Interferometric method for estimating the sound source velocity in an oceanic waveguide

The noise immunity of an algorithm for estimating the radial component of sound source velocity, based on the data on frequency shifts of field interferencemaxima, has been investigated. A limiting estimate for the input signal-to-noise ratio, which determines the algorithm efficiency, is obtained. The results of computational and natural experiments are reported.     

Correlation reception of a diffraction sound field in an oceanic waveguide

Intensity matched processing is considered for a diffracted signal when the object is detected by the main lobe of the shadow scattered sound field. The consideration is based on the few-parameter model that adequately describes the characteristics of the desired signal. It is shown that, in the absence of noise and fluctuations in the parameters of the medium, the proposed algorithm provides the determination of the exact parameters of the signal and the moving object.     

Diffraction of sound pulses on an elastic spherical shell submerged in an oceanic waveguide

The paper is devoted to simulating an acoustic field scattered by an elastic spherical shell placed in a waveguide with a fluid attenuating bottom. The emitted signal is a wideband pulse with a Gaussian envelope. The normal wave method is used in the frequency domain for calculating the field of a point source in a free waveguide and the shell scattering coefficients. Movement of the receiver along a vertical straight line located behind the shell makes it possible to obtain a “three-dimensional” image of the field scattered by the shell. In this representation, the horizontal axis is time; the vertical axis is the submersion depth of the receiver; the intensity shows the amplitude of the received signal. Such three-dimensional structures make it possible to analyze the dependence of the complex diffraction structure of the acoustic field on receiver depth. In the considered numerical example, a thin, elastic, spherical shell is located near the attenuating fluid bottom.     

Efficiency of sound field focusing in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning the focal point in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically considered. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of the periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path relative to the propagation direction of internal waves.     

Interference immunity of an interferometric method of estimating the velocity of a sound source in shallow water

We describe an algorithm for estimating the radial component of the velocity of a sound source based on information about frequency shifts of the interference maxima of the field and consider the problem of its interference immunity. We obtain the limit estimate for the value of the input signal/noise ratio when the algorithm is working effectively. We present results of computational and field experiments using a single receiver and a horizontal array. We compare the experimental data with analytic interference immunity estimates.     

Waveguide invariant focusing for broadband beamforming in an oceanic waveguide

The performance of broadband sonar array processing can degrade significantly in shallow-water environments when interference becomes angularly spread due to multipath propagation. Particularly for towed line arrays near endfire, elevation angle spreading of multipath interference often results in masking of weaker sources of interest. While adaptive beamforming in a series of narrow frequency bands can suppress coherent multipath interference, this approach requires long observation times to estimate the required narrowband covariance matrices. To form wideband covariance matrices which can be estimated with less observation time, plane-wave focusing methods have been used to avoid interference covariance matrix rank inflation. This paper extends wideband focusing to the case of coherent multipath interference. The approach presented here, called waveguide invariant focusing (WIF), exploits a robust relationship for the frequency dependence of horizontal wave number differences. Unlike matched-field methods, WIF does not model multipath wave fronts but rather makes the interference appear to occupy the same rank-one subspace across frequency. This permits formation of wideband covariance matrices without interference rank inflation. Simulation experiments in a realistic ocean environment indicate that adaptive beamforming using WIF covariance matrices can provide a significant array gain improvement over conventional adaptive methods with limited observation time.     

Flat-response sound source technique for using the two-microphone method in an impedance tube

In this letter, to improve the acoustic impedance measurement in a tube using the two-microphone method, a sound source technique is developed that is capable of generating incident sound waves of uniform intensity over the entire frequency range of interest. The basic principle is to use a digitally synthesized compensation input signal to achieve a flat response of the sound source provided by one or more loudspeakers. The present sound source technique has been demonstrated to be very effective and easily implemented with the current impedance tube testing systems.     

Efficiency of sound field focusing at a long distance in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning with the focal spot at long distances in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically investigated. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path with respect to the propagation direction of internal waves. The effect of perturbation on the stability and efficiency of focusing is discussed. A comparative analysis of the data obtained for long and short distances is performed.     

Modal Doppler theory of an arbitrarily accelerating continuous-wave source applied to mode extraction in the oceanic waveguide

A Doppler-based method for using a moving narrow-band source to extract the modes of acoustic propagation in a range-independent shallow ocean waveguide over a partial-water-column spanning vertical line array (VLA) is introduced. Because the modal components propagate at distinct frequencies in the case of uniform radial source motion, the modal depth functions may be isolated and extracted from a frequency decomposition of the field. Because Doppler broadening due to radial source accelerations degrades the effectiveness of the extraction method, the method incorporates a technique to compensate for Doppler broadening. As the basis for the compensation technique, a theory is introduced for describing the VLA field from an accelerating cw source. By connecting the range of the source at the time a signal feature is emitted (the retarded time) to the range of the source at the time the signal feature arrives at the receiver (the contemporary time), the theory incorporates the Doppler effects associated with the finite group velocities of the modal components. The mode extraction method and compensation technique are applied to simulation and ocean data.     

On the virtual source method for an earth-ionosphere waveguide with inhomogeneous upper wall

We consider the influence of a smooth, with respect to the wavelength, upper wall curvature of a plain Earth-ionosphere waveguide on the propagation of electromangetic waves excited by a dipole source. Since the height disturbance is one-dimensional, it is possible to construct a system of virtual sources in an infinite medium, which excite the same total field as the field of a dipole source in the waveguide. The effective inhomogeneous refractive index of such a system is obtained. We estimate the number of reflections for which Kirchhoff’s principle can be used. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 4, pp. 296–303, April, 2000.     

A method for estimating the orientation of a directional sound source from source directivity and multi-microphone recordings: principles and application

Taking into account directivity of real sound sources makes it possible to try solving an interesting and biologically relevant problem: estimating the orientation in three-dimensional space of a directional sound source. The source, of known directivity, produces a broadband signal (in the ultrasonic range, in this application) that is recorded by microphones whose position with respect to source is known. An analytical method to process the recorded signals and estimate source orientation is developed in this paper. Experiments testing method performance in estimating source orientation were performed both in a laboratory environment with a Polaroid transducer as source and in a flight room with a Myotis daubentonii bat. In the first case, results showed the estimation method to be accurate and pointed out its limitations. The latter case is significant as an example biological application of the method for extracting behavioral features from bats; results are compared with alternative calculations based on microphone root-mean-square (rms)-pressure values.     

The optimum source depth distribution for reverberation inversion in a shallow-water waveguide

An approach for extracting the modal backscattering matrix from reverberation data in shallow water was proposed recently (Shang, Gao, and Tang, 2002). The kernel matrix of the inversion is constructed using the square of the modal functions. The singularity of this matrix (or the stability of the inversion) is the crucial issue to be considered. In this paper, we discuss this issue analytically for a Pekeris waveguide with the limited mode number M. The method that we used for singularity analysis is to calculate the maximum value of the determinant of this kernel matrix. We found that there is an optimum source depth distribution corresponding to the maximum value of the determinant of the kernel matrix. That means that, by choosing the optimum source depth distribution, we can get the most stable inversion. The conclusion is that, under a quite tolerant condition, the matrix is not singular, and the backscattering matrix can be inverted. Published in Russian in Akusticheskiĭ Zhurnal, 2007, Vol. 53, No. 5, pp. 682–686. The text was submitted by the authors in English.     

Physical modeling of nonlinear sound wave propagation in oceanic waveguides of variable depth

Results of laboratory experiments aimed at studying the spatial distribution of the difference-frequency acoustic wave field in a shallow-water waveguide with a sloping bottom are presented. It is shown that, in an inhomogeneous waveguide, the direction toward the radiation maximum in the angular spectrum of the low-frequency wave continuously varies as the rib of the wedge is approached, whereas, in a homogeneous waveguide, the angular spectrum is shaped. A spatial filtering of low-frequency modes produced by a parametric radiator and reflected from the coastal wedge is experimentally realized. The results of the experiment are confirmed by numerical modeling. Problems of the physical adequacy of the experimental results obtained under actual and laboratory conditions are discussed.     

The complex equivalent source method for sound propagation over an impedance plane

The sound field caused by a monopole source above an impedance plane can be calculated by using a superposition of equivalent point sources located along a line in the mirror space below the plane. Originally, such an approach for representing the half-space Green's function was described by Sommerfeld at the beginning of the last century, in order to treat half-space problems of heat conduction. However, the representation converges only for masslike impedances and cannot be used for the more important case of reflecting planes with springlike surface impedances. The singular part of the line integral can be transformed into a Hankel function, which shows that surface waves are contained in the whole solution. Unfortunately, this representation suffers from the lack of validity at certain receiver points and from restrictions on wave number and impedance range to ensure the necessary convergence. The main idea of the present method is to use also a superposition of equivalent point sources, but to allow that these sources can be located at complex source points. The corresponding form of the half-space Green's function is suitable for both masslike and springlike surface impedances, and can be used as a cornerstone for a boundary element method.     

一种水平变化可穿透波导中声传播问题的耦合简正波方法

通过利用标准简正波程序KRAKEN计算本地简正波解及耦合矩阵, 进一步发展了求解水平变化波导中声场的全局矩阵耦合简正波方法(Luo et al., "A numerically stable coupled-mode formulation for acoustic propagation in range-dependent waveguides," Sci. China-Phys. Mech. Astron. 55, 572 (2012)), 使得该方法可以处理具有可穿透海底及随深度变化声速剖面等实际问题, 并提供声场的完全双向解. 本文还给出了双层波导中耦合矩阵的解析表达式, 并利用其验证了本方法中耦合矩阵数值算法的精度. 最后, 利用改善后的全局矩阵耦合简正波模型(DGMCM)计算了美国声学学会(ASA)提出的可穿透楔形波导标准问题, 将所得数值解与参考解比较, 结果表明DGMCM方法可以精确处理水平变化波导中声传播实际问题. 关键词： 耦合简正波理论 全局矩阵方法 可穿透楔形波导     

Determination of the source coordinates using signal processing algorithms matched to the oceanic waveguide

Efficiency of the waveguide-matched algorithms of source localization is experimentally investigated. The algorithms determine the source coordinates from the differences in the times of signal propagation over different ray paths.     

Antenna array in the sound field of an arctic waveguide

An antenna array placed in an arctic-type acoustic waveguide is considered. The guided sound field consists of a signal generated by a point source and an isotropic interfering noise produced by the ice cover. The array is operated in a specific regime: the output signals correspond to individual modes of the sound field. The signal received by the array is subjected to correlation processing with a finite averaging time. It is shown that, depending on the processing method, the signal-to-noise ratio can reach 40–60 dB for a realization duration of 1 min, and the quality of the signal detection is increased.     

Sound source localization in an ocean waveguide

A robust algorithm for sound source localization in shallow sea based on a 2D Fourier transform of source motion interference patterns is developed. A spectrogram in time-frequency coordinates obtained after a Fourier transform contains localized areas of spectral density intensity of constructively interfering modes and a distributed area of noise spectral density. Signal spectral density is localized in an area whose linear sizes are determined by the lowest frequency and spatial scales of field variability. The peak input signal-to-noise ratio is estimated for a single receiver when robust detection is ensured and estimates of speed and initial distance to the receiver are close to the actual values. The high robustness of the algorithm is based on the coherent summation of mode amplitudes occurring at different times. Experimental results demonstrating the effectiveness of the algorithm are presented. For higher than peak signal-to-noise ratios, random estimates of the parameters are close to their statistically average values.