摘要:

Continental boundaries have a shallow sloping bottom that changes to a steeper slope. Our laboratory acoustic models (simulations) have (1) a 11° slope that changes to a 50.5° slope and (2) a 35° slope change to a 59° slope. The surface and bottom of the acoustic model are dry wall construction board. The source is a spark and the receiver is a small microphone. The Biot–Tolstoy exact time-domain solution was used. [I. Tolstoy and C. S. Clay,Ocean Acoustics(American Institute of Physics, New York, 1987), 2nd ed., Appendix 5, and C. Feuillade and C. S. Clay, J. Acoust. Soc. Am.96, 501–514 (1994)]. The image-reflection and diffraction components depend on source and receiver locations. In model (1), source and receiver were within the 11° wedge and gave reverberations that were mainly from the 11° wedge. Source and receiver beyond the shelf break gave reverberations that were mainly from the 50° part of the model. In model (2), the source was up slope and receiver was near the break to observe diffractions. The finite number of arrivals (images) obeyed the Biot–Tolstoy rule. Experimental signals matched the theory in amplitudes and arrival times. Most reverberations were caused by multiple reflections in the wedge waveguide. The amplitudes of the image-reflection paths were an order of magnitude larger than the diffraction arrivals.

ISSN:0001-4966    

DOI:10.1121/1.420320

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

This paper presents the results of acoustic and optical measurements of individual breaking waves in the surf zone. Two hydrophones, horizontally separated and deployed in 2 m of water off La Jolla Shores beach, California, were used to measure the source spectrum of breaking surf, and characterize propagation through the surf zone over length scales of order 10 m. The acoustic data show an acoustically active region within a wave which propagates shoreward. The production of sound within the active region is associated with the formation of bubble plumes. Above 500 Hz, the sound is consistent with the radiation from individual bubble oscillations. Pictures were taken of the bubble plumes formed beneath the breaking surf, providing estimates of the plume size, and bubble size distribution and void fraction within a plume. The density of bubbles scales asa−2.5fora1 mm,whereais bubble radius, and total void fractions of 0.3–0.4 were measured. Theoretical calculations show that radiation from bubbles within a plume is strongly damped, and only a thin shell of bubbles on the exterior of the plume contributes to the wave noise. A “moving bubble sheet” source model for the wave noise is presented which shows fair agreement with the acoustic measurements.

ISSN:0001-4966    

DOI:10.1121/1.420321

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Analytic expressions are derived for the variation of modal wave numbers due to variations in five basic properties of a layered environment: the compressional wave speed, shear wave speed, density, and thickness of any layer, and the depth of an interface between layers. Both fluid and solid layers are permitted. The layers may absorb energy, and the modal wave numbers and their variations are generally complex valued. The analytic expressions provide an efficient way to linearize the relation between the modal wave numbers and many geoacoustic parameters, for use in field sensitivity studies, range-dependent propagation models, partially linearized matched-field inversion schemes, and in the study of modes generally.

ISSN:0001-4966    

DOI:10.1121/1.420322

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The optimal (by the criterion of maximum likelihood function) algorithm is applied to the problem of estimating the wave numbers of normal modes in an ocean waveguide. This algorithm has a high-resolution property, takes into account availability of correlated spatial noise, does not require prior modal decorrelation, is suitable for a horizontal array of any shape, and permits one to attain the potential wave-number estimation accuracy limit (Cramer–Rao bound). Numerical simulations for the Pekeris model of an ocean waveguide are presented, demonstrating that the optimal algorithm can essentially (more than ten times) improve the wave-number estimation performance relative to those one for the MUSIC algorithm. Wave-number estimation by the optimal algorithm requires searching for the global extremum of some goal function. It is connected, in common, with known computing difficulties and the absence of sufficiently well-developed computing algorithms. Algorithms based on the signal eigenvectors property of the spectral correlation matrix measured on the array elements are free from listed drawbacks. The synthesis of such algorithms and an asymptotic analysis of their performance are accomplished under conditions of arbitrary correlated spatial noise. Among these algorithms there is one that permits the estimation of the wave numbers as accurately as the optimal algorithm. Experimental verification of this result has been received for the most cases of practical interest.

ISSN:0001-4966    

DOI:10.1121/1.420323

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Underwater acoustic studies of backscattering by submerged targets have generally focused on bodies with spherical and cylindrical symmetry. However, there are interests in scattering by objects which may be characterized by more angular features, with surfaces that tend to be composed of facets and edges. To investigate the scattering properties of such bodies, the backscattering by a number of elastic cubes, and irregularly shaped polyhedra, have been studied. Data were collected by measuring the band limited impulse response of the scatterers, using a broadband transducer, which operated as a transceiver, both transmitting and receiving signals. To present the scattering measurements nondimensionally a form function definition has been employed to normalize the backscattered signals. The normalized frequency has been expressed aska,wherekis the acoustic wave number, andais a characteristic dimension of the scatterer. The cube observations covered a broadkarange,ka=3–34,thereby encompassing the lowerkascattering region where the cube can be considered to be nominally rigid, through to higherkavalues where it would be expected that the elastic properties of the cube become important. The measurements on irregular polyhedra are more limited in extent, however, the results are presented for comparison with the cube data. To compare the observations with predictions, computations of acoustic backscattering for both rigid and elastic cubes are presented.

ISSN:0001-4966    

DOI:10.1121/1.420324

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The pressure responsep1of a fixed quantity of4Hegas subjected to sinusoidal volume perturbations is studied experimentally. Thermal diffusion near solid boundaries causes the gas response to be neither adiabatic nor isothermal. The data are interpreted in terms of the thermal relaxation functionfκwhich relatesp1to the spatial average of the acoustic temperature profile. Comparison of a number of geometries allows us to accurately determinefκfor a thermoacoustic pin-array stack.

ISSN:0001-4966    

DOI:10.1121/1.420325

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The scattering of elastic waves in a half-space containing a striplike crack is investigated. As a special case it seems that the crack may be surface breaking. A surface integral equation with the half-space Green tensor is employed. The key point of the method is the expansion of the Green tensor in Fourier representations with the free part of the Green tensor expanded in the crack coordinate system and the half-space part in the half-space coordinate system. The integral equation is discretized by expanding the crack opening displacement in terms of Chebyshew functions having the correct square root behavior along the crack edges. The incident field is emitted from an ultrasonic probe and a recent model for this is employed. The signal response in another (or the same) probe is modeled by a reciprocity argument and the stationary phase approximation is employed to simplify the final answer, which is thus only valid in the far field of the probes (yielding essentially a spherical wave). Numerical results are given and are compared with both other methods and with available experiments.

ISSN:0001-4966    

DOI:10.1121/1.420326

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Phased array theory is combined with the Rayleigh–Sommerfeld diffraction integral to predict measured acoustic fields generated by a single-source ultrasonic transducer. The idea is to treat a single-source as a “phased array,” which is composed of many small elements. The goal is to find the excitation source for the phased array, that is, the amplitude and phase for each array element, which produces an acoustic field similar to the experimentally measured field generated by the single-source transducer. A pressure field measured at a given plane parallel and close to the face of the transducer in degased water was used to calculate the excitation source of the equivalent phased array using an inverse technique. The excitation source of the equivalent phased array was then used to calculate the acoustic field from this measurement plane to the far field. It was demonstrated that this phased array approach accurately predicted the location of major grating lobes and the general distributions of the near and far pressure fields for four different transducers. This equivalent phased array method (EPAM) used to model a single-source transducer should be useful in both diagnostic and therapeutic ultrasound applications.

ISSN:0001-4966    

DOI:10.1121/1.420327

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Linear frequency-modulated (chirped) acoustic signals have been generated using a pulsed laser spatially modulated by an absorption mask at the surface of a test material. By distributing the laser energy over an area, instead of focusing it to a point or line source, the peak power density of the laser source can be kept below the damage threshold of the material. The corresponding chirped ultrasonic surface wave packet produced by the source, although extended in time, is detected and processed using a matched filtering technique which compresses the packet into a pulse, thus preserving temporal resolution for accurate time-of-flight measurements. Matched filter processing of the chirped wave packet has been compared with the same processing applied to a narrow-band tone burst wave packet. Processing of the chirped signal permits easy separation of overlapped return echoes which could not be resolved when narrow-band signals were used. Finally, by compressing the energy within a chirped signal to a single detection spike, an apparent 15-fold enhancement in signal-to-noise ratio is observed.

ISSN:0001-4966    

DOI:10.1121/1.420328

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Structural acoustic control of stochastic disturbance sources based on static compensation of multiple-input, multiple-output colocated transducer arrays is addressed in this work. Prior feedback controller designs for structural acoustic systems have been based on linear quadratic regulator theory or linear quadratic Gaussian theory, however, the need for a state estimator limits the practical implementation of such control systems on plants characterized by model uncertainty and high modal density. Results presented in this work serve to demonstrate that direct output-feedback control can be successfully employed to attenuate the structural acoustic response of a panel subjected to exogenous input disturbances. Colocated transducer pairs were implemented with a fully populated constant feedback gain matrix from sensor outputs to control inputs. The colocated transducer array serves to enhance robust stability, while the constant feedback gain matrix provides a practical means of implementation. The effects of instrumentation density, control effort penalty, and model order on predicted performance are considered.

ISSN:0001-4966    

DOI:10.1121/1.420329

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP