Omnidirectional Point Source Research Articles

Source location in an urban setting using time reversal with small sensor arrays

The finite-difference, time-domain (FDTD) method was used with time reversal to explore acoustic pulse source location in an urban setting with the source outside the array of sensors. The FDTD program simulates two-dimensional sound propagation from an omnidirectional point source, including reflection and diffraction from buildings. Pressure versus time is recorded at various hypothetical sensor locations. During time-reversal playback, each sensor becomes an omnidirectional point source generating a time-reversed version of the pressure it received. A number of sensor arrays were modeled in various urban settings to identify the limitations of using time reversal to locate a non-line-of-sight source outside the array. In general, as the number of sensors and spatial extent of the array increases, the ability to locate the source increases as well. A 19-sensor hexagonal array 8 m across, for example, is capable of locating a source on the opposite side of a building. Two small four-sensor arrays 10 m from each other are adequate for around-the-corner source location. A single small array of three or four sensors spaced 1 to 2 m apart was generally insufficient to locate the source.

Averaged response of a horizontal linear antenna in a shallow sea

Response of a horizontal linear receiving antenna combining the powers of incident normal waves in a shallow sea is studied. The acoustic field is produced by an omnidirectional monochromatic point source. The antenna aperture is comparable to or smaller than the sea depth.

Directivity pattern of neurosurgical endoscopic ultrasonic probes

The objective of this paper is to present recent investigations in characteristics of the sound field generated by neurosurgical endoscopic ultrasonic probes (NEUPs) for minimally invasive surgery. The importance of this information has been investigated and discussed taking into account following facts: 1. According to the International Standard IEC 61847:1998 basic acoustical output characteristics of ultrasonic surgical equipment is declared for and measured in an acoustical free field. The standard generally treats the ultrasonic probe as an omnidirectional point source of the zero order (monopole source). 2. In real conditions, operations with NEUPs are performed within the acoustical near field. Having in mind that the cavitational and hydrodynamic effects are dominant, two theoretical boundary conditions can be present. The first one takes place when operations are performed near the “soft” acoustical boundary (tissue/air), and the second one is near the “rigid” acoustical boundary (tissue/bone). Reflections of sound waves from boundaries have influence on the characteristics of the ultrasonic probe (transducer) and on the sound field. In such cases spherical waves of the first and second order are generated. Directivity of sound sources takes shape in the far field and is easier to measure there. On the basis of measured directivity patterns, the influence of different operational conditions (immersion depth of the probe tip, boundary type, acoustical impedance of the medium etc.) on the radiated sound power and spatial distribution of the sound pressure can be estimated.

Sound diffraction by a partially inclined noise barrier

A new analytic method is presented for predicting the performance of a partially inclined barrier. The insertion loss of the barrier is predicted by summation of multiple diffractions occurring at the top and corner points, at which convex and concave edges are formed. Kouyoumjian and Pathak's diffraction theory (Kouyoumjian RG, Pathak PH. A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface. Proceedings of the IEEE 1974;62:1448–61) is employed to construct diffraction coefficients for both convex and concave edges, whereas the barrier thickness is neglected and an omni-directional point source is assumed. A scale model experiment is performed in an anechoic room. Using measured speaker directivity and theoretical directivity formula does not show any significant changes in prediction. However, including additional diffraction at edge points due to finite thickness leads to improvement by removing many small oscillations in the frequency domain. The comparisons of theoretical and experimental insertion losses show reasonable agreements. For the two-dimensional case, predictions with and without thickness effect are compared to numerical result obtained by boundary element method (BEM), in which the smoothing phenomenon is more clearly observed and good agreement is found.

New design concepts for the construction of an omnidirectional sound source

This paper presents a study for the optimum design and construction of an omnidirectional point source. The point source model of theoretical acoustics is used as an elementary unit for electroacoustic measurements in the area of room and free-field acoustics. The spherical radiation requirement for the entire acoustic spectrum and the finite dimensions of the point source model contradict the construction in practice. This is because of high-frequency beaming and the existing dimensions of radiators. A new design approach of using horn loaded cone loudspeakers and properly placed high-frequency diffusers was used to minimize these problems. Twelve radiators loaded on pentagon-shaped horns were placed in a dodecahedron topology to build a practical point source with a useful frequency response of 80–16<th>000 Hz and an omnidirectional radiation pattern up to 12<th>000 Hz. The results display improvement in the radiation pattern compared to those of the already existing point sources, due to the use of horns and diffusers. Increase in the sound pressure level is also displayed because of the obvious increase of the directivity index of each individual radiating element.

Sound-Propagation Curves in Industrial Workrooms: Statistical Trends and Empirical Prediction Models

Predictions of workroom noise levels are based on predictions of the workroom sound-propagation curve, SP( r) – the variation with distance r from an omnidirectional point source of the sound-pressure level Lp ( r) minus the source sound-power level Lw SP( r) = Lp ( r)- Lw. While more accurate approaches such as ray tracing exist, from a practical point of view there is considerable scope for developing simple empirical prediction methods. In fact, several such models exist. However, these have short-comings which warrant the development of a new model. The approach taken here was to predict the slope(s) and absolute level(s) of the sound-propagation curve, approximated by one or more straight-line segments. With this in mind, octave-band sound-propagation measurements were made in a number of empty and fitted workrooms. The curves were approximated by one or two straight-line segments. The intercepts and slopes of the segments were then determined, and their averages and standard deviations calculated. The statistical trends provide information on the behaviour of sound in industrial workrooms. The results are used to develop simple empirical prediction models.

LSVOCN: A pulse-propagation model for a linear, space-variant ocean

A full-wave, pulse-propagation model for three-dimensional wave propagation in a Pekeris waveguide based on linear systems theory is presented. The randomly rough ocean surface and bottom are accounted for via coherent (average) reflection coefficients. Attenuation due to absorption in all three fluid media is included. Transmit and receive planar arrays with beam steering can be simulated and, as a result, vertical arrays and a single, omnidirectional point source are automatically included. A built-in signal generator can simulate arbitrary amplitude and angle-modulated carriers. Outputs from this model include plots of the magnitude and phase of the ocean surface and bottom reflection coefficients, the complex acoustic field across the receive array. Because of its highly modular structure, the model can also be used to generate pulse-propagation solutions using any time-harmonic solution such as normal mode theory or the parabolic equation method. Preliminary computer simulation results are presented. [Work supported by ONR, Code 11250A, and the Naval Postgraduate school.]

Normal mode sound field of a directional radiator

In this paper, the sound field of a general type of directional radiator in a stratified medium is treated, and the concept of directivity is applied to calculation of the normal modes. The result shows that the normal mode field of a directional radiator can be obtained by supplementing the normal mode expression of an omnidirectional point source with the directional excitation function, which is dependent on the position and directivity of the radiator. In addition, the normal mode fields of radiators with vertical-symmetrical, vertical-antisymmetrical, single-sided and sharp directivities are calculated, respectively. For a vertical line array in a homogeneous water layer, if the source distribution is proportional to the eigenfunction of some normal mode, the zeros of the directional excitation function correspond precisely to the directions of the eigenrays of other normal modes.

Aperture antenna effects after propagation through strongly disturbed random media

A strongly disturbed layer of ionization irregularities that is used as a propagation channel for radio waves can degrade the propagating wave and thereby affect the resulting measurements at the receiving antenna. The antenna aperture itself also affects measurements of the received signal by its inherent averaging process. Here an analytic solution for the two-position, two-frequency mutual coherence function, valid in the strong-scatter limit, is used to characterize the propagation channel. The channel itself consists of a thick slab of anisotropic electron density irregularities that are elongated in the direction parallel to the earth's magnetic field. Analytic expressions are obtained that give the effect of the aperture antenna on measurements of received power, decorrelation time (or distance), mean time delay, time delay jitter and coherence bandwidth. These quantities are determined as functions of the aperture diameter and of the angle between the magnetic field and the direction of propagation. It is shown that in strong turbulence aperture averaging can be a significant factor in reducing the received power by angular scattering loss, increasing the observed signal decorrelation time via aperture averaging, and reducing the time delay jitter by suppression of signals received at off-boresight angles. Results are presented for two cases. One-way propagation through an ionospheric communication channel is considered where both transmitter and receiver utilize aperture antennas. This result is easily extended to the case that one of the antennas is an omnidirectional point source, corresponding to the usual ease of transionospheric satellite communication from a small satellite antenna to a large ground based receiver. The second case involves transmission and reception of a radar signal that travels through a disturbed ionospheric channel to a target located in free space. This ease is applicable to the situation of a large antenna aboard a space based radar or to the case of a ground based defense radar.

NORMAL-MODE SOUND FIELD OF DIRECTIONAL RADIATOR

In this paper, the sound field of a more general directional radiator in a stratified medium is treated, and the concept of directivity is applied to calculation of the normal modes. The result shows that the normal-mode field of a directional radiator can be obtained by supplementing the normal-mode expression of a omnidirectional point source with the directivity-excitation-function, which is dependent on the position and directivity of the radiator. In addition, the normal-mode fields of the radiators with vertical-symmetrical, vertical-antisymmetrical, single-side and sharp directivity are calculated, respectively.For the vertical line array in a homogeneous water-layer, if the source distribution is proportional to the eigenfunction of some normal mode, the zeros of the directivity-excitation-function are just corresponding to the directions of eigen-rays of other normal modes.

The prediction of sound fields in non-diffuse spaces by a “random walk” approach

A Markov process, representing random walks of acoustical phonons in an enclosure, is used to predict the steady state high frequency sound pressure levels in complex internal spaces, excited by an omni-directional point source. In the theory, developed from work by Gerlach, one considers the three dimensional random walks of phonons inside an enclosure of any internally complex geometry. The sound pattern is derived by considering the probability that a phonon, leaving some source, will radiate to a particular wall, undergo a certain path of successive reflections, and be radiated to a detection point. The spatial density of phonons, at a given location, arising from a large ensemble of phonons traversing different random paths, gives rise to an intensity. Knowledge of the sound intensity at a suitable number of detection points, in a regularly spaced lattice, enables the sound field inside an enclosure to be estimated. The advantages of this approach, over others is that, in determining the location of each phonon, and hence the sound intensity, room shape and mutual relationships among internal partitions, and, subsequently, the past history of the phonon, are all taken into account.

Multiple-reflection diffuse-scattering model for noise propagation in streets

The sound field generated by an omnidirectional point source in an infinitely long, straight street is considered. The field is assumed to be the sum of a multiply-specularly reflected field and a diffuse field that is fed from scattering at the walls at each reflection of the specular field. It is shown that scattering is important close to the source. The sound level depends on the width of the street and the height of the walls and on the reflection and scattering coefficients of the walls.

Scattering of acoustic point‐source fields by random surfaces

Estimates are obtained for the pressure field reflected from random rough surfaces when the surfaces are insonified by a high‐frequency, omnidirectional point source. A geometric acoustics or ray‐tracing approach is taken together with the assumptions that there is no shadowing of the surface and that each ray undergoes a single reflection. A stationary phase approximation of the Helmholtz integral is used to estimate the field associated with each ray. The emphasis of the paper is on accounting for the phase differences, that is, the interference effects, between the possibly very many geometric rays from source to surface to receiver.Subject Classification: 20.30.

Scattering of acoustic point source fields by sinusoidal and by random surfaces

Estimates are obtained for the pressure field reflected from both sinusoidal and also random rough surfaces when the surfaces are insonified by a high-frequency, omnidirectional point source. A geometric acoustics or ray-tracing approach is taken together with the assumptions that there is no shadowing of the surface, and that each ray undergoes a single reflection. A stationary phase approximation of the Helmholtz integral is used to estimate the field associated with each ray. The emphasis of the paper is on accounting for the phase differences, that is, the interference effects, between the possibly very many geometric rays from source to surface to receiver.

Normal-mode theory of underwater sound propagation from directional multipole sources

Normal-mode theory of underwater sound propagation, which so far has been established only for omnidirectional point sources, is developed here for the case of sources with arbitrary spatial extent and directionality. This is accomplished by expanding the source in a series of spherical multipole distributions. From the Green's function representation, the sound field is then obtained for arbitrary multipolarity of the source, and it simplifies considerably for a point multipole source. Our results are illustrated by a calculation of range-focusing effects in a channel with parabolic velocity profile using a dipole source.

Normal-mode analysis of underwater-sound propagation with directional sources and receivers

Standard normal-mode analysis of underwater-sound propagation assumes only an omnidirectional point source. Other writers have proposed that, for use in calculations, a virtual line-array of discrete point sources, arranged vertically, can well approximate the directivity pattern, in the vertical plane, of actual sources. It seems better, however, to use an equivalent vertical line array with a continuous distribution of strength and phase. The equations for relating the actual source and the line source are given, and their solution is discussed. Identical methods allow the calculation of reception by a directional receiver. A related topic, use of a vertical array of discrete elements to stimulate or receive a single mode, is also discussed. An illustrative directivity pattern is presented, for an array adjusted to select the first mode in a shallow-water problem.

The precipitation of airborne salts in the Haifa Bay, Israel : Loewengart S., 1964. Israel Jl. Earth-Sci., 13: 111–124.

In this paper, the sound field of a general type of directional radiator in a stratified medium is treated, and the concept of directivity is applied to calculation of the normal modes. The result shows that the normal mode field of a directional radiator can be obtained by supplementing the normal mode expression of an omnidirectional point source with the directional excitation function, which is dependent on the position and directivity of the radiator. In addition, the normal mode fields of radiators with vertical-symmetrical, vertical-antisymmetrical, single-sided and sharp directivities are calculated, respectively. For a vertical line array in a homogeneous water layer, if the source distribution is proportional to the eigenfunction of some normal mode, the zeros of the directional excitation function correspond precisely to the directions of the eigenrays of other normal modes.

An Approximate Solution to the Acoustic Radiation of a Finite Cylinder

Utilizing the assumption of an omni-directional point source and varying degrees of acoustical transparency associated with a finite cylindrical shell with rigid end caps, the authors have derived two separate approaches to the problem of far-field sound radiation emanating from a normal mode of vibration. In addition, a third approach is obtained by expanding the expressions of Laird and Cohen and applying them to a shell-band source on an infinite cylinder. Several different environments are considered for each approach and for one of these environments, the results are compared to the experiment. The comparison of these theories indicates that for acoustic wavelengths considerably greater than structural wavelengths and dimensions, the assumption of omni-directional point sources and acoustical transparency is reasonable even though the source is curved and “unbaffled.” At higher frequencies, the pressure levels and directivity in the far field are approximately independent of the transparency or opaqueness of the source, thus indicating that an “equivalent” transparent cylinder of infinitesimal spherical sources could give, at most, only three- or four-db cancellation effects when a single mode excitation basis is assumed.