Volume 104, Issue 2, August 1998
Index of content:
- STRUCTURAL ACOUSTICS AND VIBRATION 
104(1998); http://dx.doi.org/10.1121/1.423335View Description Hide Description
A structure can simultaneously support different types of structural waves. For active control of vibration in a beam structure, it is necessary to consider all wave types simultaneously. Thus it is necessary to be able to measure the vibration amplitude associated with all wave types at any beam cross section as well as the total vibratory power transmission associated with all wave types. In this paper, the theoretical basis is outlined for a new method which allows measurement of all the required quantities using appropriately oriented and located accelerometers. Experimental results are also provided to illustrate the usefulness of this method.
Condenser microphone model. I. Application of the T-matrix method of Waterman to acoustic scattering from an elastic obstacle104(1998); http://dx.doi.org/10.1121/1.423336View Description Hide Description
Acoustic scattering of a plane wave from a condensermicrophone is investigated and the eigenfrequencies of this coupled acoustic system are calculated. The wave is incident parallel to the axis of a rigid cylinder, closed at the bottom by an elastic membrane. To take into account the normal modes inside the cylinder a Green’s function method is applied. Outside the scattering and interaction of the incident plane wave with the microphone is treated analytically using the transfer-matrix method of Waterman. Good agreement of the eigenfrequencies with measurement is achieved, but only for obstacles which are not too small, i.e., the wavelength is comparable to the dimensions of the microphone (radius a of the microphone membrane is greater than 0.015 m).
Full numerical solution for the far-field and near-field scattering from a fluid-loaded elastic plate with distributed mass or stiffness inhomogeneity104(1998); http://dx.doi.org/10.1121/1.423337View Description Hide Description
The scattering of a plane acoustic wave by a fluid-loaded thin elastic plate of infinite extent with a distributed mass or stiffness inhomogeneity is investigated. This paper is a follow up to previous work done by the same authors on the scattering from a fluid-loaded plate with a distributed mass inhomogeneity. In this paper both stiffness and mass distributed inhomogeneities are considered and a complete description of the full numerical solution is presented. Furthermore, both near-field and far-field scattering results are presented in this paper. The presence of the distributed inhomogeneity modifies the wave number transform of the equation of motion of the fluid-loaded plate to a Fredholm integral equation. This integral equation has singularities at the roots of the dispersion equation. To obtain a complete numerical solution of the Fredholm integral equation, a singularity subtraction technique is used, which is similar in essence to the hybrid analytic/numerical approach for the solution of the scattering from a fluid-loaded elastic plate [J. Acoust. Soc. Am. 95, 1998–2005 (1994)]. The solution to the resulting Fredholm integral equation of the second kind is obtained using the Nyström approximation. The results for the far-field scattering are for an oblique angle of incidence and for monostatic scattering. The results show that mass distributed inhomogeneities are stronger scatterers than distributed stiffness inhomogeneities for frequencies below the critical frequency of the plate. Above the critical frequency, both types of inhomogeneities have similar scattering strengths. Also included are results for different types of inhomogeneity distributions. This work was sponsored by ONR.
104(1998); http://dx.doi.org/10.1121/1.423338View Description Hide Description
A multi-level optimization approach for the design of feedforward active structural acousticcontrol (ASAC) systems is presented. The formulation takes advantage that both the structural response and the acoustic radiation from a controlled structure can be completely defined in the modal domain. All the physical parameters that define the control inputs and the error sensors are defined in the modal domain in terms of the unit modal control forces and the modal error sensor components, respectively. The upper level of the optimization solves for the optimum modal parameters that minimize the total radiated acoustic power. Then, these optimum modal parameters are used in a set of lower level or physical domain optimization problems to determine the physical characteristics of the actuators and sensors to be implemented. Since the response of the system is evaluated in the upper level using a modal approach, the formulation permits the implementation of numerical techniques and/or experimental data during the design process. Therefore, the proposed methodology can be used for the design of control systems for realistic structures with complex disturbances in an efficient manner. The design formulation is illustrated for the case of a simply supported plate excited by an off-resonance disturbance.