Showing papers on "Point source published in 1980"

Sound source reconstructions using a microphone array

Abstract: A square microphone array with 256 elements has been constructed along with interfacing electronics to study low‐frequency (1–5 kHz) sound sources in air using the principles of acoustical holography. The array is used in the nearfield of a radiating object and the sound source structure of that object is reconstructed with an on‐line minicomputer. Reconstruction of the source structure of a point source and an unbaffled, free rectangular plate point excited below its coincidence frequency are presented. The latter shows clear evidence of ’’corner’’ and ’’edge’’ modes in which the respective areas of the plate are the dominant radiating sources. A new imaging process which is not limited in resolution by the wavelength of the radiated sound is introduced.

Static elastic‐gravitational deformation of a layered half space by point couple sources

Abstract: A method for computing the deformation in a layered, self-gravitating half space due to internal point sources is formulated. The coupled elastic-gravitational field equations appropriate to the first-order perturbation problem are presented. Although the first-order perturbation problem is not strictly coupled to a physically reasonable zeroth-order problem, the perturbation equations together with boundary conditions present a well-posed problem. The general homogeneous solution for deformation in a uniform infinite medium is found in the form of eight independent solution vectors. It is easily shown that the solution to the coupled elastic gravitational equations reduces to that for the uncoupled equations in the limit of vanishing density. The solutions for various internal point sources in a layered half space are constructed out of the solution found for the infinite medium by the use of well-known techniques. The half space is divided into a series of horizontal layers, and a matrix method is used to propagate the solution from one layer to the next. The point source is represented by a discontinuity in several of the various propagated quantities across a plane z = constant. The solution is ultimately determined by the requirements of vanishing normal tractions on the free surface and vanishing of all perturbation quantities at infinite depth. Finally, expressions are given for the direct calculation of surface displacements from several types of point sources.

Discovery of a compact X-ray source at the center of the supernova remnant RCW 103

Abstract: A point source of X-ray emission has been detected at the center of the supernova remnant RCW 103 using the Einstein Observatory. The 10 arcsec radius error circle is centered at alpha 16h13m47.8s, delta = -50 deg 55 arcmin 05 arcsec (1950) and contains two candidate stars of approximately 20th magnitude. No coincident point source of radio emission is known, but it is pointed out that the X-ray object lies 1.2 deg away from the centroid position of a gamma-ray source, CG 333 + 0. The observed flux from the object at the earth is approximately 7 x 10 to the -13th ergs/sq cm s in the 0.6-2 keV band. It is likely that the object is either a hot neutron star or an X-ray emitting pulsar. In contrast to the Crab and Vela pulsars, however, no extended X-ray emission associated with the point source is discernible. If the object is a hot neutron star, it has a blackbody temperature of 2 x 10 to the 6th K, and would represent the first detection of surface radiation from a neutron star.

Photon point source buildup factors for air, water, and iron

Abstract: Buildup factors for photons in infinite homogeneous samples of air, water, and iron have been calculated by a moments method code. The photons were assumed to be emitted from a point source. Comparisons of these results to values obtained earlier, both by experiment and by calculation, show reasonable agreement except in some instances of deep penetration. The parameters in the Berger empirical formula for buildup factors have been evaluated from the present work. The Berger formula is shown to fit the calculational results for nuclei of low atomic number at energies above 1 MeV and below 0.06 MeV. In mid-energy range, differences of as much as 40% are observed. The formula appears to provide excellent fit to the data for nuclei of medium atomic number.

Noise reduction by barriers on finite impedance ground

Abstract: The sound field due to a point source behind a barrier on ground of finite impedance has been calculated from five theories that differ mainly in their theoretical approach to diffraction and the model for ground impedance. These predicted values for the sound field have been compared with results measured outdoors using plywood barriers on different combinations of hard and soft ground. Each of these theories allows for interference due to differences between several paths of propagation, determined by the geometry of the source, receiver, barrier, and ground. One of these theories that shows good agreement with measurements, has been extended to calculate the sound spectrum level behind a barrier due to an incoherent line source, and further, to calculate the overall or A‐weighted sound level for a known source spectrum. Results suggest that there is a significant effect, due to the presence of the ground, that is much greater than that due to absorptive properties of the barrier. Results also predict sound level reductions that differ from predictions using well‐known barrier theories (most noticeably a smaller insertion loss): these differences can be of the order of 10 dB(A) depending on geometry, source spectrum, and acoustical condition of the ground.

Arrangement for locating radiating sources

Abstract: A radiation sensor for determining the angular location of a light source and locating a direction defined by a plurality of sources fastened to a movable structure such as a helmet-mounted sight visor. The sensor comprises a housing with an optical mask which is provided with transparent zones of circular shape, with the result that the omnidirectional radiation transmitted by a point source is confined within a cone. Radiation-detecting means constituted by linear arrays of photosensitive elements are disposed in a plane parallel to the mask so that the center of the projected circle corresponding to the angular direction of the source is determined by the positions of sensitized elements of the detecting strips.

A new method of measuring radio source parameters of a partially polarized distributed source from spacecraft observations

Abstract: An experimental system is analyzed which is designed to determine the angular and polarization properties of low frequency radio sources from measurements made on a spinning spacecraft. The system has been optimized to provide high accuracies for sources at high as well as low elevation angles. Theoretical expressions are derived for the response of this system to a partially polarized point source. Integrations are then carried out to get the system response to a uniform circular distributed source. Data processing techniques are derived so that computer simulations can be carried out to investigate the accuracy of this technique. It is shown that using 24 measurements of a partially polarized source (with Q = U = V = 0.5), taken in one spacecraft rotation, the resulting rms errors in angular position are less than one degree and the errors in determining the Stokes parameters are generally 1-10% for a wide range of source elevations.

High fidelity loudspeaker system for aurally simulating wide frequency range point source of sound

Abstract: The loudspeaker system utilizes an ellipsoidal reflector of substantially the same diameter as the low frequency (LF) driver, positioned on-axis of the LF driver, as a reflective dispersing means for increasing the angular dispersion of acoustical energy, especially in the upper range of the LF driver. The ellipsoidal reflector is also utilized as a means to mount the higher frequency (HF) drivers in such a way that their axes diverge generally uniformly away from a point near the center of the ellipsoid so that the sound field of the higher frequency drivers is also well dispersed. The result is that the ellipsoid causes acoustical energy both from the LF driver and the HF drivers to appear to emanate from a point near the center of the ellipsoid, simulating the sound field generated by a single wide frequency range point source of sound.

Spreading Of Light Beams In Ocean Water

Abstract: Laboratory measurements of the irradiance distribution due to a laser beam and the radiance distribution due to a point source in a hydrosol have been made and the results analyzed. The analysis indicated that after proper accounting for absorption, the distri­ butions may be scaled as a function of the scattering length rather than the more commonly used parameter, the attenuation length. The two distributions have also been shown to be equivalent as required by the optical reciprocity theorem. The measurements have given new insight into understanding the spreading of light underwater.IntroductionIn the study of the characteristics of the natural light field underwater, it is sometimes necessary to look at simple lighting geometries in order to understand some of the basic physical questions which arise. This is especially true in the case of the lighting distribution near the air-water interface or around artificial underwater light sources. In the boundary layer just below the air-water interface and within a couple of optical distances of it, the initial radiance distribution present at the interface (due to multiple scattering processes in the atmosphere) undergoes rapid and complex changes as a function of the inherent optical properties of the water column.These changes, mainly due to the shape of the volume scattering function of the hydrosol, have been extremely difficult to predict analytically due to the characteristics of the equation used to describe the propagation of light underwater. Solutions of this equation, the equation of radiative transfer, while being extensive and using many different approximation techniques, have yet to give adequate insight into the changes in the distri­ bution which occur due to spatial variations of the optical properties of the hydrosol.The same problem also occurs when studies are made of the lighting distribution around artificial light sources underwater. Although the initial lighting or radiance distribu­ tion is usually much less complex and more easily defined, the same analytic difficulties are present and prevent a full understanding of the character of the lighting distribution in the hydrosol.In-order to circumvent these theoretical difficulties experimental measurements were made of the underwater irradiance and radiance distributions produced respectively by a uni-directional point source (or a laser beam) and an omni-directional source. The measurements were made in a laboratory tank where the optical properties of the hydrosol could be varied and controlled.The resulting distributions, essentially the so-called beam spread and point spread, are important since theoretically knowledge of these distributions allows a user to compute the lighting distribution produced by any arbitrary source through use of the super-position or convolution integral1 '*.The hydrosol used in the laboratory experiments was adjusted so that its volume scattering function duplicated as closely as possible the volume scattering function of natural oceanic water. However, there were differences between the two hydrosols, therefore the final results presented should not be looked upon as a precise representation of the lighting distribution occurring in natural waters under similar lighting geometries. The primary objective of the research was to develop an understanding about the character of the lighting distribution underwater due to simple sources and the changes which occur as the optical properties of the hydrosol are varied. The measurements and their analysis has provided this understanding.Inherent optical propertiesIn characterizing the optical properties of a hydrosol, there are four important inherent properties. These are a, the volume attenuation coefficient; s, the total volume scattering coefficient; a, the absorption coefficient; and a, the volume scattering

Buoyant line and point source (BLP) dispersion model user's guide. Final report

Abstract: The BLP dispersion model was developed specifically for aluminum reduction plants. It is part of the UNAMAP version 4 collection. Aluminum reduction plants are a complex arrangement of emission sources, composed of parallel, low-level, buoyant line sources called potrooms interspersed, typically, by short point sources. In addition to theoretical considerations, BLP is based on extensive wind tunnel simulations of two reduction plants and an SF6 field study tracer program at one of the plants. The model has been verified using two years of SO2 measurements at the second reduction plant. The major features of the BLP dispersion model are: UTM or line source oriented coordinate system; multiple point source and finite buoyant line source capability; finite buoyant line source plume rise, plume enhancement due to multiple line sources; vertical wind shear in plume rise formulations for both point and line sources; transitional plume rise; incorporation of building downwash in both dispersion and plume rise calculations for point and line sources; terrain adjustment plume path coefficients; time-dependent pollutant decay; source contribution concentrations; and flexible post-processing package.

Real-time recording of light patterns in heavy hydrocarbons: a theoretical analysis.

Abstract: It has been experimentally shown in recent papers that the surface profile of a thermocapillary liquid submitted to heating by a nonuniform light beam undergoes a time-dependent deformation. The profiles corresponding to illumination with (a) a Gaussian laser beam, (b) a point source, (c) a fringe system, and (d) a circular Gaussian speckle pattern are calculated. This study sets some theoretical bases for practical utilization of highly viscous liquid films as transient photographic receivers.

Use of generalised resolution methods to locate sources in random dispersive media

Abstract: Random dispersive media such as the shallow-water sound channel cause the far field of a point source to be random in time and space. Consequently, resolution methods such as d.f.t., `maximum-likelihood estimator? or`maximum-entropy method? are useful only to a limited extent for estimation of source parameters such as range, bearing or depth. This is because those methods are based on the assumption that the signal wavefront is coherent. The resolution methods can easily be generalised to the situation in which the desired signal is random. A shallow water sound propagation model is used to demonstrate the application of generalised power estimators to the problem of locating point acoustic sources in shallow water.

Method and apparatus for determining acoustic properties in the earth

Abstract: A method and apparatus for determining the acoustic properties of the earth utilizes one or more identical, non-interacting, first spaced arrays of point sound sources and one or more identical, non-interacting, second spaced arrays of point sound sources with corresponding arrays of the first and second sources containing the same number of point sound sources in the same spatial relationship, with the spacing between the point sources of the second array or arrays being a factor α greater than the spacing between corresponding point sources of the first array or arrays. The energy of the radiation emitted by each point source of the first array(s) is greater than that emitted by the corresponding point sources in the second array(s) by a factor of α 3 .

New methods of X-ray diffraction spectrometry. I. Image formation in curved crystals

Abstract: Image formation in a dispersive system, consisting essentially of a perfect cylindrically bent crystal in association with a point source, has been studied both for transmission and reflection. The Laue and Bragg conditions have been calculated together with the dispersive power. The treatment has been extended to include two crystals. In the single-crystal case and also for most versions of two-crystal combinations a point or line focus is required to obtain highly resolved spectra. A few versions of the two-crystal arrangement can be made to be focusing, thereby enabling the use of an extended radiation source.

X-Rays from the Surface of Neutron Stars

Abstract: Observations of supernova remnants with the Einstein Observatory were discussed by Drs Fabbiano and S Holt Holt (1979) concentrated on line emission, while Fabbiano did so on continuum radiation both from the extended nebula regions and from point sources She reported on observations of possible point sources in four supernova remnants In this paper, we report on the results of the attempts to observe such point sources in five more supernova remnants, as well as nine radio pulsars, which have already been made with the Einstein Observatory We discuss what could come out of careful point source investigations of the Crab pulsar and the Vela pulsar with the Einstein Observatory and other X-ray space programs scheduled for the near future (AXAF, etc) Emphasis is placed on the possible theoretical implication of the outcome of such investigations

Ultraviolet pumping of Si/+/ fine-structure levels

Abstract: We describe an efficient method for the numerical solution of the multiline, multilevel radiative transfer problem and investigate the effects of radiative pumping through eight uv levels with 16 lines in the range 990 A < lambda < 1820 A, on the population of the doublet fine-structure levels of Si/sup +/. We provide analytic results for optically thin media, and numerical results for a grid of models representing optically thick, homogeneous, plane-parallel nebulae illuminated by a point source. The grid spans a wide range of values for the electron density, the strength of the uv source, and the line optical depth of the medium.

An algorithm for profile reconstruction in the presence of a known background slab

Abstract: An iterative inversion algorithm is given which facilitates the reconstruction of a plane-stratified profile from scattering data in the presence of a background slab. The inversion procedure, formulated within the bounds of the so-called, ‘distorted wave Born’ model, is given for both an incident plane wave and a point source excitation. While an exact inversion is achieved for an incident plane wave, the point source configuration is inverted asymptotically, thus excluding wave ‘trapping’ effects.

Note on Spectral Diffusivity Theory

Abstract: The diffusion equation with a wavenumber-dependent diffusivity is derived as an approximation to the statistical theory of plume dispersion from a continuous point source. The travel time explicitly appearing in the statistical theory is implicitly included in the wavenumber-dependent diffusivity K(k). By suitable choice of this function it is possible within a few percent to reproduce the centerline concentration. Due to the approximate nature of the method, however, it yields a non-Gaussian concentration shape close to the source. Results for the case of a crosswind extended source are compared to other commonly used methods for this problem. Some considerations are presented on the proper choice of averaging time for the concentration field.

Digital generation of accurate synthetic seismograms

Abstract: A point source insonifies a one mile deep, 200 layer, compressional velocity earth model, and the backscattered response is computed at surface receiver elements. The computational method is based on a plane wave decomposition of the spherical wave emanating from the source. Cylindrical symmetry is assumed in computing the surface responses. Physical and numerical phenomena affecting accuracy are illustrated: aliasing, critical angle effects, absorption and a new phenomenon we call tunneling. The accuracy is demonstrated by comparison with real field seismograms for an area consistent with the computational assumptions. This research is directed toward improving accurate seismic inversion techniques for geophysical data.

The contribution of normal modes in the bottom to the acoustic field in the ocean

Abstract: The effects of normal modes in the bottom on the acoustic field in the ocean are examined. The ocean bottom model consists of a slow isovelocity layer overlying an isovelocity half‐space to simulate the characteristic sound velocity drop at the water–bottom interface. Attention is focused on the perfectly trapped modes which are excited in the layer by inhomogeneous waves emitted by a point source in the water column. The relative normal mode contribution to the total acoustic field in the water is calculated analytically for a near‐bottom source/receiver geometry and evaluated for representative ocean bottom examples. It is shown that, for combined source/receiver heights less than a wavelength, the field is dominated by the leaky mode contribution at short ranges (≲2 km) and the trapped mode contribution at long ranges (≳2 km). For fixed bottom parameters, the trapped mode contribution increases exponentially with decreasing combined source/receiver height. It is also shown that, for a fixed layer waven...

Temperature stresses in an infinite disk having a heat source in the vicinity

Abstract: Thermoelastic stresses in a thin infinite disk heated by radiation from a point source situated at a finite distance are studied.

Comparison Of Coherent And Incoherent Imaging In The Location Of Point Sources

Abstract: The position location accuracy of a point source in the fields of view of two telescopes with the same focal plane is considered. It is shown that for detectors sized to equal the distance to the first zero of the coherently added system, there is little, if any, difference in accuracy between a system that adds the flux coherently as opposed to a system that adds it incoherently.

Spectral broadening of waves propagating in a random medium

Abstract: Formulas for the cross‐correlation and spectral density functions of the scalar wave field radiated by a random point source in a weakly inhomogeneous three‐dimensional time‐dependent random medium are derived. The medium is assumed to be statistically homogeneous and isotropic and to be statistically independent of the source. The analysis is based on a modification of the smoothing method. An approximate expression for the power spectrum of the wave as a function of the source‐field point distance (or propagation distance) is obtained for the case in which the characteristic frequency of the source is much greater than that of the medium. This expression shows that the wave spectrum approaches a limiting form, which is referred to here as the fully developed spectrum, with increasing propagation distance. It is also found that the total signal power is conserved as the spectrum evolves. Results obtained for the case of a narrow‐band source indicate that the spectral bandwidth increases initially as the square root of the propagation distance, but that at larger distances it approaches a limiting value. Numerical results obtained for the narrow‐band case show a progressive broadening of the wave spectrum with increasing propagation distance and/or with increasing strength of the randomness of the medium, in agreement with observations.

Energy deposition from an anisotropic point source in an infinite medium

Abstract: Using a simple model of scattering we have obtained an exact solution of the transport equation for a point anisotropic source in an infinite, homogeneous medium. The energy deposition function is derived and numerically evaluated for values of mass ratio (target/projectile) equal to 0.2, 0.4, 0.6, 0.8, 1,2,5 and 10. The results show the marked anisotropy of the distribution as the mass ratio decreases. We have also derived an expression for the fraction of energy deposited in the medium z<0 as a function of the radial co-ordinate (x2 + y2)1/2. This function is useful for studying the region of influence of the energy sputtered from a point source.