Showing papers on "Acoustic source localization published in 1982"

Acoustic Tomography

Abstract: High-resolution measurements of the density field in the ocean are prohibitively expensive if traditional ship-borne instruments are used. Tomography uses acoustic remote sensing to infer ocean structure, and avoids many of the limitations of direct measurements. Sound pulses follow distinct trajectories through the water from source to receiver, and the travel time for a given pulse is a known functional of the sound speed field. This functional can be inverted to recover an estimate of the sound speed field through which it passed. The inversion is accomplished with either detenninistic linear matrix inversion or stochastic optimal estimation, and the sound speed field estimate returned can be converted to an estimate of density. A numerical simulation of the pilot tomography experiment is presented to demonstrate that tomography can be effective in reproducing significant ocean features.

Method and assembly to locate detonations

Abstract: 1. A method of taking bearings on sharp reports, in which the acoustic pressure oscillations from each report are picked up at at least two measuring points (MS; MS1, MS2, MS3) equipped with at least two sound detectors (M1, M2, M3), the time differences in the arrivals of the acoustic pressure oscillations at the sound detectors (M1, M2, M3) at the same measuring point are determined for each measuring point (MS; MS1, MS2, MS3) and the corresponding time differences are transmitted to one central evaluation point (AS) for the determination of the place of origin (SO) of the report, characterised in that the time of arrival of the acoustic pressure oscillations of each report at the individual sound detectors (M1, M2, M3) at each measuring point (MS; MS1, MS2, MS3) is established and stored, that the time differences in the arrivals of the acoustic pressure oscillations at the sound detectors (M1, M2, M3) at the same measuring point (MS; MS1, MS2, MS3) are stored after the determination for each measuring point (MS), and that at the location of each measuring point (MS; MS1, MS2, MS3) from the time data and time difference data the direction of incidence of the sound (r; r1, r2, r3) and the time of incidence of the sound for the measuring point (MS; MS1, MS2, MS3) in question are determined electronically, after which these data are transmitted to the central evaluation point (AS) for the determination of the place of origin of the sound (SO) and of the time of origin of the sound.

Fluctuations in sound transmission through internal waves associated with the thermocline: A computer model for acoustic transmission through sound velocity fields calculated from thermistor chain, CTD, XBT, and acoustic backscattering

Abstract: Data sets obtained from towed thermistor strings, instrumented tow bodies, conductivity temperature depth sensors, expendable bathythermographs, sound velocimeters and high‐frequency acoustic backscattering systems provide detailed information on the temporal and spatial variability of the sound velocity field in the upper ocean. The data sets show that internal waves and other small scale fluid phenomena such as instabilities can significantly perturb the sound velocity field and consequently sound propagation. Little attention has been focused on the magnitude of short‐range, high‐frequency acoustic propagation variability caused by internal wave field and/or water mass variability. A ray trace program package has been developed which numerically integrates with a fourth‐order Runge–Kutta method the time dependent eikonal equations through a range dependent sound velocity field. The sound‐speed profiles, which are calculated from field data, are entered into a matrix with sizes larger than the numerical...

A method for measuring the change of ultrasound velocity in a liquid

Abstract: A method for measuring the change of sound velocity from a reference state in a liquid is described. Two interdigitial transducers on a piezoelectric substrate are in contact with the liquid, and are used as an input and an output transducer for constructing a liquid delay device. The phase difference between an input and a delayed output signal is detected as a dc voltage via a phase comparating circuit. The temperature dependence of the sound velocity in water, measured by the present method, agrees well with the published data. The influence of hydrostatic pressure on the sound velocity in water is given.

Nonlinear theory of shocked sound propagation in a nearly choked duct flow

Abstract: The development of shocks in the sound field propagating through a nearly choked duct flow is analyzed by extending a quasi-one dimensional theory. The theory is applied to the case in which sound is introduced into the flow by an acoustic source located in the vicinity of a near-sonic throat. Analytical solutions for the field are obtained which illustrate the essential features of the nonlinear interaction between sound and flow. Numerical results are presented covering ranges of variation of source strength, throat Mach number, and frequency. It is found that the development of shocks leads to appreciable attenuation of acoustic power transmitted upstream through the near-sonic flow. It is possible, for example, that the power loss in the fundamental harmonic can be as much as 90% of that introduced at the source.

Acoustic Transmission Imaging for Flow Diagnostics

Abstract: Wave propagation through a given flow field can be utilized to obtain information about the flow. Acoustic waves in particular offer the possibility of measuring velocity fields, because sound waves are convected with the flow, and temperature fields, because the acoustic index of refraction is a strong function of temperature. This work concentrates on situations where the first effect is dominating compared to the latter one. A sound system capable of "illuminating" a flow field with ultrasonic waves and measuring the amplitude and phase distribution of the transmitted wave as a function of time was constructed. A large area transmitter and a linear receiving array were used as transducers. Parallel signal processing and interleaved data conversion and acquisition result in a maximum frame rate of 10 kHz. The feasibility of measuring velocity disturbances with ultrasound was demonstrated by transmitting sound through a vortex, which was generated in a duct by an airfoil swirl generator. Assuming an exponential fit for the tangential velocity component, inner core radius and circulation can be determined directly from the phase change of the transmitted wave due to the vortex. A more accurate representation of the radial velocity profile can be found by digital reconstruction via the Abel inversion formula, which allows reconstruction of rotationally symmetric objects from line projections. Even though the flow field under investigation was steady, this is neither a restriction of the technique, nor of the apparatus. However, the repetition rate for consecutive data frames depends on the operating procedure.

Sound Velocity in Critical Binary Mixtures

Abstract: This paper describes the significance of sound velocity measurement in exploring the acoustic anomaly of critical binary mixture and the methods for measuring ultrasonic and low frequency sound velocities in such a mixture.

Acoustic Determination of Sub-Bottom Density Profiles Using a Parametric Sound Source

Abstract: Pulse-echo techniques may be well suited to acquire continuous recordings of sub-bottom density profiles. To obtain an accurate estimate of the sub-bottom reflectivity function, an acoustic system should have a broad frequency spectrum for adequate depth-resolution (a), sufficient energy at low frequencies for a pre-specified depth-penetration (b) and a narrow beamwidth to minimize the influence of scattering noise (c).

Non-Contact Measurement of Velocity Vector Distribution by Numerical Reconstruction of Reflected Sound Field

Abstract: A new method aimed at non-contact velocity measurement of moving objectsis proposed. In this method, ultrasonic wave is emitted, the echo field reflected by moving objects is detected by receiver array, and recorded for a certain period of time. The velocity vector field is reconstructed numerically from the recorded signal using digital beamforming techniques. Whole distribution of velocity vector components, both the longitudinal and transverse components can be measured simultaneously. The possible applications are non-invasive measurement of blood flow, observation of high temperature gas, or investigation of moving powder, etc. In this paper, 2-dimensional case is discussed for feasibility study, however, the principle can be extended to 3-dimensional measurement. The measurement system emits monochromatic sinusoidal ultrasonic wave through a single transmitter and it detects the echo by a linear array of receivers. Two velocity estimating algorithms are proposed. The accuracy of velocity measurement by the each algorithm is evaluated as the square root of the minimum obtainable variance of the estimate. Several fundamental relations for selection of parameters are obtained. The experimental measuring system is built utilizing ultrasonic wave in air (40kHz). Computer simulation and experimental results prove the feasibility of the method. 1. は じ め に

Propagation of nonlinear acoustic waves in gallium

Abstract: We study nonlinear phenomena which occur during strong sound signal propagation in superpure Ga and are due to the perturbation of the electron subsystem in the metal. Although the observations are in qualitative agreement with theoretical concepts in a number of aspects, there are effects which still lack adequate theoretical interpretation, in particular, an oscillatory dependence of the harmonic amplitudes on magnetic field which is linear with respect toH1/2, and appreciable nonlinear distortions of the sound pulse envelope accompanied by the acoustic noise generation.

Collinear acoustico‐optic diffraction using a diffuse acoustic field

Abstract: The acoustic waves propagating in a low‐loss crystal travel long distances after numerous reflections by the walls before vanishing and thus form a complex sound field in the crystal. From this sound field called ’’diffuse’’ it is possible to select different acoustic modes at given frequencies by means of flat and parallel, disposed normally to pure‐mode axes of the crystal, pairs of reflecting walls which form acoustic Fabry‐Perot interferometers. Some acoustic modes selected from the diffuse field are used for realizing collinear acousto‐optic diffraction and thus, it is shown that the overlapping in space of acoustic and light beams may be achieved in this way. A model for determining the intensities of the selected modes is established. The model is used for calculating the efficiency of the collinear diffraction.

Acoustic propagation in the upper sound channel

Abstract: This paper deals with acoustic propagation in sound channels occurring naturally in the ocean, allowing for the presence of random scattering irregularities. These irregularities are generally very weak and are often large compared to the acoustic wavelength. The parabolic equations for propagation of the moments of the wave field can be used in these circumstances and an approximate solution has been obtained for the second moment which is valid even in the case of multiple scatter. Although the physical meaning of the solution is clear, the expressions can be difficult to evaluate for most sound channels, and special care must be exercised in the neighborhood of caustics and foci. A detailed description is given of the methods used to evaluate the solution of the second moment equation with a particular discussion of the numerical techniques used. Results are presented for a sound‐speed profile modeled by a cubic polynomial corresponding to channeling conditions observed in the North Atlantic. It is fou...

Ocean acoustic tomography: Currents

Abstract: Synoptic maps of the geostrophic current structure of the mesoscale field can be constructed from the three-dimensional density field provided by ocean acoustic tomography with unidirectional acoustic transmissions. Reciprocal acoustic transmissions can extend the technique by permitting one to directly measure the current field, including the barotropic component. A preliminary reciprocal acoustic transmission experiment at long range (300 km) and low frequency (400 Hz) is planned for autumn 1982.

On the localization of underwater acoustic sources in a constant‐velocity waveguide

Abstract: A method to detect and to locate underwater acoustic sources in the constant‐velocity waveguide is developed in the present work With the attenuation of sound waves through multiple reflections at the waveguide boundaries and the sound pressure decay in the neighbor medium of the waveguide included in consideration, the normal‐mode method is first used to determine the sound pressure distribution in the waveguide Then, the cross‐correlation method is applied to determine the distribution of detection factor (DF) The detection factor indicates whether the calculated values for an assumed location of the sound source has a good correlation to a set of sound pressure measurements taken from the receivers Better correlated signals will provide higher DF values The location of the real source can thus be determined Practical examples of different arrays of three and six receivers are employed to develope algorithm for locating an acoustic source of low‐frequency (300 Hz) signal generated in the