摘要:

Temperature dependence of ultrasonic attenuation and ultrasonic Gruneisen parameter are investigated in alkali halides in the range 80–300 K. The calculations use a computer program, developed by the authors, for implementing Nava’s modified formulation of Woodruff’s theory. These calculations are done for NaCl, NaF, and LiF for longitudinal and shear waves along the [100], [110], and [111] directions. Compared to calculations based on original Mason formulation, it was found that in most cases the results are in better agreement with the experimental data. The results also help to throw some light on relative merits of different theories of attenuation.

ISSN:0001-4966    

DOI:10.1121/1.423240

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A theory on acoustic streaming generated by Lamb waves propagating in a thin composite membrane was developed. Using this theory, the mass-transport velocity in water, which is loaded on a 4-μm-thick membrane, due to Lamb waves at 4.7 MHz was calculated. The results are in good agreement with the experimental values measured by Moroney et al. [Appl. Phys. Lett.59, 774 (1991)].

ISSN:0001-4966    

DOI:10.1121/1.423241

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

An array of 32 microphones spanning 675 m of range was used to measure the frequency coherence for pulse signals propagating through atmospheric turbulence near the ground. Frequency coherence is the correlation between the fluctuations in received signals of different frequencies as expressed through the two frequency mutual coherence function evaluated at the same point in space and time. The experiments were conducted for frequencies between 200 and 1000 Hz under both downward and upward refracting propagation conditions. Measurements and theory are shown to be in good agreement for line-of-sight propagation. In the acoustic shadow region the frequency coherence bandwidth shows a strong dependence on mean frequency. The effects of refraction and diffraction, which cause wandering in the pulse arrival time and broadening of the pulse width, respectively, are clearly distinguished. The measurements provide experimental characterization of signal fluctuations for sound fields propagating outdoors. They are important for acoustic remote sensing and detection applications and for the validation of theoretical and numerical developments in sound propagation modeling.

ISSN:0001-4966    

DOI:10.1121/1.423242

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The numerical implementation of the Green’s function parabolic equation (GFPE) method for atmospheric sound propagation is discussed. Four types of numerical errors are distinguished: (i) errors in the forward Fourier transform; (ii) errors in the inverse Fourier transform; (iii) errors in the refraction factor; and (iv) errors caused by the split-step approximation. The sizes of the errors depend on the choice of the numerical parameters, in particular the range step and the vertical grid spacing. It is shown that this dependence is related to the stationary phase point of the inverse Fourier integral. The errors of type (i) can be reduced by increasing the range step and/or decreasing the vertical grid spacing, but can be reduced much more efficiently by using an improved approximation for the forward Fourier integral. The errors of type (ii) can be reduced by using a numerical filter in the inverse Fourier integral. The errors of type (iii) can be reduced slightly by using an improved refraction factor. The errors of type (iv) can be reduced only by reducing the range step. The reduction of the four types of errors is illustrated for realistic test cases, by comparison with analytic solutions and results of the Crank–Nicholson PE (CNPE) method. Further, optimized values are presented for the parameters that determine the computational speed of the GFPE method. The computational speed difference between GFPE and CNPE is discussed in terms of numbers of floating point operations required by both methods.

ISSN:0001-4966    

DOI:10.1121/1.423260

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In April 1994, the USAF Armstrong Laboratory, in cooperation with USAF Test Pilot School, conducted an experimental study of controlled focus boom generated by supersonic maneuvers. The objective of this study was to collect focus and postfocus booms and to assess the ability of aircrews to control the placement of the focal region during basic maneuvers. Forty-nine supersonic passes were flown and included level linear acceleration, level turn, accelerating dives, and climbout-pushover maneuvers. These flights were flown under calm and turbulent atmospheric conditions. Turbulent conditions had a defocusing effect which caused distortions in the focus region and resulted in smaller maximum overpressures. Sonic booms were collected by up to 25 boom event analyzer recorders (BEARs) placed in a 13 000-ft linear array. The BEAR units were spaced 500–2000 ft apart with the denser spacing at the expected focal region. This spacing was chosen to evaluate the thickness of both the focal and postfocal regions. The target location varied from 2000–5000 from the uptrack end of the array. Of the 49 flights, a focus boom was placed within the array 37 times and within±3000feet of the target point 27 times, demonstrating the ability to place controlled focus booms.

ISSN:0001-4966    

DOI:10.1121/1.423261

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

In this paper the acoustic absorption due to an orifice plate in a duct supporting a mean flow is studied theoretically. Absorption takes place as the acoustic field energizes a vortex field which is generated at the orifice rim. A linearized approximation is made to the absorption mechanism. This work presents an analytical extension of Howe [Proc. R. Soc. London, Ser. A366, 205–233 (1979)]. The latter deals with unsteady high Reynolds number flow through a circular aperture in a thin infinite screen. To the author’s knowledge, such an extension has not been previously made. The problem is formulated analytically insofar as it is possible and numerical results are presented. A Green’s function series expansion is used in the formulation. A difficulty arises with the convergence of this expansion. It is solved by a renormalization technique, which has been developed for this problem. The technique appears to be a novel method for dealing with convergence problems associated with term by term differentiation of Green’s function series expansions. To provide a check on the solution, it is shown that when the radius of the duct tends to infinity the present expression for the Rayleigh conductivity of the orifice plate limits to the expression obtained by Howe for an aperture in a thin infinite screen. With respect to the numerical results, it appears that for orifice mean flow Mach numbers≲0.2,an orifice to duct open area ratio of 0.3 provides near optimal average absorption, for the band of frequencies limited by the first symmetric mode cutoff frequency.

ISSN:0001-4966    

DOI:10.1121/1.423262

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Experimental data (environmental propagation loss and beam response) obtained in the Levantine Basin of the Mediterranean Sea are presented and compared to calculations performed with a multilayer, normal-mode computer model. The comparisons show good agreement between measured propagation loss and computer calculations. Beam response data show that the primary source of signal gain degradation is the bifurcated and broadened beam response. The degraded response pattern results from the combined effects of acoustic pressure field variations and motional array shape deformation. Calculations performed with the multilayer, normal-mode model show similar degraded response patterns caused by array tilting. These results indicate that system response and performance characteristics can be calculated for a specific sound channel as a function of the array shape and motion.

ISSN:0001-4966    

DOI:10.1121/1.423263

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Sounding of the plankton aggregation which consists of identical statistically uncorrelated particles by a plane monochromatic wave has been studied. Analytical formulas for scattered signal intensity and interference coefficient have been obtained for an arbitrary plankton spatial distribution form. The role of interference of waves scattered by individual plankton targets has been under investigation. The influence of the plankton probability density function form on the sounding signal mean intensity has also been considered.

ISSN:0001-4966    

DOI:10.1121/1.423285

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The feasibility of inverting acoustic field statistics to obtain the parameters of a stochastic internal wave model is demonstrated using numerical simulations. For weak scattering satisfying the Rytov approximation, the parameters of a generalized form of the Garrett–Munk internal wave model can be obtained. A hierarchy of experiment scenarios has been studied. Scenarios range from a densely populated vertical receiving array to single-point measurements. In each case, the intrinsic range-averaging of acoustic measurements provides integral constraints on the environmental model. The success of the inversion improves with increasing experimental complexity. With a vertical array, up to four parameters of the internal wave model can be recovered. For the simplest situation, two parameters can be fit with reasonable accuracy. The implications of these results for understanding oceanographic processes are discussed.

ISSN:0001-4966    

DOI:10.1121/1.423264

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Properties of the seafloor affecting sound propagation can be inferred from the horizontal wave numbers of bottom-interacting modes of sound propagation. To this end, the modal wave numbers can be extracted from the harmonic sound field recorded by a single hydrophone, when either the source or hydrophone are towed at constant depth, giving in effect a well-sampled synthetic aperture array. This method has been used to determine the bottom properties in a range-independent environment, by first applying a Hankel transform to get a representative wave-number spectrum in which the wave numbers of the dominant modes feature prominantly [S. D. Rajan et al., J. Acoust. Soc. Am.82, 998–1017 (1987)]. In range-dependent environments, however, the modes vary owing to changes in bathymetry and seafloor properties, making it impossible to speak of a single set of range-independent modal wave numbers as the Hankel transform method requires. In this paper, a phase demodulation technique is derived from the adiabatic mode model of sound propagation whereby range-dependent wave numbers can be extracted from data collected in range-dependent environments using the same experimental setup as in range-independent environments.

ISSN:0001-4966    

DOI:10.1121/1.423265

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP