Index of content:
Volume 103, Issue 3, March 1998
- STRUCTURAL ACOUSTICS AND VIBRATION 
A numerical approach to determining the transient response of nonrectangular bars subjected to transverse elastic impact103(1998); http://dx.doi.org/10.1121/1.421284View Description Hide Description
The objectives of the studies presented in this paper are to propose an efficient numerical approach to determining the transient response of nonrectangular bars subjected to transverse elastic impact. It is shown that the response of nonrectangular bars, such as T- and I-beams, subjected to transverse impact is composed of frequencies corresponding to the cross-sectional modes of vibration of the nonrectangular bar. In this study, eigenvalue analyses of plane-strain finite-element models of T- and I-sections were performed first to solve for the cross-sectional mode shapes and the corresponding natural frequencies. Three-dimensional resonant analyses were used to verify the existence of the cross-sectional modes obtained from the eigenvalue analyses. Three-dimensional finite-element models were also used to determine the transient response of T- and I-beams subjected to transverse elastic point impact. Numerical results were verified by experimental studies on a 0.5-m-deep concrete T-beam and a 0.72-m-deep concrete I-beam. Good agreement was obtained between the numerical and experimental results. It is concluded that the proposed numerical models are valid and efficient for determining the transient response of nonrectangular bars subjected to transverse impact.
103(1998); http://dx.doi.org/10.1121/1.421285View Description Hide Description
This paper presents an exact analytical solution to the vibrational intensity of torsional waves in three-dimensional solid and hollow cylindrical bars of finite length. As the torsional response in circular cylindrical bars to the excitation of pure torsional moments is uncoupled with the motion of other degrees of freedom, its characteristics can be investigated independently. After verifying the orthogonal properties of the eigenfunctions of the torsional waves in a bar with fixed-free ends, the steady state response of the bar to a torsional moment was obtained analytically. The solution of the response was then utilized to calculate the vibrational intensity. Examples are used to illustrate the features of the torsional intensity in a hollow cylindrical bar.
103(1998); http://dx.doi.org/10.1121/1.421286View Description Hide Description
The dynamic and acoustic response of an unbaffled rotating disk, subjected to a space-fixed harmonic lateral force, is investigated. The plate is supposed to be clamped at the inside and free at the outer boundary. The exciting force being space-fixed, the mass and stiffness matrices, obtained from the variational method combined with the Rayleigh–Ritz approach in the rotating frame, are converted back to the nonrotating frame, where the generalized force and the displacements of the plate are evaluated. The related acoustic pressure, neglecting fluid loading, is determined using a boundary element formulation. Special attention has been given to ensure correct modeling of the unbaffled condition on the radiation of the plate. Results are presented to show the effect of rotation for both dynamic response and acoustic radiation. A practical case is also investigated: the circular saw. To simulate the exciting force of the saw, a measurement of the force induced by one tooth has been done. Then, a retarded-time sum has been performed to account for the exact number of teeth.
Active control of sound radiation from a plate using a polyvinylidene fluoride volume displacement sensor103(1998); http://dx.doi.org/10.1121/1.421301View Description Hide Description
This paper presents a new volume displacement sensor (made of shaped strips of PVDF film) and the experimental implementation of this sensor in an active control system. A design strategy for a PVDF sensor detecting the volume displacement induced by a vibrating 2D structure is presented. It is based on the modal representation of the plate response. It actually consists in designing a PVDF sensor, composed of several shaped PVDF strips bonded to the surface of the structure, in such a way that the output signal of the sensor is directly proportional to the volume displacement. The design methodology is based on the experimental measurements of the plate mode shapes (eigenfunctions) and is valid for any type of boundary conditions. The experimental implementation of such a volumetric sensor in an active control system is then presented. The experimental results obtained validates this new type of volume displacement sensor.
103(1998); http://dx.doi.org/10.1121/1.421287View Description Hide Description
An important issue in sensor studies is to define sensor configurations such that the reduction of a performance measure based on the sensormeasurements will guarantee a decrease in a global response measure of the system. This paper presents a study of sensors along this line. In the paper, the concept of global sensorsystems is introduced. The condition for such a sensorsystem is derived. This concept is then applied to structural and acoustic control problems. It is shown in the paper that the nodal placement of sensors always leads to a global sensorsystem, and allows a single sensor gain to scale the sensor based performance measure in such a way that it is identical to the true global performance measure of the system response. Tonal and narrow-band problems are also considered where a global sensorsystem may be redundant. The rank of a reduced performance matrix is used to determine the number of sensors needed in these applications. The theory of the paper is demonstrated by using simple examples. Further work needs to be done to extend the present study to more complex systems. Nevertheless, the study presented in this paper may have significant implications in designing practical sensorsystems for both structural and acoustic control problems.