摘要:

The scattering of sound by turbulence redistributes the acoustic energy flow in space. For sound propagation with a given geometry, such a redistribution can appear as an energy loss in the received part of a beam. Such a loss now is calledexcessattenuation. The following analysis determines the amount of excess attenuation in the signal obtained in the configuration of a typical monostatic echosonde. Such estimates of excess attenuation are of great importance for accurate quantitative acoustic remote sensing of atmospheric parameters.

ISSN:0001-4966    

DOI:10.1121/1.384436

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In the case where the source of sound is a turbo‐jet engine fan the solution of Fresnel–Kirchhoff’s diffraction integral contains an additional term. That term may be neglected when the source of sound is a single harmonic point source. Formulas, including this term, show that behind a barrier the fan noise is amplified instead of being reduced. The amplification effect occurs when the rim of the barrier is located in areas of high or rather maximum sound levels. It was found out that double‐wall barriers prevent sound amplification. This additional abatement of sound in the shadow zone of the screen is partially caused by absorption of diffracted waves within the open double‐wall ’’cavity.’’ A semi‐empirical formula was developed to estimate this absorption. As basic configuration for experimental studies a DC‐10 half model with a CF6‐turbo fan engine was selected to be tested in model scale of 1 :10. All test runs were conducted in an anechoic chamber of the DFVLR, Department of Technical Acoustics, Germany. The agreement between theory and experiment is satisfactory.

ISSN:0001-4966    

DOI:10.1121/1.384437

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper describes experimental results on the scattering of sound by turbulence in water. The turbulence is created by a 1‐in.‐diam submerged water jet, nozzle velocity 648 cm/s. The sound frequency is 1 MHz. The spectral broadening of the monochromatic signal (due to the scattering of the sound by the turbulent eddies) is compared for different forward scattering angles in the case of incident plane and cylindrical waves. A theoretical model is presented to explain the plane‐wave results. Results for the scattering of cylindrical waves are compared with experiments performed by Candel and Julienne [AIAA Paper No. 76‐545, 3rd AIAA Aero‐Acoustics Conference, Palo Alto, California (July 20–23, 1976)] in air with spherical wave sources.

ISSN:0001-4966    

DOI:10.1121/1.384438

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A model is developed for the calculation of the spatial properties of the noise field produced in a stratified ocean by the action of wind at the surface. The random noise sources are represented by correlated monopoles distributed over an infinite plane located an arbitrary depth below the surface. Wave‐theoretical methods are applied to derive expressions for the intensity and spatial correlation of the noise field. A normal‐mode representation of the noise field is used to reduce these expressions to forms which allow physical interpretation and are suitable for numerical computation. Examples are given of intensity profiles and spatial correlation in the vertical for three generic sound‐speed profiles. The results show that the sound‐speed profile and the presence of the bottom can be important in determining the spatial properties of the noise field. An example is given of a calculation of the horizontal spatial correlation using the fast field program (FFP).

ISSN:0001-4966    

DOI:10.1121/1.384439

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The effects of random fluctuations in an ocean current on underwater cw sound transmission between a bottomed source and receiver are determined for an ocean channel with a linear, depth‐dependent sound speed. A horizontal, depth‐dependent current is considered whose components are random processes. Effects of such a current on ray geometry are determined and six basic current‐induced ray states are found. Under certain conditions, including the assumption that the sound‐speed gradient is larger than current‐component gradients, only one ray state may arise. The geometry of this state is expressed explicitly in terms of the current. Approximations for travel times, total‐field intensity, and their first and second moments are obtained. These moments depend significantly on properties of the source–receiver current component. Intensity moments are predicted using ocean‐current data. For selected parameter values, a difference between relative mean intensity and relative intensity without current of as much as 9 dB and a standard deviation of relative intensity as large as 3 dB are found. These moments are rapidly varying with transmission range; however, useful bounds are derived which are slowly varying and which display an unusual behavior near certain critical ranges.

ISSN:0001-4966    

DOI:10.1121/1.384440

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A phase‐coded signal with 64‐ms resolution was transmitted at 10‐min intervals for a 48‐day period between an acoustic source moored at 2000‐m depth and a bottom mounted receiver at ∠3000‐m depth and at ∠900‐km range. About 16 multipaths were resolved. They were stable in the presence of ocean fluctuations and could be identified (with some exceptions) from ray theory. The precision to which daily travel‐time fluctuations along multipaths could be measured was better than 10 ms. The resolution, stability, identification, and precision is adequate for acoustic monitoring of mesoscale ocean variability by measuring travel‐time variations along ray paths.

ISSN:0001-4966    

DOI:10.1121/1.384441

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The determination of the continuous spectral contribution to the transmission loss in ocean acoustics can be very difficult. While it is applicable to very general sound‐speed profiles, a uniform asymptotic method, i.e., one that is valid as the proper modes pass through cutoff, is proposed and illustrated for the Pekeris model. The method exploits the fact that there are singularities near the integrand of an integral representation of continuous spectrum and that these singularities determine the asymptotic behavior of the continuous spectrum. Numerical examples illustrate the influence of the proper, real improper, and complex improper modes on the continuous spectrum. The technique avoids the sometimes costly numerical evaluation of the continuous spectral contribution; however, the improper eigenvalues must be found.

ISSN:0001-4966    

DOI:10.1121/1.384442

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In an experiment using a cw source suspended from a drifting ship, the phase angles of the signals at four hydrophones laid on the bottom in a horizontal line were compared. The experimental technique used was a new one which canceled out the Doppler shift of the signal by using the combined outputs of all the hydrophones as a phase reference signal. This technique made it possible to compare the phase angles of signals arriving over multiple paths. The mean cosines of the phase differences were obtained as a function of separation for four pairs of hydrophones. An empirical expression was selected to fit the experimental results and permit interpolation to obtain values for other hydrophone spacings.

ISSN:0001-4966    

DOI:10.1121/1.384443

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper is concerned with a method for solving vibration problems of a thin elastic plate with arbitrary shape. The exact solution of an equation of motion is utilized and the boundary conditions along both arbitrarily shaped curved and straight line boundaries are satisfied by means of the Fourier expansion method. Numerical calculations are carried out for the clamped or simply supported elliptical and parabolic plates.

ISSN:0001-4966    

DOI:10.1121/1.384444

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Torsional wave motion in an infinite right circular hollow piezoelectric cylinder belonging to (622) crystal class is investigated when the cylindrical surfaces are either traction‐free or subjected to relative displacement. Open and short‐circuit resonant frequency equations are formulated. Fundamental resonant frequency curves are given for traction‐free cylinders. The theory is applied to the vibrations of annular accelerometers.

ISSN:0001-4966    

DOI:10.1121/1.384445

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP