Index of content:
Volume 108, Issue 3, September 2000
- STRUCTURAL ACOUSTICS AND VIBRATION 
108(2000); http://dx.doi.org/10.1121/1.1286515View Description Hide Description
Resonances of shell structures play a prominent role in determining their acoustic characteristics. Among the most important of such characteristics are radiated noise, scattering, and self-noise levels. The nature of the structural resonances, for example the number of modes within a particular frequency band or the overall spatial dependence of the response, determines the importance of these phenomena in various contexts and what steps may be taken to modify their acoustic impact. In the past, knowledge of the resonant response of highly idealized systems has been used to guide the development of quiet platforms. The results of a scale model investigation of the effects of increased complexity on the nature of the resonances of submerged shell structures are reported. Effects presented here are flexural Bloch mode resonances, localized resonances resulting from structural irregularity, and the effects of non-axisymmetry on these phenomena. The implications of these results on acoustic design and the development of systems is discussed.
108(2000); http://dx.doi.org/10.1121/1.1286514View Description Hide Description
Efficient sensing schemes for the active reduction of sound radiation from plates are presented based on error signals derived from spatially weighted plate velocity or near-field pressure. The schemes result in near-optimal reductions as compared to weighting procedures derived from eigenvector or singular vector analysis of the radiation operator. Efficient control configurations are suggested using a, possibly analog, front-end implementing a bank of spatial weighting functions and a digital controller with a minimized number of input and output channels. The performance of different weighting functions is compared, as well as the performance of different frequency-dependent filtering functions. Design rules are given for the sensor spacing, the number of weighting functions, the number of actuators, and the corresponding controller dimensionality.
The vibration behavior of railway track at high frequencies under multiple preloads and wheel interactions108(2000); http://dx.doi.org/10.1121/1.1288408View Description Hide Description
The track foundation is preloaded by multiple wheel loads due to the train weight and, as the pad and ballast are nonlinear, their stiffness depends upon the preload in them. Due to the influence of these resilient components of the track, the track vibration is affected by the wheel loads. It is also affected by the wheel/rail interactions. In this article the preloads in the pad and ballast are calculated by considering the nonlinear properties of the track foundation, and thus the preloaded pad and ballast stiffnesses are determined. The vibration properties are explored for the track under multiple wheel loads and multiple wheel/rail interactions by comparing the results from different track models with and without these effects. It is found that the point receptance of the track is reduced and the vibration decay rate is enhanced at low frequencies due to the wheel loads. The effects of the wheel/rail interactions are most significant for frequencies 400–2000 Hz. Because of the wheel/rail interactions, the point receptance fluctuates and the vibration decay is enhanced in the regions around the wheels.