Index of content:
Volume 120, Issue 4, October 2006
- STRUCTURAL ACOUSTICS AND VIBRATION 
120(2006); http://dx.doi.org/10.1121/1.2261244View Description Hide Description
Numerical approaches based on finite element discretizations of Biot’s poroelasticity equations provide efficient tools to solve problems where the porous material is coupled to elastic structures and finite extent acoustic cavities. Sometimes, it may be relevant to evaluate the radiation of a poroelastic material into an infinite fluid medium. Examples include (i) the evaluation of the diffuse field sound absorption coefficient of a porous material and/or the sound transmission loss of an elastic plate coupled to a porous sheet, (ii) the assessment of the acoustic radiation damping of a porous material coupled to a vibrating structure. The latter is particularly important for the correct experimental characterization of the intrinsic damping of the material’s frame. Up to now, the acoustic radiation of a porous medium into an unbounded fluid medium has usually been neglected. The classical approaches for modeling free field radiation of porous materials (i) assumes the interstitial pressure at the radiation surface to be zero or (ii) fixes the radiation impedance to an approximate value. This paper discusses the limitations of these assumptions and presents a numerical formulation for evaluating the sound radiation of baffled poroelastic media including fluid loading effects. The problem is solved using a mixed FEM-BEM approach where the fluid loading is accounted for using an admittance matrix. Both numerical examples and a transmission loss test are presented to illustrate the performance of the technique and its applications.
120(2006); http://dx.doi.org/10.1121/1.2266574View Description Hide Description
A model reduction method is developed to estimate modal parameters of fluid-loaded structures. The method uses a matrix-free formulation of rational Krylov projection to construct reduced order models of the fluid-loaded structures from forced responses at selected interpolation frequencies. Due to small sizes of the reduced order models, eigenpairs of the associated eigenvalue problems are available at a very low computational cost. Resonance frequencies, modal damping ratios, and mode shapes of the original systems are recovered from the forced responses and the eigenpairs of the reduced order models. Nonphysical modes introduced by the model reduction process are filtered out by a modal damping ratio test and double checked by the condition number of the dynamic stiffness matrix. Efficiency and accuracy of the present method are demonstrated on a benchmark model of a water-loaded plate clamped in a baffle.
Active vibration isolation experiments using translational and rotational power transmission as a cost function120(2006); http://dx.doi.org/10.1121/1.2228839View Description Hide Description
Active vibration isolation experiments were conducted using a transducer that measures translational and rotational power transmission from a vibrating mass, through a single-axis active isolator and into a beam. The transducer is capable of measuring forces and moments along six axes and an accelerometer array measures its motion. By combining the measured force and velocity signals the translational and rotational power transmission was measured. Comparisons were made of the effectiveness of several cost functions for minimizing the vibration transmitted into the beam. The results show that active vibration isolation using power transmission as a cost function to be minimized is limited by the phase accuracy of the transducers. The best results were obtained from the minimization of the weighted sum of force and velocity.