摘要:

Here a high-frequency acoustic experiment through a turbulent flow in Cordova Channel is described which is specifically designed to measure the contribution of scalar and vector fluctuations to the total scattered signal over a 48-h period. The effective refractive index fluctuations are determined by both the random changes in sound speed (scalars) and the random motion of the medium (vectors), and so in this paper it is demonstrated that reciprocal acoustic transmission is a technique which unambiguously separates their effects. The effective refractive index structure parameter,Cηeff2is defined as the sum of scalarCηs2and vectorCηv2contributions through the equationCηs2+11/6Cηv2.The effective refractive index structure parameter is also measured from the forward propagation acoustical scintillation variances, thus providing an independent check on the interpretation.

ISSN:0001-4966    

DOI:10.1121/1.421116

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Amplitude and phase fluctuations of monochromatic acoustic signals traveling through diffuse mid-ocean ridge hydrothermal vent plumes are modeled using existing theory in an attempt to find suitable frequencies and path lengths for plume monitoring. Weak-scattering solutions are evaluated numerically, with model parameters adjusted to match observed plume characteristics. Constraints required for weak-scattering solutions to be valid can be met for transmission ranges of 500–2000 m and frequencies of 20–80 kHz. Therefore, because fluid structure and scattering strength are more closely linked for weak scattering than for stronger scattering, inversion for fluid statistical properties may be possible, enabling diffuse vent monitoring. Such monitoring would be subject to geometric assumptions such as transmission entirely within a statistically homogeneous plume. Performance-limiting phase fluctuations have also been computed for a 13–17 kHz geodetic survey system.

ISSN:0001-4966    

DOI:10.1121/1.421092

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A perturbation model for high-frequency sound scattering from an irregular elastic sea bed is considered. The sea bed is assumed homogeneous on the average and two kinds of irregularities are assumed to cause scattering: roughness of the water–sea bed interface and volume inhomogeneities of the sediment mass density and the speeds of compressional and shear waves. The first-order small perturbation approximation is used to obtain expressions for the scattering amplitude and bistatic scattering strength. The angular dependence of the scattering strength is calculated for sedimentary rock and the influence of shear elasticity is examined by comparison with the case of a fluid bottom. Shear effects are shown to be strong and complicated.

ISSN:0001-4966    

DOI:10.1121/1.421093

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

It is known that marine sediments can support both compressional and shear waves. However, published work on scattering from irregular elastic media has not examined the influence of shear on sea bed scattering in detail. A perturbation model previously developed by the authors for joint roughness-volume scattering is used to study the effects of elasticity for three sea bed types: sedimentary rock, sand with high shear speed, and sand with “normal” shear wave speed. Both bistatic and monostatic cases are considered. For sedimentary rock it is found that shear elasticity tends to increase the importance of volume scattering and decrease the importance of roughness scattering relative to the fluid case. Shear effects are shown to be small for sands.

ISSN:0001-4966    

DOI:10.1121/1.421094

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A wideband extension of the Optimum Uncertain Field Processor (OUFP) is presented. Combined with Monte Carlo estimation methods, this processor provides a fast, efficient, and robust technique for Matched-Field Processing (MFP). Under a simulated Hudson Canyon environment, a quantitative, probabilistic analysis of the sensitivity of the Optimum Matched-Field Processor (OMFP) to various kinds of environmental mismatch is shown. Similarly, the performance of the OUFP is calculated. Finally, the optimum wideband OUFP is applied to a data set collected from the Hudson Canyon, the results tabulated, and then compared to a standard Bartlett processor. The performance of the optimum wideband OUFP and the suboptimum Bartlett processor are in very good agreement with the performance predicted from simulation results.

ISSN:0001-4966    

DOI:10.1121/1.421117

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

An optimal approach to tracking a moving source in the presence of environmental variability is presented. This tracking algorithm, called the optimum uncertain field tracking algorithm (OUFTA), incorporates a model for the source motion as well as the uncertain ocean environment. The performance of the OUFTA is evaluated over a range of signal-to-noise ratios (SNRs) as a function of the number of observations by examining its ability to correctly estimate both the current source position and the entire source track. The improvement in performance provided by the OUFTA is illustrated by comparison to two suboptimal tracking algorithms. Results show that incorporatinga prioriknowledge of both the source dynamics and the ocean model provides the most accurate estimates of the source track.

ISSN:0001-4966    

DOI:10.1121/1.421095

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Tracking techniques may be used to reduce the ambiguity of an acoustic source’s position in matched-field processing, particularly for low signal-to-noise ratios. An efficient tracking algorithm was recently described, in which the performance would be comparable to that of an exhaustive tracker; that is, the probability that the source track is examined is close to unity for cases of interest. The current paper describes an efficient technique to estimate the probability that the source track is examined. The procedure involves the use of noise-only data to define statistical thresholds for the strongest peaks in the ambiguity surface. Once these thresholds are defined, the required probabilities may be estimated by evaluating the ambiguity function for signal-plus-noise data within a small region surrounding the known source location, and identifying any peaks exceeding the thresholds. Results of simulations obtained for a slanted array indicate that this approach provides an effective way to estimate the probability that the source track is examined.

ISSN:0001-4966    

DOI:10.1121/1.421118

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The three-dimensional time-average temperature distribution in a pore of a thermally isolated thermoacoustic stack is calculated. A boundary-value problem is formulated in the acoustic and short-stack approximations from the equation of conservation of energy using literature results for the time-average energy flux. In the central region of the pore, the solution for the time-average temperatures of the wall,Tw,and of the gas along its center line,Tg,share a common profile, linear in the axial coordinate,z.Near the pore ends, where the energy flux approaches zero, the axial gradient ofTgapproaches the critical temperature gradient over a distance of order the acoustic displacement amplitude. The axial gradient ofTwapproaches zero over a much smaller distance, provided the wall has small thermal conductivity. The transverse heat-flux density,q,is nonzero only near pore ends. Under certain conditions,q=h1(Tg−Tw),whereh1is proportional to the thermal conductivity of the gas divided by the thermal penetration depth. The constant of proportionality, of order unity, depends on pore width and Prandtl number. Results agree favorably with recently published numerical calculations.

ISSN:0001-4966    

DOI:10.1121/1.423109

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A new electrostatic film transducer is described. This transducer uses a flexible sheet of elastomeric microstructures to achieve a wide range of internal impedance and driving pressure. A simple single degree of freedom model is used to predict the performance of the device and the predictions have been confirmed by measurements made on a prototype device.

ISSN:0001-4966    

DOI:10.1121/1.421119

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A numerical study comparing the use of two structural sensing approaches for sound radiation control is performed on a baffled rectangular plate. The first sensing approach implements an array of accelerometers whose outputs are filtered to construct an estimate of the sound pressure radiated at given angles in the far field. The second method uses the same array of point sensors to estimate the net volume acceleration of the plate. Results show the improved performances of the sensor observing far-field sound radiation over a volume acceleration based on sensor.

ISSN:0001-4966    

DOI:10.1121/1.421112

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP