Ray chaos and ray clustering in an ocean waveguide

We consider ray propagation in a waveguide with a designed sound-speed profile perturbed by a range-dependent perturbation caused by internal waves in deep ocean environments. The Hamiltonian formalism in terms of the action and angle variables is applied to study nonlinear ray dynamics with two sound-channel models and three perturbation models: a single-mode perturbation, a randomlike sound-speed fluctuations, and a mixed perturbation. In the integrable limit without any perturbation, we derive analytical expressions for ray arrival times and timefronts at a given range, the main measurable characteristics in field experiments in the ocean. In the presence of a single-mode perturbation, ray chaos is shown to arise as a result of overlapping nonlinear ray-medium resonances. Poincare maps, plots of variations of the action per ray cycle length, and plots with rays escaping the channel reveal inhomogeneous structure of the underlying phase space with remarkable zones of stability where stable coherent ray clusters may be formed. We demonstrate the possibility of determining the wavelength of the perturbation mode from the arrival time distribution under conditions of ray chaos. It is surprising that coherent ray clusters, consisting of fans of rays which propagate over long ranges with close dynamical characteristics, can survive under a randomlike multiplicative perturbation modelling sound-speed fluctuations caused by a wide spectrum of internal waves.     

Wave analysis of ray chaos in underwater acoustics

The dispersion of a wave packet in an acoustic medium is considered in the paraxial wave approximation, where the effective potential, due to variation of the speed of propagation, varies both with depth and propagation distance. The analysis of the resulting parabolic equation, similar to the Schrodinger equation, clearly demonstrates the role of ray chaos in enhancing the dispersion of the initial packet. However, wave coherence effects are also seen that suppress the effects of the ray chaos in a manner analogous to the effects of quantum chaos. (c) 1999 American Institute of Physics.     

Interrelation between ray and mode field representations in an acoustic waveguide

Some well-known results obtained by Brekhovskikh, Felsen, Tindle, Guthrie, and other authors during their studies of the interrelation between modes and rays in a plane-layered waveguide are briefly discussed. The relationships that extend these results to the case of a waveguide with large-scale inhomogeneities of the refractive index are analyzed. It is shown that in the presence of inhomogeneities, the groups of the constructively interfering modes, as in the case of a plane-layered waveguide, also form contributions of individual rays. In this case, fluctuations of the amplitudes of normal modes can be described by simple ray formulas, which are the mode analogs of the relationships of geometric optics and the smooth perturbation method.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 115–121, January–February, 1995.     

Phase conjugation in an acoustic waveguide

A method for holographic observation of sound sources and passive inhomogeneities in acoustic waveguides is proposed. The method is based on the principle of coherent phase conjugation. A possibility of visual observation of both sound sources and stationary inhomogeneities of different kinds is experimentally verified.     

Statistics of the ray travel times in oceanic acoustic waveguides under chaos conditions

Data processing for the numerical calculation of the ray paths in both range-dependent and range-independent oceanic acoustic waveguides is performed. The obtained statistical data are used to plot empirical distribution functions of the chaotic-ray random parameters. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 3, pp. 196–211, March 2007.     

Theory of passive localization for underwater sources based on acoustic ray modeling

I.IntroductionLongrangeMFPpassivelocalizationisahotpointinunderwateracousticsstudyinrecentyears.Thekeyworkistointroducethesoundfieldsinformationindeterminingthesourcelocation.MFPlocalizationutilizesmuchmoreinformationofseaenvironmentandthereforeobtainsmuchbetterperformancewhencomparingwiththetraditionalmulti-subarraypassivelocalization.PeoplehaveplungedilltothestudyofMFPlocalizationwithgreatzeal.ThestudiesofMFPlocaJizationconcentrateontheoreticalresearchandsimulations,whilesuccessfulexpe…     

A spherical acoustic source in an ideal waveguide

The acoustic field of a spherical source in an ideal waveguide is considered with allowance for the diffraction by the source. The consideration uses the previously obtained results on the diffraction by a spherical source in a halfspace with ideal boundaries. The resultant field is shown to be representable as an infinite sum of the fields of some equivalent sources. The errors that appear when the number of these sources is limited are estimated. The field produced by the sphere in an ideal waveguide is calculated with and without allowance made for the scattering.     

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.     

Wideband time-reversal imaging of an elastic target in an acoustic waveguide

Time-reversal is addressed for imaging elastic targets situated in an acoustic waveguide. It is assumed that the target-sensor range is large relative to the channel depth. We investigate the theory of wideband time-reversal imaging of an extended elastic target, for which the target dimensions are large relative to the principal wavelengths. When performing time-reversal imaging one requires a forward model for propagation through the channel, and the quality of the resulting image may be used as a measure of the match between the modeled and actual (measured) channel parameters. It is demonstrated that the channel parameters associated with a given measurement may be determined via a genetic-algorithm (GA) search in parameter space, employing a cost function based on the time-reversal image quality. Example GA channel-parameter-inversion results and imagery are presented for measured at-sea data.     

Design of an acoustic bending waveguide with acoustic metamaterials via transformation acoustics

Transformation acoustics is employed to design an acoustic bending waveguide. A two-dimensional square area with anisotropic and homogeneous material properties is transformed into a fan-shaped area with anisotropic and inhomogeneous material properties to rotate the direction of beam propagation. An alternating layered structure is considered to approximate a medium with anisotropic material properties. From the calculation results, the transformation medium can be realized by an alternating layered structure consisting of water and fluid with negative mass density. We propose that an acoustic metamaterial composed of three layers in water background can be designed to replace negative mass density fluid. The effective mass density and bulk modulus of the system that is composed of the acoustic metamaterial and water are dependent on the incident frequency and the geometric size of the acoustic metamaterial. We tune the geometric size of the acoustic metamaterial to approach the corresponding mass density distribution of the negative mass density fluid at a specific frequency. Thereby, the acoustic bending waveguide designed by using transformation acoustics can be achieved by the acoustic metamaterials.     

A tunable gas sensor using an acoustic waveguide

A tunable gas sensor using surface acoustic waves (SAW) is described. It is designed on the basis of a waveguide delay line fabricated on a piezoelectric substrate made of 128° Y-cut LiNbO₃. A voltage applied between the waveguide and two electrodes causes a local change in the properties of the substrate near the waveguide and differently affects the sensor’s response to the vapors of various analytes. Some results of the experimental study of the sensor, which show the change in the selectivity under the effect of voltage, are presented. The analytes used for testing include a number of alcohols and deionized water. The possibilities for employing such a sensor in the sensor arrays of gas analyzers of the electronic nose type are discussed.     

Simulation of the lower boundary of an acoustic waveguide

The paper discusses simulation of the lower boundary of an acoustic waveguide. The applicability and relationship of rigid bottom and fluid half-space models are studied as a function of the bottom sediment parameters. The influence of bottom losses on the sound field characteristics in a waveguide is studied. Examples of numerical simulation are given.

     

Energetic analysis of an actively controlled one-dimensional acoustic waveguide

The aim of this work is to show the energetic behavior when an active noise controller is applied in a one-dimensional waveguide, namely an ideal duct under the first critical frequency. In order to model the duct, a spectral element method, which is shown to be more practical for analyzing pipe networks than other commonly used analytical models, was used. The model used here consists of a duct with two sources, the primary source at one end of the duct, and the secondary source at the middle section. The error sensor was placed downstream from secondary source, and the other end of the duct was open with no flange. Three optimal control methods were applied: minimization of the potential energy density, minimization of the active intensity, and minimization of the total acoustic power radiated by the sources. It was observed that the three control methods achieved the same final result, and when the volume velocity of the secondary source was driven to the optimal volume velocity, neither the primary source nor the secondary source radiated any acoustic power. Furthermore, the controlled duct was equivalent to a duct opened-ended at the secondary source position with radiation impedance equal to zero.     

Non-linear acoustic radiation by an underwater vibrating spherical shell

I.IntroductionMostofthepublishedstudiesontheacousticwaveradiationfromelasticbodyconcen-tratedon1ineartheory.However,iftheamplitudeislargeandthefrequencyishigh,theresultingsoundwavepropagatinginthemediumwillgenerateconsiderablenonlineardistor-tion.Sothest.di.s[1]inthisfieldarelimited.Inthispaperwetreatedasphericalshell.Itconsistsoftwoaspects:oneisthestructuralresponseofasphericalshellinacousticfield.Becausethenonlineareffectofacousticwavecumulateswithpropagatingdistance,itisnegligibleonthesph…     

Piston acoustic radiator in an impedance screen of a Pekeris waveguide

A model problem of seismic hydroacoustics is studied for a piston radiator inserted in an impedance screen that coincides with the lower boundary of a Pekeris waveguide. The radiation resistance of the piston is numerically calculated as a function of the screen type and parameters of the radiator and the waveguide.     

Focusing of a sound beam in an underwater waveguide using a vertical antenna

We propose a method for calculating the amplitude-phase distributions of signals on the elements of an emitting antenna for generating narrow wave beams and field focusing at given points of the waveguide. The efficiency of the method is confirmed by the data of a field experiment conducted in Lake Ladoga.     

Space-time structure of the low-frequency acoustic field in an underwater sound channel

Experimental data are presented on the fine structure of the sound field in an underwater sound channel for low and infralow sound frequencies. The experiments are performed in the Black Sea, on a 600-km-long path, with explosive sound sources. The intensity, space-time, and frequency characteristics of the sound field are analyzed. The geometric dispersion of the first normal wave is experimentally studied. The role of the channel inhomogeneities in the violation of the sound field coherence is determined for different frequency bands. On the basis of the experimental data, the vertical distribution of the critical frequencies of the waveguide is obtained, and the validity limits are established for the wave and ray calculation methods. The applicability of the phase methods for calculating the sound fields in waveguides with dispersion is discussed. The frequency-angular dependence of the effective sound attenuation coefficient in an underwater waveguide is revealed and explained.     

Efficiency of face-plated underwater acoustic transducers

The possibility of obtaining the efficiency of simple disc transducers, broad-banded by use of face-plates, from input electrical conductance measurements under air-loading is explored. For an air-backed ceramic disc with a Perspex face-plate it was found that the main limitation to the accuracy of efficiency estimates is due to departures from uniformity of the face-plate thickness.     

Statistical properties of chaos in Chebyshev maps

Statistical properties of fully developed chaotic maps in the form of Chebyshev polynomials are calculated exactly. We derive analytic expressions for characteristic functions, moments, and moment functions and mention a number of other properties. We also determine higher-order moment functions, which are important for a characterization of the non-gaussian processes exhibited by many maps.     

Experimental study of the horizontal correlation of an acoustic field in an inhomogeneous shallow-water waveguide

We present the results of experiments on measuring the characteristics of a wideband low frequency (10–200 Hz) acoustic field in an inhomogeneous shallow-water waveguide in the Barents Sea area. Correlation functions of the signals received by hydrophones of autonomous bottom stations with independent reception, which are horizontally spaced 18 and 112 km apart, are measured. A pneumoacoustic radiator with an overpressure of 10.5 MPa and a volume of 7.5 l was used as a sound source. During the measurements, the signal source was shifted from the line of receivers to distances 10–225 km. The dependences of the correlation coefficient of the received signals on the radiation frequency and the interferometer-baseline length and orientation are studied. It is shown that pronounced local minima in the interval 10–40 Hz due to the difference of the inhomogeneities in the propagation channels and, therefore, the frequency characteristics of the medium are present in the frequency dependences of the correlation coefficient against the background of a monotonic decrease with increasing frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 3, pp. 208–215, March 2009.