摘要:

A theoretical model was developed to evaluate the reduction of structure-borne noise that is generated by a line force applied on an infinite plate using an air-voided elastomeric baffle. The vibrating plate is covered with an elastomeric baffle layer to reduce the noise generated by the structural vibration. The vibrating plate is perfectly bonded to the elastomeric baffle layer. The outer surfaces of the vibrating plate and the elastomeric baffle are in contact with air and water, respectively. The analysis for modeling is based on the theory of elasticity and pertinent boundary conditions. Effects of various parameters such as baffle layer dimensions and material properties on the noise reduction are presented.

ISSN:0001-4966    

DOI:10.1121/1.418292

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

An integro-modal approach is presented in this paper for computing the acoustic properties of irregular-shaped cavities. The method consists of discretizing the whole cavity into a series of subcavities, whose acoustic pressure is decomposed either over a modal basis of regular subcavities or over that of the bounding cavities in the case of irregular-shaped boundaries. An integral formulation is then established to ensure continuity of both pressure and velocity between adjacent subcavities using a membrane with zero mass and stiffness. To some extent, the method provides a combined approach retaining the advantages of both the acoustoelastic method (AEM) and the Green’s function method (GFM). Numerical and experimental results are presented demonstrating the efficiency and accuracy of the suggested technique. Comparisons with other existing methods are also made. It is shown that good accuracy on the computation of cavity modes can be obtained using a very limited number of subcavities.

ISSN:0001-4966    

DOI:10.1121/1.418346

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The properties of tube wave (Stoneley wave) propagation in a fluid-filled borehole penetrating a permeable rock are modified by the presence of a mudcake which may form on the borehole wall. A theoretical description of these effects characterizes the bulk properties of the porous rock using the Biot theory and the mudcake as an elastic impermeable layer. A finite membrane stiffness simulates the mudcake partial sealing mechanism. Within the context of this model, the mudcake can reduce, but not eliminate, the permeability effects on the tube wave slowness and attenuation. Numerical examples relevant to a typical sandstone of porosity 20% and permeability 200 mD, show that (a) the membrane stiffness acts to reduce the permeability effect on slowness and on attenuation and (b) a finite thickness of mudcake shifts the phase slowness by a (nearly) frequency-independent amount, with little effect upon the attenuation. An expression for the low-frequency limit clarifies the important combinations of mudcake parameters.

ISSN:0001-4966    

DOI:10.1121/1.418347

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In this article, a computational method is proposed for computing the Helmholtz integral equation for acoustic radiation and scattering problems with a three-dimensional body shape. Unlike the previous boundary element method, the integration of acoustic properties is based on the global body surface instead of local elements. The approximation function of body geometry and the weighting function of numerical integration are independently chosen. When the singular kernels of Helmholtz equation are regularized as bounded, discontinuous functions, the order of integration polynomials for acoustic properties can be arbitrarily chosen, therefore, the accuracy and efficiency of computation can be increased.

ISSN:0001-4966    

DOI:10.1121/1.418348

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A bounded form of the hypersingular Helmholtz integral equation for 3-D acoustic problems is developed in this paper. All integrals with singular kernels are regularized within each regular surface element, in the sense of Kellogg, with twice continuous differentiability. The standard Gaussian quadrature formula is applied without any special treatment for all the computational points. The collocation points are chosen to be the Gauss–Legendre nodes and no interpolation function is assumed for acoustic variables.

ISSN:0001-4966    

DOI:10.1121/1.418349

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

A closed-form analytic solution has been derived for an arbitrarily oriented dipole placed above an impedance plane. Asymptotic approximations for the total sound field can be written in a form similar to the classical formula for a monopole. Analytic approximations derived for horizontal and vertical dipoles give predictions that agree well with those obtained from a program of the fast field type. The asymptotic results have been confirmed also by laboratory measurements. Compared with numerical methods, the analytic approximations offer the combined advantages of easier physical interpretation and a much reduced computational time. It is found that the variation of the sound-pressure level from a vertical dipole above an impedance plane differs significantly from that due to a monopole above this plane, particularly as a result of differences in the ground wave component. However, the excess attenuation due to a horizontal dipole is found to be rather similar to that for a monopole except for source and receiver heights that are comparable with range.

ISSN:0001-4966    

DOI:10.1121/1.418293

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

In this paper, oscillation modes of single jet, twin jet, and multijet with square configuration were investigated by using optical and acoustical observations. In the case of a single jet, axisymmetrical, helical, and lateral oscillation modes appeared with increasing pressure ratio of the jet. In the case of twin jets, two counter rotating helical oscillation modes and reversely lateral oscillation modes along a line through the center line of the two jets were observed for two different pressure ratios. In the case of multijets with square configuration, the quadrupolelike oscillation mode, four counter rotating helical oscillation modes, and a symmetrical oscillation mode were observed with increasing pressure ratio. The first and the last oscillation modes of the multijet with square configuration are unexpected based on observations for a single jet.

ISSN:0001-4966    

DOI:10.1121/1.418294

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

To study the frequency-dependent spatial and temporal variabilities of sound propagation in coastal regions two experiments were conducted by transmitting sound impulses in the form of M sequences centered from 0.6 to 22 kHz. The site of the first experiment was the Atlantic Generating Station (AGS) where the source–receiver range was 214 m. The site of the second experiment was Delaware Bay where the range was approximately 760 m. The mean water depth was 14–15 m in both locations. Oceanographic data and acoustic data were collected simultaneously in both experiments. It was found that the temporal coherence of the propagated broadband signal changes significantly with pulse center frequency, as well as varying with geographic location and time. Trends of increasing signal decorrelation (between consecutive pulses) with increasing center frequency, and increasing signal decorrelation for paths with increasing number of interface interactions, were observed. It was found that for lower center frequencies there is hardly any decorrelation in signal over several hours, while for signals with center frequencies only a few kHz higher there was substantial decorrelation over times as short as 10 to 20 min. No significant spatial decorrelation of the signal was observed over the hydrophone spacing of a few meters for these experiments.

ISSN:0001-4966    

DOI:10.1121/1.418350

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

The three-dimensional acoustic scattering from a vertical, impenetrable cylinder in a waveguide is studied. The analytical solution of the problem, for a Dirichlet or a Neumann boundary condition on the scatterer, has been derived recently by Athanassoulis and Prospathopoulos [J. Acoust. Soc. Am.100, 206–218 (1996)] in the form of a double-infinite normal-mode series, representing the total acoustic field. In order to extend the applicability of this solution to higher frequencies, the total field is decomposed into the incident and the scattered parts. A series expansion for the scattered field is obtained, and the critical parameter controlling its azimuthal convergence is shown to be the nondimensional wave numberkabased on the radiusaof the cylinder. The general term of the series starts to decay exponentially immediately after the azimuthal index has exceeded the critical valueka,a fact justifying the introduction of the concept ofazimuthal-evanescent modes. By exploiting the above decomposition, the direct numerical summation of the normal-mode series becomes feasible up toka≈1000.For calculations at even higher frequencies(ka→∞),asymptotic expressions are derived by using appropriate integral representations of Bessel and Hankel functions, in conjunction with the method of stationary phase. The asymptotic analysis shows that the scattered field is obtained as a superposition of 2-D point sources lying on the boundary of the vertical cylinder, with appropriate amplitudes and phases. Excellent agreement between asymptotic and direct summation numerical results has been demonstrated, at moderate frequencies, where both representations are expected to be valid.

ISSN:0001-4966    

DOI:10.1121/1.418295

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP

摘要:

Compressional(P)-wave attenuation, shear(S)-wave attenuation, and shear(S)-wave velocity measurements, compiled for fine-grained, unconsolidated sediments, show trends that do not support empirical relationships [Hamilton, J. Acoust. Soc. Am.68, 1313–1340 (1980); inAcoustics and Ocean Bottom(F. A. S. E. Specialize Conf., Madrid, 1987)] commonly used for geoacoustic modeling. The exception is compressional-wave attenuation data, from 10 Hz to 500 kHz, that essentially follow a frequency dependence off1(±2standard deviations).P-wave attenuation with depth is difficult, if not impossible, to predict given the variability in the data. Examination of several attenuation/depth profiles indicates that those of Mitchell and Focke [J. Acoust. Soc. Am.67, 1582–1589 (1980)] may be the best choice for predictingP-wave attenuation with depth at the present time. The assumption thatS-wave andP-wave attenuations are proportional [Hamilton, J. Acoust. Soc. Am.60, 334–338 (1976c); inAcoustics and Ocean Bottom(F. A. S. E. Specialize Conf., Madrid, 1987)] is not supported by the data.S-wave attenuations calculated from effective stress mimmick the data, however, suggesting this as a better method for predictingS-wave attenuation with depth.S-wave velocity/depth profiles fall into high- and low-velocity groups that (1) reflect gross differences in sediment texture, and (2) demonstrate the importance of sediment characterization in order to select an appropriate velocity/depth function.

ISSN:0001-4966    

DOI:10.1121/1.419374

出版商:Acoustical Society of America    

年代:1997

数据来源: AIP