摘要:

A 1960 experiment is examined in which sound from three underwater explosions near Perth, Australia, was detected near Bermuda. A recent attempt [Munketal., J. Phys. Ocean.18, 1876 (1988)] to calculate propagation paths for this event included rotational flattening of the Earth and horizontal refraction determined from the vertical sound speed minimum. That calculation left Bermuda in a shadow zone. The current work invokes adiabatic mode theory to include refraction due to horizontal variations in the vertical mode structure. These results include separate horizontal rays for each of the first few vertical modes, using an archival data set of 230 ocean sound profiles to generate the modes numerically. Where appropriate, interaction with bathymetry is included. This solution possesses two eigenray groups: Group A passes just south of the Cape of Good Hope, at which point group B is almost 1000 km to the south. Intermediate rays are blocked by islands. Group A proceeds unimpeded to Bermuda for a total time of flight of 13 354±5 s, while group B interacts slightly with bathymetry off Brazil, arriving at 13 403±9 s, and suffering roughly 7–12 dB more bottom attenuation. The spread in these arrivals overlaps satisfactorily with experimental data (main arrival at 13 364±5 s; pulse train half‐width 15 s; second arrival 30±5 s later roughly 10 dB below the first arrival).

ISSN:0001-4966    

DOI:10.1121/1.402062

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

It is known that if single frequency continuously transmitted ultrasound or electromagnetic energy is reflected from ‘‘straight line flow,’’ defined here as one or more scatterers moving with constant velocity along an infinite straight line, the Doppler effect will shift the echo spectrum center frequency from the transmitted value, and broaden its bandwidth. It is proved thatifsuchstraightlineflowisshiftedlaterallyorinrangeanywhereinthefield,i.e.withoutchangeoforientation,itsDopplerbandwidthremainsunchanged. (The ‘‘Doppler bandwidth’’ is here defined as the frequency difference between the extrema of the echo power spectrum.) The theorem holds true even though the time domain echo changes dramatically with motion of the flow path, and is believed to be valid for electromagnetic as well as ultrasound waves. Its implications with respect to flow measurement, as well as preliminary experimental and computational confirmation, will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.402063

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

It is shown experimentally and theoretically that waves propagate around circular cylindrical shells with a phase velocity that is less than the phase velocity of sound in water. These waves are called Scholte–Stoneley waves. First this paper presents results on air‐filled aluminum pipes of different ratios of the internal radiusbto the external radiusa. The frequency resonances and the ordersnof the resonances increase with theb/aratio. The frequency range of the observed resonances is fixed for a given pipe. This is related to the evolution of the resonance width with frequency. The frequency resonances and their width are calculated with the use of the resonance scattering theory. The so obtained results agree quite well with the experimental ones. The Scholte–Stoneley wave, which propagates around the shell, is compared to the antisymmetric Lamb wave, which appears on a thin plate when the plate is imbedded in water. Second, the influence of the internal fluid on the Scholte–Stoneley resonances is studied. Three different liquids are used: water, chloroform, and glycerol. The experimental results agree once more with the theoretical ones.

ISSN:0001-4966    

DOI:10.1121/1.402389

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper develops in more detail the theory for a fully three‐dimensional version of an earlier boundary integral equation method (BIEM) formulation, for the computation of the scattering of underwater sound from compact deformations of an oceanic waveguide’s surfaces. The method allows for three‐dimensional sources and boundary deformations in an otherwise uniform waveguide. The technique involves only integrations over the compact surface of the waveguide deformation. The implementation is illustrated for a seamountlike deformation in an oceanic waveguide.

ISSN:0001-4966    

DOI:10.1121/1.402064

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The Rayleigh–Fourier and extinction theorem method mathematical formalisms are established and compared in the case of acoustic scattering in a waveguide with impedance boundary conditions and one nonplanar surface. It is shown that the two methods are theoretically equivalent by using the properties of the solution given by the extinction theorem method. This result, which is confirmed by a perturbation approach, may be of practical importance in view of the better computational efficiency of the Rayleigh–Fourier method.

ISSN:0001-4966    

DOI:10.1121/1.402065

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Radiation of sound from a submerged and freely flooded tube, closed at one end and open at the other, follows well‐known relationships if the exit dimension is much smaller than a wavelength. If the tube is instead filled with a gas and submerged in water, the radiated power is higher than that of the flooded tube for the same internal acoustic pressure. If the density‐sound‐speed product, ρc, is much higher in the liquid than in the gas, as would normally be the case, then the radiated power is greater by the factor 1/0.36(ka)2, wherekis the wave number in the water andais the tube radius. This provides an ideal setting for construction of an underwater acoustic source using a standing‐wave thermoacoustic driver since this driver is force limited rather than displacement limited. The performance is ultimately limited by the hydrostatic pressure and the dimensions of the tube.

ISSN:0001-4966    

DOI:10.1121/1.402066

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

This paper addresses the scattering of seismoacoustic energy from rough water–solid interfaces. A two‐dimensional elastic finite difference method with a velocity‐stress formulation is used for all of these models, including all phases, multiples, interface, and interference waves. Initial sinusoidal seafloor models at 15 grid points per wavelength (ppw) show that even medium slopes can greatly increase the amount of energy (especiallyPtoSconverted energy) which is transmitted into the bottom. Scattering from microroughness (stair‐step definition of a sloping seafloor) is significant for 10‐m steps with a pressure pulse source of 10‐Hz center frequency. Additional models with much finer grids (30 and 60 ppw) were computed to decrease the microroughness scattering and to check the accuracy of scattering from 10‐m steps. Most of the microroughness scattering disappears for the 60‐ppw model. Scattering strength is similar for all models with 10‐m microroughness. Reciprocal models at 15 and 30 ppw (both with 10‐m stair steps) show very good agreement. An important conclusion of this work is that the specification of complex seafloors requires a much finer grid than is necessary for numerical stability and accuracy. This will be true for other numerical methods as well.

ISSN:0001-4966    

DOI:10.1121/1.402067

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

In this paper a closed‐chamber technique for measuring the complex free‐field sensitivity and directivity patterns of long underwater acoustic arrays is described. Performing these free‐field calibrations normally requires a large body of water to ensure that the reflections from the sides, surface, and bottom have a negligible effect on the measurement. The chamber contains acoustic projectors and monitoring hydrophones located along its length. The projectors are driven to produce an acoustic pressure field in the chamber that is equivalent to a free‐field plane acoustic wave propagating from any angle of incidence. This is accomplished by first measuring the transfer matrix for the calibration system. The transfer matrix relates the sound pressures at the hydrophone locations to the electrical drives to the projectors. The required projector drives, both amplitude and phase, are then determined by inverting the transfer matrix. A prototype calibration system has been developed. The system can operate over the frequency range from 10 to 600 Hz and is capable of simulating depths to 410 m. Directivity patterns obtained using the prototype for a test line array are in good agreement with theoretically predicted patterns.

ISSN:0001-4966    

DOI:10.1121/1.402068

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Associative feedforward neural networks with no hidden layers were applied to the problem of localizing a source in range and depth using the acoustic signal arriving at a vertical array of sensors. A highly processed form of the signal (excitations of an orthogonal basis) was used as input in order to increase the robustness of the trained network. The output layer consisted of one unit for each possible range and one unit for each possible depth of the source. The networks were trained with a signal‐to‐noise ratio (S/N) at the hydrophone of 50 dB, and then their performance was evaluated with S/Ns of 50 and 0 dB. Network weights were found for narrow and broad target shapes that correspond to narrow and broad beamshapes. The narrow target produced the beamformer with the lowest sidelobes and highest gains with acceptable but somewhat higher sensitivity to noise. Performance in the region for which the network was trained compared favorably with minimum variance beamforming.

ISSN:0001-4966    

DOI:10.1121/1.401860

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Measurements of reverberation and attenuation as a function of frequency have been made in a pine forest at ranges from 10–100 m, paying particular attention to diurnal variation in meteorological conditions and effects associated with receiver location. Up to 1 kHz, the measured excess attenuation is modeled tolerably well by a conventional point source calculation using a two‐parameter ground impedance model. It is found that the observed variations in meteorological profiles do not produce significant changes in the measured 100‐m attenuation at any frequency. It is proposed that high‐frequency attenuation and reverberation for ray paths below the foliage canopy are dominated by multiple scattering among the tree trunks. This is modeled numerically by a stochastic particle bounce procedure. The results of calculations indicate the relative importance of multiple scattering in reverberation and attenuation. However, the model is unable to verify the strong height dependence shown by the measured attenuation. The results suggest an interaction between ground effect and trunk scattering that is more complicated than simple addition as proposed hitherto. It is shown that the measured attenuation is considerably underpredicted by the Dutch–Scandinavian noise prediction scheme. The A‐weighted imission level of road traffic noise transmitted through 100‐m woodland of this type is significantly lower than that transmitted over 100‐m pasture.

ISSN:0001-4966    

DOI:10.1121/1.401861

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP