Volume 110, Issue 2, August 2001
Index of content:
- GENERAL LINEAR ACOUSTICS 
110(2001); http://dx.doi.org/10.1121/1.1378355View Description Hide Description
The analysis of aeroacoustics propagation is required to solve many practical problems. As an alternative to Euler’s linearized equations, an equation was established by Galbrun in 1931. It assumes the flow verifies Euler’s equations and the perturbation is small and adiabatic. It is a linear second-order vectorial equation based on the displacement. Galbrun’s equation derives from a Lagrangian density and provides conservative expressions of the aeroacoustics intensity and energy density. A (CAA) method dealing with the numerical resolution of Galbrun’s equation using the finite-element method(FEM) is presented. The exact solution for the propagation of acoustic modes inside an axisymmetric straight-lined duct in the presence of a shear flow is known and compared with the FEM solution. Comparisons are found to be in good agreement and validate a first step in the development of a CAA method based on the FEM and Galbrun’s equation. The FEM is then applied to an axisymmetric duct including a varying cross section and a nonuniform flow with respect to both the axial and the radial coordinates. The expression of the aeroacoustics intensity implemented in the FEM provides an accurate in-duct power balance.
The reflected and transmitted acoustic field excited by a supersonic source in a two media configuration110(2001); http://dx.doi.org/10.1121/1.1381020View Description Hide Description
The acoustic wave field due to a supersonic motion of a rigid object over a half-space is investigated. The analysis presented leads to closed form expressions for the reflected and transmitted conical waves as separate contributions. The linearized acoustic field equations are applied to obtain representations for the fields in a combined Laplace–Fourier transform domain. To these representations, which are mapped into the proper form, we apply the Cagniard–de Hoop technique in order to find closed form time–domain solutions for the reflected and transmitted acoustic fields. Attention is focused on supersonic effects, so the analysis concentrates on application of the Cagniard–de Hoop technique to obtain closed form space–time domain expressions from the contributions of poles, appearing in the transform domain representation for the reflected and transmitted acoustic wave field. It turns out that these pole contributions, next to wave solutions of the conical type also can give rise to headwaves, associated with the reflected conical wave. Numerical results for the reflected as well as the transmitted conical wave field are presented for an air–sea configuration with supersonic source velocities up to MACH 5.
110(2001); http://dx.doi.org/10.1121/1.1381021View Description Hide Description
The theory of the dynamic bulk modulus, of a porous rock, whose saturation occurs in patches of 100% saturation each of two different fluids, is developed within the context of the quasi-static Biot theory. The theory describes the crossover from the Biot–Gassmann–Woods result at low frequencies to the Biot–Gassmann–Hill result at high. Exact results for the approach to the low and the high frequency limits are derived. A simple closed-form analytic model based on these exact results, as well as on the properties of extended to the complex ω-plane, is presented. Comparison against the exact solution in simple geometries for the case of a gas and water saturated rock demonstrates that the analytic theory is extremely accurate over the entire frequency range. Aside from the usual parameters of the Biot theory, the model has two geometrical parameters, one of which is the specific surface area, of the patches. In the special case that one of the fluids is a gas, the second parameter is a different, but also simple, measure of the patch size of the stiff fluid. The theory, in conjunction with relevant experiments, would allow one to deduce information about the sizes and shapes of the patches or, conversely, to make an accurate sonic-to-seismic conversion if the size and saturation values are approximately known.
110(2001); http://dx.doi.org/10.1121/1.1382616View Description Hide Description
Compressional wavevelocity and attenuation were measured at frequencies of 200–1500 Hz on seafloor sediments at Lough Hyne, Ireland, using a mini-boomer source and hydrophone array. Velocity and attenuation were also measured in the laboratory at 200–800 kHz on a 1 m long sediment core taken from the site. The in situ results indicate an average sediment phase velocity of about 1600 m/s and sediment quality factor of 10–20. The laboratory core measurements give an average phase velocity of and quality factor of The poorly sorted, Lough Hyne sediments are highly attenuating and highly dispersive when compared to values published in the literature for well-sorted, marine sediments such as clean sands and marine clays. The results are consistent with the few published data for poorly sorted sediments, and indicate that intrinsic attenuation is highest when the mass ratio of mud to sand grade particles is close to unity. It is proposed that compliance heterogeneities are most abundant when mud and sand grade particles are present in roughly equal proportions, and that the observations support local viscous fluid flow as the most likely loss mechanism.
Frequency analysis of the acoustic signal transmitted through a one-dimensional chain of metallic spheres110(2001); http://dx.doi.org/10.1121/1.1385179View Description Hide Description
This work investigates the propagation of acoustic pulses through a chain of elastic spheres embedded in air. This study is an extension of the works realized on individual sphere by several authors for measuringelastic constant and internal friction with a monofrequential acoustic excitation. The frequency analysis of the experimental transmitted train waves exhibit maxima which were correlated to different types of free vibration modes: the Rayleigh modes the torsional modes and the spheroidal modes These resonances may be generated separately according to the polarization of the excitation pulse.