摘要:

Acoustic measurements in and above porous materials (500‐μm‐diam glass beads) have been used to determine rigid‐frame porous medium parameters. The propagation constant for the acoustic signal propagating through the air in the pores was calculated from probe microphone measurements made inside the porous medium. Reflection measurements have been used to calculate the characteristic impedance of a thick porous medium. A model relating porous medium properties to the propagation constant and impedance was used to analyze the data from both the reflection and transmission measurements. Both reflection and transmission measurements are needed to determine the rigid‐frame porous medium parameters. Porous medium parameters can be calculated using the model and the data. The calculated porous medium parameters from the transmission and reflection measurements are compared. Some possible explanations for the discrepancy and suggestions for further work are presented.

ISSN:0001-4966    

DOI:10.1121/1.414710

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

A single, efficient scheme developed by Sridhara and Crocker [J. Acoust. Soc. Am.95, 2363–2370 (1994)] was used to predict the radiated sound pressure and muffler insertion loss by incorporating a simple logical statement in the computer codes. The scheme was verified through a series of experiments. A 0.06‐m‐diam loudspeaker and a single cylinder, reciprocating‐type vacuum pump were used as sound sources during the tests. The source impedance was measured using the direct method. A 0.114‐m‐diam and 0.3‐m‐long simple expansion chamber and a 0.006‐m‐diam and 0.025‐m‐long side‐branch resonator were used as mufflers. Sound‐pressure level measurements for the prediction scheme were made for several source–muffler and source–straight pipe combinations. The predicted and measured sound‐pressure levels were compared for all cases considered. In all cases, correlation of the experimental results and those predicted by the developed expressions was good at low frequencies. Predicted and measured values of muffler insertion loss were also compared. The agreement between the two quantities was good.

ISSN:0001-4966    

DOI:10.1121/1.414711

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The numerically implemented angular spectrum approach (ASA) is investigated based on the evaluation of the 2‐D fields radiated by striplike planar sources and 1‐D focusing linear‐phased arrays with different aperture sizes and on the comparison with those obtained by using the analytical solution derived with the point spread function method. Since the discrete Fourier transform (DFT) of a finite‐size source produces an angular spectrum with spatial frequency aliasing and discretization of the source in even‐numbered samples causes a half‐sample length phase shift error in the angular spectrum, the effects of the aliasing and the phase shift on calculation accuracy are dealt with. The results show that the frequency aliasing causes an overestimation of the near field and the phase shift distorts the results. A numerical algorithm for eliminating the aliasing is proposed and correction of the phase shift is confirmed to be necessary. The algorithm proposed can completely remove the errors and obtain exact results. The methods used in this paper can be directly applied to the case of 3‐D fields. The investigation of the characteristics of the numerically implemented ASA as a function of angular range is carried out in the companion paper [P. Wu, R. Kazys, and T. Stepinski, ‘‘Analysis of the numerically implemented angular spectrum approach based on the evaluation of two‐dimensional acoustic fields. Part II. Characteristics as a function of angular range,’’ J. Acoust. Soc. Am.99, 1349–1359 (1996)].

ISSN:0001-4966    

DOI:10.1121/1.414712

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The numerically implemented angular spectrum approach (ASA) is investigated in terms of the characteristics of the ASA as a function of angular range over which the angular spectrally decomposed plane waves are superimposed to construct an acoustic field radiated by a source. The investigation is based on the evaluation of the 2‐D fields radiated by striplike planar sources and 1‐D linear phased arrays with different aperture sizes and on the comparison with those obtained by using the analytical solution derived with the point spread function method. The convergence and the spatial aliasing of the ASA’s results are systematically dealt with. A region well converging to the analytical solution as the angular range increases is found to exist before the aliasing comes up. Analytical formulas to determine the boundaries of this region are given. Based on the instantaneous frequency concept, an analytical expression for optimal selection of the angular range used is derived. Hence the sampling frequency can be chosen to be optimal and adaptive to the discretization size of the source plane without worrying that the number of the samples for the discretization size should be chosen to be a power of two that the fast Fourier transform (FFT) usually needs. An excellent agreement of the ASA’s results with the analytical solution is demonstrated when using the optimal selection of the angular range for the specified parameters. The analysis made in this paper can be extended to the case of 3‐D fields.

ISSN:0001-4966    

DOI:10.1121/1.414713

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The variations of linear and nonlinear elastic parameters as a function of initial stress and material structure are considered using a model of a granular medium with fluid pore filling. Examples of such variations for some geological conditions interesting for seismoprospecting are presented. It is demonstrated that the nonlinear parameter may be used in exploration seismology as a much more sensitive characteristic compared with conventionally exploited linear moduli.

ISSN:0001-4966    

DOI:10.1121/1.414714

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

This paper reports the result of an experimental study of nonlinear emission from gas‐filled bubbles with particular emphasis on the subharmonic emission. The gas bubbles are Infoson, a contrast agent for use in echocardiology consisting of small gas‐filled microspheres with a mean diameter of approximately 4 μm. Pulsed signals with center frequencies of 3.5 and 4 MHz were transmitted through a cloud of bubbles and the level of the subharmonic component was measured as a function of the level of the exciting signal. No sharp threshold, as expected from theory, is found. The amplitude of the subharmonic response increases, however, approximately as the cube of the driving pressure when the driving pressure is in the region of 50–100 kPa.

ISSN:0001-4966    

DOI:10.1121/1.414715

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

The well‐studied problem of estimating the angle of arrival of plane waves in ocean acoustics is revisited. While beamforming is the classical technique, the most recent methods use parametric models of the signals, and maximum likelihood estimation (MLE). Yet, in all cases, fluctuations in the required signals due to medium perturbations have been neglected. In this paper, the advantages of using the correct second moment for waves propagating in random media (WPRM) to model the fluctuations of the arriving signals are investigated. In this way, the physics of WPRM is incorporated in this parametric model of the observations. This model is then used as the basis for the MLE. Finally, the Cramer–Rao lower bound versus fluctuation strength, angle of arrival, and the noise power is provided. Also developed are iterative ML algorithms based on the Newton–Raphson method for a single source and extended to multiple sources. The method is applicable for plane wave arrivals at any known array geometry with arbitrary medium autocorrelation function. Simulation results show a dramatic improvement in the precision of the estimated angle of arrival.

ISSN:0001-4966    

DOI:10.1121/1.414716

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Existing models to obtain acoustic intensity and phase fluctuations due to thermal structure in the ocean, such as that developed by Chernov, are limited in their application. They are valid only for an isotropic medium and where the range is much greater than the correlation distance of the thermal structure. Extensions to short range and for an anisotropic medium are described here, both for the stochastic wave and ray models. Numerical estimates of acoustic fluctuations are easily evaluated from these theoretical expressions.

ISSN:0001-4966    

DOI:10.1121/1.414717

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Data collected from the Office of Naval Research‐special research program (ONR‐SRP) bottom reverberation research cruises consist of various environmental measurements (sound‐speed profiles, bottom properties, etc.), extensive bathymetric mapping of the 300 km×150 km natural laboratory, and high‐quality acoustic reverberation data recorded in both monostatic and bistatic geometries.The analysis of the acoustic data and the effects of propagation are investigated. Accurate GPS tracking of the participating research vessels provided the precision necessary to attempt to correlate ‘‘spiky’’ reverberation events with bathymetric features. Understanding the limitations of our ability to resolve such features—due to imperfect signal processing, environmental variability, and complex multipath structures—is the main objective of this work. Using a simplified time‐to‐range conversion, the measured reverberation data can be displayed over the local bathymetry. This process shows good correlation between large‐scale [O(km)] high‐level returns and bathymetric ridge structures. Employing a PE propagation model in a quasi‐cw manner, many of the large‐scale reverberation features can be shown to correlate well with predicted high‐level ensonification of the total field [low transmission loss (TL)]. Extending the model predictions to include time domain broadband pulse propagation, it can be shown for ranges greater than ∼20 km that multipaths and forward scatter produce complex ensonification patterns which are unresolvable in time and/or range. The effect of this on the ability to correlate finer scale returns and bathymetric features will be discussed. Preliminary analysis of data from a bottom mounted vertical array in the vicinity of 1/2 CZ exhibits strong evidence for the existence of these predicted multipaths and provides a means of confirming the relative intensities predicted by propagation models.

ISSN:0001-4966    

DOI:10.1121/1.414718

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP

摘要:

Asymptotic (in the geometric limit) phase errors associated with parabolic approximations to the one‐way Helmholtz equation are investigated. In order to estimate the phase accuracy of various parabolic approximations, the canonical Hamiltonian formalism is used to derive ray equations. It is shown that among the class of parabolic approximations that allow the split‐step Fourier algorithm to be used, the wide‐anglec0‐insensitive approximation [Tappertetal., J. Acoust. Soc. Am.97, 2771–2782 (1995)] has full second‐order accuracy. Other approximations in this class, such as the Thomson–Chapman and the Berman–Wright–Baer approximations, are shown to have second‐order errors, as does the standard first‐order parabolic approximation. Numerical calculations support the expectation that among this class of parabolic approximations the wide anglec0‐insensitive approximation is superior.

ISSN:0001-4966    

DOI:10.1121/1.414719

出版商:Acoustical Society of America    

年代:1996

数据来源: AIP