摘要:

Current versions of the U.S. DOT Federal Highway Administration (FHWA) traffic noise model assume 500 Hz as a representative sound frequency and apply an air absorption attenuation rate of 0.54 dB per 1000 ft for the distance associated with the maximum noise level. According to the American National Standards Institute, the air attenuation rate at 500 Hz is approximately 1 dB per 1000 ft for a wide temperature and relative humidity range. However, the rate of decrease of the composite A‐weighted traffic noise level depends on the spectral distribution of car and truck noise, and varies with distance. A more representative air attenuation for traffic noise is obtained by including this effect during integration as source distance varies to obtain the attenuated sound exposure level (SEL) since the traffic noise level is determined from single event SEL and number of events. In many cases, an attenuation due to air absorption of 0.5 to 3.0 dB may be warranted, as compared to less than 0.5 dB assigned by the traffic noise model. These considerations are appropriate only for the ‘‘hard site’’ case since the ‘‘soft site’’ attenuation factor assumed by the model includes extra attenuation due to shielding and air absorption. Air absorption and roadway truncation by terrain shielding may account for most of the traffic noise level attenuation now attributed to ‘‘ground absorption’’ in some cases.

ISSN:0001-4966    

DOI:10.1121/1.410432

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

Some exact analytic expressions of Green’s functions are obtained for wave propagation in range‐dependent inhomogeneous media where the refractive index profile can be a function of radial or horizontal distance. These solutions are obtained by starting with a onefold integral representation for the solution that was derived from the path integral. The study is extended to the cases involving either a soft or hard boundary. These solutions are particularly useful for testing the validity of approximate or numerical solutions for wave propagation through range‐dependent media, and for construction of more general solutions for problems with boundaries, through the use of integral equation techniques.

ISSN:0001-4966    

DOI:10.1121/1.410433

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

For attenuation described by a slowly varying power law function of frequency, α=α0‖ω‖y, classical lossy time domain wave equations exist only for the restricted cases wherey=0 ory=2. For the frequently occurring practical situation in which attenuation is much smaller than the wave number, a lossy dispersion characteristic is derived that has the desired attenuation general power law dependence. In order to obtain the corresponding time domain lossy wave equation, time domain loss operators similar in function to existing derivative operators are developed through the use of generalized functions. Three forms of lossy wave equations are found, depending on whetheryis an even or odd integer or a noninteger. A time domain expression of causality analogous in function to the Kramers–Kronig relations in the frequency domain is used to derive the causal wave equations. Final causal versions of the time domain wave equations are obtained even for the cases wherey≥1, which, according to the Paley–Wiener theorem, are unobtainable from the Kramers–Kronig relations. Different forms of the wave equation are derived including normal time, retarded time, and parabolic (one and three dimensional). These equations compare favorably with those from the literature corresponding toy=0, 0.5, 1, and 2.

ISSN:0001-4966    

DOI:10.1121/1.410434

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The combination of time domain source imaging techniques and arrays to consider the localization of an impulsive source in a wedge waveguide, with a free surface and rigid bottom, is investigated. An impulsive signal is transmitted from an unknown location, received by an array of hydrophones, and stored. The signals are time reversed and transmitted into the model wedge by a transducer at each receiver location. Model transmission functions are used to compute the pressures at a set of locations. The field calculations use an image construction from D. Chu’s exact solution for a density contrast wedge [J. Acoust. Soc. Am.86, 1883–1896 (1989)]. A map of the peak pressures gives an image of the source location. Particular attention is given to the peak amplitudes, sidelobe amplitudes, and spatial resolution as a function of the number and placement of receivers. Source localization is enhanced by the range dependency of the environment, which eliminates ‘‘range’’ sidelobes, even for a single hydrophone. Adding receivers along an arc eliminates ‘‘angle’’ sidelobes; but adding receivers along an arc or radial gives little reduction in overall background level. Adding receivers parallel to the wedge axis gives sharp, unambiguous azimuthal imaging.

ISSN:0001-4966    

DOI:10.1121/1.410435

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The near‐field scattering of plane waves by a submerged thin elastic cylindrical shell witnesses a shadow boundary generated by thegeometricalacoustics field, which is not encountered in the far field except in the immediate vicinity of the forward scattering direction. Even in the lit zone, the incident and reflected rays may appear close and are not well separated. Based on these physical observations a modified ray acoustic approach is proposed here that combines the two spectral integrals that would give rise to separate incident and reflected fields when reduced by saddle point evaluation. The resulting single integral descriptive of the geometrical acoustics field can be cast as an integral version of the harmonic series form for the total (incident plus scattered) field, thereby facilitating its numerical computation. Averting the asymptotic integration procedure, and hence the contour deformation involving possible capture of the membrane wave poles, the new algorithm predicts the total near field uniformly over theentireazimuthal domain, which is now synthesized by the geometrical acoustics field and the shell‐guided phase‐matched compressional and shear wave field. Extensive numerical examples for the field dependency on frequency as well as on azimuthal and aspect angles are presented; they indicate that the modified ray acoustic solution leads to remarkable accuracy forka≥1 or so (wherekis the fluid acoustic wave number andais the shell mean radius). By reciprocity, the method presented in this paper applies as well to excitation by an axially phased line source located atfinitedistance (on‐ or off‐surface).

ISSN:0001-4966    

DOI:10.1121/1.410436

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The presence of conspicuous penumbra regions in near‐field wave scattering renders asymptotic treatment of ray integrals—by Fock’s theory, for instance—extremely involved, especially for elastic shell–fluid boundary conditions. A modified ray acoustic model that avoids these intricacies as well as those associated with reflected‐leaky wave transition regions has recently been proposed, and proved very accurate at moderate and high frequencies for the thin cylindrical shell prototype [J. M. Ho, J. Acoust. Soc. Am.96, 515–524 (1994)]. Following this approach, this paper discusses the ray synthesis of the on‐ and off‐surface acoustic response of a submerged empty thin spherical shell insonified by an acoustic plane wave. In particular, the two spectral integrals that would yield the incident and reflected ray fields by saddle point reductions are combined into a single integral representing the modified geometrical acoustics field, which may be so rearranged that it is merely the continuum form of the discrete (normal mode series) solution for the total field, thereby facilitating its numerical implementation. The complementary leaky compressional wave fields are described by spectral integrals characterizing multiple guided wave circumnavigations around the shell and are evaluated by residue calculus as usual. It is shown through extensive numerical results that, for the thin spherical shell, the modified ray algorithm also accurately reconstructs the acoustic near field in theentirelatitudinal domain providedka≥1 or so (wherekis the fluid acoustic wave number andais the shell mean radius).

ISSN:0001-4966    

DOI:10.1121/1.410437

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A new differential formulation is presented for acoustic wave scattering from rotationally symmetric penetrable bodies. The numerical implementation of this formulation is fairly simple, and comprises basically the construction of the state‐transition matrix of a system of differential equations and the solution of a matrix equation. The validity and the accuracy of the numerical scheme are tested considering objects of known scattering behavior. Other numerical applications are also presented to demonstrate the generality of the method in handling scatterers of arbitrary shapes and material compositions.

ISSN:0001-4966    

DOI:10.1121/1.410438

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

A computationally efficient approach to the calculation of the transient field of an acoustic radiator was developed. With this approach, a planar or curved source, radiating either continuous or pulsed waves, is divided into a finite number of shifted and/or rotated versions of an incremental source such that the Fraunhofer approximation holds at each field point. The acoustic field from the incremental source is given by a 2‐D spatial Fourier transform. The diffraction transfer function of the entire source can be expressed as a sum of Fraunhofer diffraction pattern of the incremental sources with the appropriate coordinate transformations for the particular geometry of the radiator. For a given spectrum of radiator velocity, the transient field can be computed directly in the frequency domain using the diffraction transfer function. To determine the accuracy of the proposed approach, the impulse response was derived using the inverse Fourier transform for commonly used radiator geometries. The results obtained agree well with published data obtained using the impulse response approach. While the approach does not lead to a general or exact solution, its computational efficiency and accuracy compares favorably to those of the point source method and the impulse response approach for many useful cases.

ISSN:0001-4966    

DOI:10.1121/1.410439

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The linear pressure fields generated by rectangular apertures at ultrasonic frequencies are considered, experimental results and theoretical predictions are presented both for focused and unfocused cases. Measurements were made in water using a circular transducer (38 mm in diameter) with various rectangular apertures in front of it. Focusing was achieved by the addition of perspex lenses. The aperture dimensions and focal lengths were chosen to be comparable with those encountered in medical ultrasound systems. The pressure field generated by the aperture was sampled using a 1‐mm‐diam bilaminar polyvinylidene difluoride (pvdf) hydrophone. Predictions for diffraction losses due to a rectangular transmitter and a circular receiver of finite size are also shown. The general behavior of the pressure field of a rectangular aperture is discussed and good agreement is shown between experimental measurements and theoretical predictions within the known limitations of both.

ISSN:0001-4966    

DOI:10.1121/1.411397

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP

摘要:

The acoustical performance of a submerged linear array of spherical shells in shallow water is examined by combining theT‐matrix method of solving for multiple acoustic interactions among separate bodies with a model for the ocean as a fluid layer over a half‐space of a distinct fluid. The system of source and reflected waves is analyzed by standard approximate contour integration techniques, valid for the far field of the array. Calculations using the results show the effects of shallow water on array performance.

ISSN:0001-4966    

DOI:10.1121/1.410440

出版商:Acoustical Society of America    

年代:1994

数据来源: AIP