Volume 107, Issue 5, May 2000
Index of content:
- STRUCTURAL ACOUSTICS AND VIBRATION 
107(2000); http://dx.doi.org/10.1121/1.428637View Description Hide Description
When acoustic-structure interaction is analyzed according to the wave number-based version of the surface variational principle (SVP), considerable computational effort is required to generate each coefficient in the quadratic sum forming the variational quantity. Examination of the wet-surface impedance, which represents the spectrum of pressure amplitudes generated by a specified spectrum of surface velocity amplitudes, reveals that many cross-impedance terms are very small. This suggests that some coefficients need not be computed. The article introduces a priori criteria for selecting the impedance terms to be omitted, based on the supersonic cutoff wave number. Each truncation scheme is assessed by comparing its predictions to the convergent SVP solution. It is shown that, for the nonsymmetric azimuthal harmonics, the field quantities and the radiated power are well predicted if subsonic waves are ignored. In contrast, for the axisymmetric component, substantial errors (6 dB or more) for radiated power arise, unless a broad spectrum of subsonic waves are included in the formulation. The complex power is mostly reactive, being associated with an evanescent field, even for the supersonic spectrum. The study shows that small interactions between subsonic waves in the axisymmetric case can result in the high wave number spectrum being associated with substantial radiation contributions.
Acoustic radiation from an elastic baffled rectangular plate covered by a decoupling coating and immersed in a heavy acoustic fluid107(2000); http://dx.doi.org/10.1121/1.428638View Description Hide Description
The vibroacoustic behavior of an elastic, simply supported rectangular plate covered by a locally reacting decoupling layer supporting thickness deformation is presented. The model simulates the vibration and acoustic response of the system immersed in water and subjected to a point force disturbance. A simplified version of the theory is derived in the limiting case of a large decoupling (low mechanical impedance of the layer/high frequency). An appropriate vibratory indicator, representative of the acoustic attenuation provided by the decoupling treatment, and independent of the structure dimensions, is also investigated from the perspective of small-scale laboratory characterization
107(2000); http://dx.doi.org/10.1121/1.428639View Description Hide Description
This paper presents analyses and implementation of the reconstruction of acoustic pressure fields radiated from a general, three-dimensional complex vibrating structure using the Helmholtz equation least-squares (HELS) method. The structure under consideration emulates a full-size four-cylinder engine. To simulate sound radiation from a vibrating structure, harmonic excitations are assumed to act on arbitrarily selected surfaces. The resulting vibration responses are solved by the commercial FEM(finite element method) software I-DEAS. Once the normal component of the surfacevelocity distribution is determined, the surfaceacoustic pressures are calculated using standard boundary element method(BEM) codes. The radiated acoustic pressures over several planar surfaces at certain distances from the source are calculated by the Helmholtz integral formulation. These field pressures are taken as the input to the HELS formulation to reconstruct acoustic pressures on the entire source surface, as well as in the field. The reconstructed acoustic pressures thus obtained are then compared with benchmark values. Numerical results demonstrate that good agreements can be obtained with relatively few expansion functions. The HELS method is shown to be very effective in the low-to-mid frequency regime, and can potentially become a powerful noise diagnostic tool.
Active control of harmonic sound transmission into an acoustic enclosure using both structural and acoustic actuators107(2000); http://dx.doi.org/10.1121/1.428640View Description Hide Description
This paper describes an analytical and experimental investigation into the active control of harmonic sound transmission in a structural–acoustic coupled system. A rectangular enclosure is considered that has five acoustically rigid walls and a flexible plate on the remaining side through which a harmonic sound wave is transmitted into the enclosure. The control system is designed to globally reduce the sound field inside the enclosure, and the roles of structural and acoustic actuators are of particular interest. Three control configurations, classified by the type of actuators, are compared and discussed. They are: (i) use of a single point-force actuator, (ii) use of a single acoustic piston source, and (iii) simultaneous use of both a point-force actuator and an acoustic piston source. It is shown both analytically and experimentally that the point-force actuator is effective in controlling plate-dominated modes while the acoustic source is effective in controlling cavity-dominated modes. Since the transmitted sound field is governed by both plate- and cavity-dominated modes, the hybrid use of both types of actuators is shown to be a desirable configuration for the active control of sound transmission into a structural–acoustic coupled system.