885095992

9:35

4SA3. Active control of sound radiation from a uniform rectangular fluid-loaded piąte. Yi Gu and Chris R. Fuller (Dept. of Mech. Eng._t Virginia Polytech. Inst. and State Univ., Blacksburg, VA 24061)

Active control of sound radiation from a simply supported rectan-gular one-sidc fluid-loaded piąte is analytically studied. The piąte is assumed to be excited by a point force at subsonic frequencies. The dynamie equation of the piąte motion is based on the in oacuo eigen-functions of a homogeneous panel as the basis for Fourier dccomposi-tion of the fluid loading. Feed-forward control is applied by either point forces or piezoelectric actuators attachcd to the platc. The amplitudes of the control forces are determined by the optima! solution of a quadratic cost function which integrates the far-field radiated acoustic pressure in a hemisphere in the fluid half-space. The results show that for on-resonance frequencies, high global reduction in radiated pressure is pos-sible with two active forces properly located on the piąte. For oflf-resonance conditions in order to provide global control an inereased number of control forces is rcquircd. However, for the Iow frequencies considered here four control forces proved adequate. The far-field di-rcctivity pattern, the modal amplitudes of the piąte vibration, the platc power spectrum in a two-dimensional wave-number domain, and the near-field intensity distribution are extensively studied. The work con-firms that the feed-forward control tcchnique will work for heavy fluid loading where the piąte modes are nonorthogonal and modę edge cou-pling is important. [Work supported by ONR/DARPA.]

9:50

4SA4. "Smart" noise control foams: A feaslbility study. J. S. Bolton, N.-M. Shiau, and E. R. Green (Ray W. Herrick Labs., School of Mech. Eng., Purdue Univ., West Lafayette, IN 47907)

A porous layer can absorb a significant amount of acoustical energy only if its thickness is comparable to the wavelength of the incident sound. Thus, a porous layer inevitably becomes a less effective sound absorber as the frequency is decreased. In this paper it will be shown through theoretical calculations that the low-frequency performance of a finite-depth layer of elastic porous materiał may be enhanced by ap-plying an appropriate force to the solid phase at the front surface of the layer. In particular, it will be shown that at any angle of incidence the solid phase may be forced in such a way to crcate a perfect impedance match with an incident piane wave, thus causing the sound to be com-pletely absorbed. Complete absorption is not possible when the incident sound field is diffuse; nevertheless, sample calculations will show that considerable enhancement of the passive absorption is still possible. Notę that the success of the approach suggested here requires a significant degree of coupling between the motion of the solid and fluid phases of the porous materiał. Thus it may be expected that partially reticulated, polyurethane foams will be susceptible to this approach owing to the degree of viscous and inertial coupling between their fluid and solid phases. Several methods for actuating the front surface of an elastic porous materiał and for sensing its surface normal impedance will be discussed.

10:05

4SA5. Numerical simulation of active noise control for three-dimensional structures: Prellminary investigation. Christopher E. Ruckman and Christopher R. Fuller (Dept. of Mech. Eng., Yirginia Polytech. Inst. and State Univ., Blacksburg, VA 24061)

In certain cases, low-frequency noise radiated by a structure into a surrounding fluid can be attenuated by applying harmonie “control forces” directly to the structure. If the system is contmllable. i.e.. if the proposed control forces are sufficiently capablc of influencing the radiation, then global radiation attenuation is possible. Recent studies show that some structures, including baffled fiat plates, are indeed conlrolla-ble in this sense. But analyzing a complex three-dimensional structure.

particularly if it is immersed in a dense fluid, can be difficult because there usually is no analytical solution for the radiated field. The present research adapts NASHUA, a generał finitc cłement/boundary element Computer program, to investigate whether radiation from three-dimensional structures can be controlled by harmonie forces. The pre-liminary test case to be presented is a thin spherical shell, chosen because it has an analytical solution. Control force locations are determined by examining the spatial Fourier transform of the surface velocity. Control force magnitudes and phases are found by minimizing the radiated power using quadratic optimal control theory. Also exam-ined are scnsitiviiy to control-force placement and phasing errors, and sensitivity to system Identification errors. The numerical and analytical results are found to be in good agreement. [Work supported by ONR/ DARPA.J

10:20

4SA6. Digital time delay network for active underwater acoustic coating. Thomas R. Howarth (HVS Technologies, 820 N. University Dr., State College, PA 16803), Xiaoqi Bao, Vijay K. Varadan, and Vasundara V. Varadan (Dept. of Eng. Sci. and Mech. Res. Ctr. for the Eng. of Electron, and Acoust. Mater., Penn State Univ., 149 Hammond Bldg., University Park, PA 16802)

A digital time delay system has recently been developed for control of an active underwater coating. The active coating contains piezoelec-tric polymer sensors and a piezocomposite aciuator encapsulated within a host polymer. The coating is for layering on submerged objects to actively reduce underwater sound reflection. The control system incor-porates time delay circuitry as opposed to the analog phase delays used in previous studies. The type of control is an open-loop velocity feed-back. This system prevents feedback instability because the acoustic reflection is decoupled from the actuator. The digital controller can characterize both the incident acoustic pressure and the reflected pressure through parallel multichannel processing. The reflected pressure is monitored at the sensors for determination of the eflcctive sound reduction. Experimental results were obtained in a water-filled acoustic pulse tubę terminated at its boundary with the active coating. Echo reduction comparisons as functions of sound reduction and bandwidth will be presented.

10:35

4SA7. Active control of edge sound radiation from a clamped beam.

Catherine Guigou and Chris R. Fuller (Dept. of Mech. Eng., Virginia Polytech. Inst. State Univ., Blacksburg, VA 24061)

Active control of sound radiation from a thin semi-infinite beam clamped at one end is analytically studied. Active control is achieved by applying either control forces (approximating shakers) or control mo-ments (approximating piezoelectric actuators) on the beam near the edge discontinuity. For a single frequency, the flexural response of the beam subject to an exciting point force and to the control forces or control moments is expressed in terms of waves of both propagating and near-field types. The control force or control moment magnitudes are derivcd by optimizing a quadratic cost function which is defined as the acoustic power radiated from one side of the baffied beam and obtained by integrating the far-field radiated acoustic intensity over a hemisphere. For single frequencies, large attenuation in radiated acoustic power and pressure is found when one and two control forces or two and four paired control moments are applied. The proper location of the control forces or control moments is determined in order to obtain the maximum decrease of the optimized radiated acoustic power. The influence of the control near-field component on the amount of attainable attenuation is studied and found to be important. The far-field radiated pressure dircctivity, the displacement of the vibrating beam, and the one-dimcnsional wave-number spectrum of the beam velocity are exten-sively studied. The work provides insight into active control of edge radiation from morę complex 2-D structures. [Work supported by ONR/DARPA]

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J. Acoust. Soc. Am.. Vol. 89. No. 4. Pt. 2. April 1991

121st Meeting: Acoustical Society of America

1915