Index of content:
Volume 116, Issue 2, August 2004
- STRUCTURAL ACOUSTICS AND VIBRATION 
116(2004); http://dx.doi.org/10.1121/1.1763957View Description Hide Description
The dispersion properties of bending wave, localized near the stress-free edge of a thin isotropic plate, are investigated for relatively high frequency. In order to clarify the nature of the wave, attention is focused to the sensibility to the improved boundary conditions and to the iterations of the main differential operator when describing by two-dimensional (2-D) approximate plate theories. As shown, the boundary conditions of next asymptotic order do not change too much the leading part of the desired wave, but the corrections of the main operator lead to dramatical changes. Finally, beginning with its simplest Kirchooff’s plate model only the leading part of wave may be subjected to asymptotic correction. For applications the wave speed calculation is reduced to two simple analytical formulas. The results agree well with experiments and finite elementtesting. The alternative approach related to the Timoshenko–Reissner–Mindlin theory is discussed.
116(2004); http://dx.doi.org/10.1121/1.1766022View Description Hide Description
A novel homogenization method for periodic structures that utilizes a local/global separation of the high and low wavenumber spectrum is presented. The low-wavenumber global problem has an infinite-order operator. The global problem is self-contained; local solutions can be reconstructed after the fact if desired. Global problems are constructed for a membrane and a plate in vacuo, each with periodic impedance discontinuities. A fluid-loading approximation is introduced in order to homogenize problems of interaction between fluid and structure. Radiating acoustic modes are contained in the smooth global problem, and the global structural operator accounts for an influence of evanescent acoustic modes. As an example, oblique sound reflection from a flexible barrier with impedance discontinuities is analyzed. Accurate results are obtained from the method.
116(2004); http://dx.doi.org/10.1121/1.1766052View Description Hide Description
A general approach to solving for the velocity response and associated pressure field of elastic solids loaded internally by a fluid is formulated. An in vacuoeigenvector expansion is used to describe the time dependent velocity field of the fluid loaded solid resulting from an arbitrary force excitation. The pressure field in the fluid is expressed using a Green’s function and fluid loading on the structure is included via the use of the impulse responses of the modal acoustic radiation impedances. The time dependent modal velocity responses are obtained from the solution to a set of coupled convolution integral equations. A Fourier transform approach is used to develop the associated equations of motion for the harmonic response. The special case of a fluid filled axisymmetric elastic cylindrical shell with a base plate is considered. Transient and harmonic velocity and pressure responses due to a bandlimited axisymmetric point force excitation applied to the base plate are discussed and compared with experimental measurements.
A linear least-squares version of the algorithm of mode isolation for identifying modal properties. Part I: Conceptual development116(2004); http://dx.doi.org/10.1121/1.1765195View Description Hide Description
The Algorithm of Mode Isolation (AMI) is an iterative procedure for identifying the number of modes contributing to a frequency response function (FRF) concurrently with identifying the complex eigenvalues and eigenvectors of those modes. The latest modifications obtain these modal properties solely by using linear least squares fits of the FRF data to canonical forms. The algorithmic operations are explained in a detailed sequence of steps that are illustrated by some sample data. The computational efficiency of AMI relative to other modal identification algorithms that fit response data to multi-degree-of-freedom modelequations is discussed.
A linear least-squares version of the algorithm of mode isolation for identifying modal properties. Part II: Application and assessment116(2004); http://dx.doi.org/10.1121/1.1765196View Description Hide Description
The latest modifications of the algorithm of mode isolation (AMI) for identification of modal properties from frequency response data are tested with synthetic data derived from an analytical model of an elastic frame in which flexure and torsion are coupled. The parameters of this model are selected to cause the occurrence of localized modal patterns in two modes having close natural frequencies. The response data is contaminated with white noise at a level sufficient to almost mask the two close modes. Results for the real and imaginary part of the eigenvalues are tabulated. The analytical modal patterns of displacement and torsional rotation are depicted graphically, accompanied by the discrete values obtained from AMI. Excellent agreement is found to occur for each mode, other than one of the pair of close modes. The poorer quality of that mode’s identified properties is shown to be a consequence of its localized modal pattern. Results for the eigenvalues obtained by the rational fraction polynomial algorithm, which is an alternative modal identification technique, are found to be substantially less accurate as a consequence of difficulty in the presence of noise.