truncating the infinity domain with minimal spurious reflections in PE models.

The horizontal correlation of long range bottom reverberation in shallow water with inclined sea floor

The performance of active sonar system is seriously influenced by bottom reverberation in shallow water waveguide. It is important to understand the horizontal correlation of bottom reverberation for active towed-array processing techniques in shallow sea. However, little work had been done for the research on horizontal correlation of distant bottom reverberation. In this paper, a coupled mode reverberation model was applied for the horizontal correlation, and it was investigated as a function of receiving position, time and frequency. Calculations show that transverse correlation is greater than the longitudinal correlation in horizontal space for distant bottom reverberation. The adiabatic mode solution is introduced to derive the mathematic mode for horizontal correlation in the range-dependent waveguide with varying depth and the numerical results indicate that the influence of inclined sea floor on the horizontal correlation should be considered.

Analysis and design of multi-pole vector sensor arrays

Super-directive sensor arrays, due to their small size and enhanced directivity, are quite promising in signal-to-noise enhancement and target direction finding. An array design method, which can build a desired beam pattern using compact multi-pole sensor arrays, is presented in this paper. The proposed method extracts multi-pole modes of different order from the spatial differentials of the sound fields which are measured by a uniform square array composed with hydrophones and particle velocity sensors. It can achieve more benefits with the employment of particle velocity sensors because of their inherent dipole directivities. The spatial differential in the extraction procedure is approximated with spatial difference and the approximation decreases the beam pattern performance. The performance degradations in multi-pole beam patterns are investigated for different incident wave frequency. Simulation and analysis show that the array gain is to a certain extent robust to uncorrelated noise.

Coupled theory for long range bottom reverberation from seabed volume in-homogeneities

It is well known that in shallow water the performance of active sonar systems is seriously influenced by bottom reverberation, which is simply the sum of scattered sound on the irregular bottom interface or of random in-homogeneities in the sediments. However, in the case of a relatively flat bottom or of bottom with a small grazing angle, the volume scattering in the sediments would play a much more important role. In this paper, the theory of coupled modes is used to model the long range bottom reverberation in shallow water caused by inhomogeneous sediments in the bottom. The distant bottom reverberation level of an impulsive source is derived. This approach, which gives results obeying the principle of reciprocity in all cases, is available for both mono-and bi-static situations.

Inversion of sound speed profile in shallow water with irregular seabed

Horizontal deflection often occurs when a ray propagates in shallow water with an irregular sea bottom. This makes it difficult to determine the eigen-ray trajectory and transmission time. A new method for seeking eigen-rays in three-dimensional space is proposed. SSP has been inverted from experimental data in the South China Sea using the fastest eigen-ray transmission times to form the cost function. In order to improve the speed of the inversion, the algorithm was transformed from a non-linear optimization to the solution of linear equations by using the perturbation method. The experimental results indicate that the precision of SSP inversion can be enhanced dramatically by considering horizontal deflections of eigen-rays. The inversion speed can be improved sufficiently for real-time monitoring of SSP with reasonable precision by using the perturbation method.

Analysis and design of a low frequency piston-like infra-sound transmitter

A kind of ultra-low frequency piston-like sound source was analyzed and designed. The circular radiator on an infinite baffle plate was used as the acoustic model of the transducer. To get higher efficiency, sound wave was radiated from the source by the vibration of both sides of transmitter with corresponding radiation impedance. The relationship among frequency, radius and the vibrating amplitude of the radiator had been derived for different source levels. The infra-sound transmitter designed in this paper was proved to be powerful, and satisfied the demand of low-frequency sound propagation experiment in shallow water.

Phase difference estimation methods for linear array

The performance of array may be seriously degraded by the difference of phase responses between the array elements. Thus it is important to compensate these differences by means of precise phase estimation. In this paper, two methods for phase difference estimation between linear array elements are presented. The first one works by applying the cost function to search the optimal source position before the phase difference is obtained. The other one establishes the observation equation at first, and then solves it by least-squares method. Algorithms for both methods proposed here are very simple, while the required experimental equipments are also not very complicated. Simulation results and experiment results show that both methods have sufficient accuracy and are easy for application.