摘要:

Shallow‐water propagation experiments conducted on the Scotian Shelf in 1978 produced very high transmission losses at low frequency for a range of only 4 km. The bottom along the propagation track consisted of a thin layer of coarse sand over granite bedrock which protruded through the sediment in certain regions. The sources of the high losses at low frequency are investigated by varying the acoustic properties of the geoacoustic model. It is shown that neither scattering nor shear effects in the granite basement accounts for this loss, but that a third mechanism, excitation of shear waves in the sediment, can cause such high losses.

ISSN:0001-4966    

DOI:10.1121/1.394180

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Transient acoustic wave fields in the vicinity of caustics in both two and three space dimensions are considered. In the vicinity of some possible caustics, the time history of the wave field due to an impulsive source includes a precursor. This noncausal behavior is reconciled with the physical principle of causality by requiring that such caustics be a part of certain higher dimensional caustics. With this constraint all noncausal arrivals are preceded by a causal arrival and the total time history is causal. This constraint is used to derive some simple general properties of transient wave fields in the short wave limit, e.g., the number of arrivals must be an odd number.

ISSN:0001-4966    

DOI:10.1121/1.394181

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper presents initial analysis results of ambient noise measurements from an active ice pressure ridge over a 3‐day period in April 1979. The site was a research ice camp 180 miles from the pole in the Eurasian Basin of the Arctic Ocean. The ridge was instrumented with calibrated hydrophones paired in both the horizontal and vertical directions. Their distance from the ridge was about 100 m and their outputs were tape recorded. Dimensional surveys of the active portions of the ridge were made at the surface. Source level linear density is deduced from ambient noise measurements near the pressure ridge by assuming the ridge to be a line source of spatially incoherent point sources. Measured Arctic transmission loss data are used to deduce an active pressure ridge average separation compatable with previously measured omnidirectional ambient noise data statistics for the month of April. The effects of surface/image interference are present in the data and allow a preliminary estimate of acoustic source depth of between 14–18 m. The ridge length was approximately 500 m and was formed when a 1 m‐thick refrozen lead was pushed on top of a thicker 4‐m ice floe. As such, this ridge was, most likely only one ‘‘subridge’’ along the length of the boundaries of the two interacting floes. Recent SEASAT SAR data from October 1978 have shown that ice pans (defined as groups of separate ice floes that move together) can vary from 10 to 100 km in linear dimension and, unless pan size changes significantly from October to April, extremely long ridges (composed of smaller discrete ridges such as the one analyzed in this study) may exist between pressured boundaries of adjacent pans. A uniform spacing of ridges was assumed for noise model calculations and, with an active ridge spacing of 37 km, model noise level estimates and measured 50 percentile noise levels of April were in good agreement. An average ridge was assumed to be 2 km in length consisting of two active 500‐m subridges. It is not known whether this ridge was typical in level, length, etc., since these were the first‐known quantitative acoustical measurements of ambient noise from a nearby pressure ridge.

ISSN:0001-4966    

DOI:10.1121/1.394182

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A theoretical model of acoustic propagation in the ocean around a conical seamount is presented. The sea surface and the surface of the seamount are assumed to be perfect reflectors, with the apex of the seamount just touching the surface of the ocean. This geometry is such that the boundary surfaces of the channel can be represented by the surfaces of a separable (spherical polar, origin at the apex) coordinate system. The solution of the Helmholtz equation for a point source in the channel is shown to consist of a sum of normal modes in which the variable is angular depth, and in which the mode amplitude functions contain the entire range/azimuth dependence of the field. These mode amplitude functions show that, in each mode, there is a sharply defined shadow zone lying behind the seamount, and surrounding the shadow is an ensonified region where the field shows very rapid spatial oscillations. This oscillatory behavior indicates that strong interference occurs within a single mode. An important feature of the model is that it specifies completely the three‐dimensional field around the seamount, giving both amplitude and phase. The theory is therefore relevant to studies of the performance of acoustic arrays in this type of environment.

ISSN:0001-4966    

DOI:10.1121/1.394183

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

An expression derived recently for the acoustic field from a point source in the channel around a conical seamount involves an integral with a highly oscillatory integrand. This integral is examined here and evaluated using the method of stationary phase. A relatively simple approximate expression for the spatial dependence of the field is obtained whose detailed properties compare very favorably with those of the original formulation.

ISSN:0001-4966    

DOI:10.1121/1.394184

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Single compact scatterers insonified at frequencies near their intrinsic radial resonance ω0act primarily as monopole sources, amplifying the local pressure field by a factor of order (kRa)−1,abeing the scatterer radius andkRthe wavenumber at ω0in the surrounding material (kRa≂1.37×10−2for air bubbles in water). This paper deals withsystemsof such scatterers (bubbles, inflated balloons, thin shells in water) which, through couplingviamultiple scatter, may themselves exhibit true resonant modes at frequenciesclosetoω0, amplifying the local pressure field by a further factor of order (kRa)−1, leading to net amplifications in excess of 103for bubbles or balloons, i.e., of over 120 dB for the intensity. This phenomenon, referred to assuperresonance, is predicted by formulas for the equivalent source strengthBof each scatterer of the system, proper account being taken of multiple scatter between all elements, using a self‐consistent scheme. It has long been known that for a pair of scatterers (doublet), or in periodic lattices,Bexhibits maxima for certain values ofx=kl(lbeing the distance between scatterers), which had been called resonances [Twersky, J. Opt. Soc. Am.52, 145–171 (1962)], but are in fact only partial (quasi) resonances. It is demonstrated here that, under suitable conditions, true resonances exist; i.e., in the absence of attenuation,Bexhibitsrealpoles(the infinities are removed by introduction of acoustic damping and finite radii). The existence of such superresonances is furthered by the presence of elastic boundaries in the fluid near the system (e.g., a thin plate or a solid half‐space). Multiple scatter coupling then occurs by means of guided flexural or surface waves, which offer a more efficient energy transport mechanism between scatterers than the acoustic modes (volume waves) of a homogeneous fluid fullspace.In the latter case, a doublet of resonant bubbles, or shells, exhibits no superresonances (poles), only partial ones (quasiresonances); it takes at least three scatterers to create a true superresonant system in a fullspace, and even so there is only one resonant spacing (kl=1.8955, ω=1.0022 ω0for an equilateral triangle of bubbles in water). But near a thin plate, both doublet and triplet configurations develop spectra of resonant spacingsxnand frequencies ωn. Near an elastic solid half‐space, Stoneley (Scholte) wave coupling creates similar effects. The role of small interface roughness on this phenomenon is also examined.

ISSN:0001-4966    

DOI:10.1121/1.394185

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A recent experimental investigation [Mazeetal., J. Acoust. Soc. Am.77, 1352 (1985)] has shown that the spectrum of the resonance frequencies of plates can be obtained using ultrasonic pulse techniques. The distortion of incident wave trains upon their reflection, when the carrier frequency is near a resonance frequency, is analyzed here; the physical origin of plate resonances can be elucidated, the observed ringing of the resonances being caused by multiple internal reflections. A series of backscattering experiments, employing wave trains with carrier frequencies on and off a resonance, has been interpreted in the mentioned fashion, with very good agreement between theory and experiment.

ISSN:0001-4966    

DOI:10.1121/1.394187

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Previous measurements have shown that there is a marked change in the attenuation of an acoustic impulse above grassland as it propagates into a meteorological shadow zone. This paper extends the theoretical calculations by considering penetration of the boundary by creeping waves. Calculated pulse waveforms for various ground impedance models are compared with the experimental results. Discrepancies between the pulse shapes and overprediction of the excess attenuation may be caused by the restrictive approximations used in applying the theory or imply that other mechanisms contribute to the sound penetration.

ISSN:0001-4966    

DOI:10.1121/1.394146

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A wave composed of weak and short waves (WS waves) in a dynamically deformed elastic material is studied theoretically by using quasigeometrical acoustics. The material considered is one dimensional and inhomogeneous with a nonlinear stress–strain relation. A general theory for analyzing the composite wave is exploited. It is shown to the second‐order approximation that the analysis of the composite wave can essentially be reduced to the analysis of a single WS wave. As an example of the general theory, a head‐on collision between two WS waves is investigated.

ISSN:0001-4966    

DOI:10.1121/1.394147

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The reflection and transmission characteristics of a grating of elastic cylinders arranged periodically in an elastic slab are examined. A simple approach synthesized from Fourier–Bessel expansions and theT‐matrix method has been used for analysis of a two‐dimensional problem involvingSHwaves only. It is observed that the nonspecular modes are generally very weak for such gratings in contrast with those for gratings made of periodic bimaterial interfaces.

ISSN:0001-4966    

DOI:10.1121/1.394148

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP