摘要:

Experimental results are presented for the sound level distribution at different distances from the source when the sound waves propagate over ground with finite impedance. The measurements have been carried out in an anechoic chamber using a point source and using materials that can be used to model both reflecting and absorbing surfaces. The results from the measurements are compared with the theoretically calculated ones. The latter are obtained making use of a one‐parameter ground model to allow for the presence of the finite impedance of the ground.

ISSN:0001-4966    

DOI:10.1121/1.401862

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The general equations of finite amplitude acoustics, including classical absorption effects and second‐order nonlinear effects, are written in a form suitable for two‐dimensional numerical solution. A finite difference scheme then is applied to numerically solve the equations. To demonstrate the method, examples are given of spherical free‐field propagation, normal plane reflection from a hard surface, and oblique spherical reflection from a hard surface for spark pulses. This method has an advantage over Burgers’ equation methods, one‐way wave equation methods, and Pestorius type algorithms in that it can predict the interaction of multiple finite amplitude acoustic waves at arbitrary propagation angles.

ISSN:0001-4966    

DOI:10.1121/1.401863

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The backscattered form function of an infinite cylinder becomes very small at a certain value of thickness for values of the dimensionless frequencyka<1. This phenomena is examined by deriving the equations of motion in terms of the overall mass and compressibility of the cylinder within a thin shell approximation. The value of the form function is determined for the most part by the first two terms in the normal mode expansion. These two terms are of comparable magnitude. Additionally, each of these two terms is shown to depend primarily on either the mass or compressibility of the cylinder. Cancellation of these two terms occurs at the value of thickness for which the backscattered form function is very small.

ISSN:0001-4966    

DOI:10.1121/1.401864

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

An integral expression is derived which gives the shear wave magnitude received by a circular plane‐polarized receiver that is identical to and coaxial with a source on the surface of an isotropic solid half‐space. Numerical evaluation of the expression shows that the shear wave diffraction profile (i.e., the received magnitude as a function of source/receiver separation) is very similar to that for compressional waves in a solid and acoustic waves in a fluid in the Kirchoff limit of largeka, wherekis the wave number andais the source and receiver radius. Under such conditions, simplified acoustic diffraction formulas may be applied to shear wave data with little loss of rigor. In the opposite limit of smallka, the shear diffraction profile differs significantly from that of compressional or acoustic waves and the shear wave diffraction formula presented herein should be employed.

ISSN:0001-4966    

DOI:10.1121/1.401865

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Previously, a fundamentally oriented analysis was presented of the complex‐kaplane pole structure of theSmatrix for a thin spherical shell for 0

ISSN:0001-4966    

DOI:10.1121/1.401866

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

A classical theorem of statistical optics, the van Cittert–Zernike theorem, is generalized to pulse echo ultrasound. This theorem fully describes the second‐order statistics of the spatial fluctuations (the spatial covariance) of the field produced by an incoherent source. As a random scattering medium is insonified, it behaves as an incoherent source. The van Cittert–Zernike theorem can thus predict the spatial covariance of the pressure field backscattered by a random medium. It is shown that this spatial covariance and the incident energy diagram are Fourier pairs. In the case of a focused illumination, the spatial covariance of the backscattered pressure field is proportional to the autocorrelation of the transmitting aperture function. This is independent of frequency and ofF/ number. Experimental results obtained with a linear array are in good agreement with theoretical expectations. The implications of this theorem in speckle reduction and in focusing in nonhomogenous media are discussed.

ISSN:0001-4966    

DOI:10.1121/1.401867

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

Many numerical implementations of the Helmholtz integral equation exist today that can predict routinely the field scattered by a volume‐holding body. One such object is the ellipsoidal ‘‘core’’ of a typical airborne or submerged vehicle stripped of its thin appendages, i.e., stripped of its finlike control surfaces, etc. The reason for these omissions has often been an inherent limitation of the cited modeling tools rather than a rational dismissal of the potential effect of these neglected appendages on the complete body’s expected scattering cross section. The limitation of existing techniques is this: The standard form of Gauss’ theorem on which they are based, that leads to the Helmholtz integral, becomes meaningless when the thickness of the shape addressed tapers down to zero even over only part of the structure. This paper exposes analytically the fundamental cause of this thin‐shape breakdown (TSB), and in the process derives an alternate boundary element formulation for its cure.

ISSN:0001-4966    

DOI:10.1121/1.401868

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The Sommerfeld–Watson transformation (SWT), when applied to problems of sound scattering from elastic bodies with separable geometries, allows the scattered field to be displayed in terms of waves rather than in terms of modes, and so gives a more meaningful picture of the scattering mechanisms at moderate and high frequencies. As typically applied, however, the SWT has left a discontinuity in the calculated field as a function of field point. This discontinuity is not inherent in the method, but is a result of not accounting for the effects of nearby poles in the evaluation of the line integral which represents the geometric part of the scattered field (essentially the specular term). This paper considers this effect in the particular case of broadside scattering from an infinite circular cylindrical elastic shell in an acoustic medium. The scattered field in the backward half‐space, as computed from the corrected wave solution, is shown to be in excellent agreement with the field calculated from the normal mode (partial wave) series for frequencies corresponding toka=1.5 and above, for a steel shell whose wall thickness is one percent of its radius. Shell theory, rather than the theory of elasticity, is used.

ISSN:0001-4966    

DOI:10.1121/1.401869

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The reflection of a monochromatic plane wave impinging from a homogeneous fluid half‐space onto a random fluid half‐space is studied using an integrodifferential equation method. The coherent reflection coefficient is presented as a function of frequency and statistical parameters characterizing the random medium.

ISSN:0001-4966    

DOI:10.1121/1.401870

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP

摘要:

The propagation of antiplane waves in an unbounded linearly elastic solid that contains a distribution of flat cracks is considered. The cracks are parallel to each other, the distribution is dilute, and the incident wave is normal to the planes of the cracks. Using the Kramers–Kronig relations, the speedcof the coherent wave in the cracked solid is derived as a function of the dimensionless frequency ωa/cT(where ω is the angular frequency,ais the half crack width, andcTis the speed of antiplane waves in an uncracked solid) and of crack density. Numerical results for the speedcare obtained, based on the assumption that the limit ofcfor high frequencies is equal tocT. These results show that the speed of the coherent wave in a cracked solid is always less than the limiting speed at high frequencies. The results of this work are valid for the entire range of frequencies.

ISSN:0001-4966    

DOI:10.1121/1.401871

出版商:Acoustical Society of America    

年代:1991

数据来源: AIP