885095998

The use of gentamicin as an antimicrobial agent has been shown to produce as an untoward effect. ototoxicity. The purpose of this study was to invcstigate difTerential effects of gentamicin ototoxicity as a func-tion of Rx timing with regard to circadian rhythms. Spraguc-Dawley rats received a daily subcutaneous dosage of 100 mg/kg of gentamicin. The rats were maintained on a light-dark 12:12 illumination cycle with light commencing at 0000. One group of rats received gentamicin at 0200 with the other group at 1400. Hearing loss was assessed with the auditory brainstem response using pure-tone stimuli of 8, 16, 24, and 32 kHz. Thcse measures werc obtained at baseline, 2 and 4 weeks after the initial dosing. Measures of blood-gentamicin lcvel and nephrotoxicity were conlrasted for the groups. Ototoxicity was greater for gentamicin when given to the rodents during their diumal rest span (0200) in comparison to their nocturnal activity span (1400). Thesc circadian dependent effects are thought to be related to rhythmic variations in the disposition and accumulation of gentamicin in cochlear tissues.

WEDNESDAY AFTERNOON, 1 MAY 1991

INTERNATIONAL D, 1:45 TO 4:05 P M.

Session 5SA

Structural Acoustics and Vibration: Structural Acoustics and Fluid-Structure Interaction

Jerry H. Ginsberg, Cochair

School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332

David Feit, Cochair

Codę 1904, Dauid Taylor Research Center, Bethesda, Maryland 20084

Chair’s Introduction—1:45

Contributed Papers

1:50

5SA1. Phase*space description of random fluid-structure interaction.

E. K. Dimitriadis, J. J. McCoy, and M. J. Beran (School of Eng. and Architecture, Catholic Univ. of America, Washington, DC 20064)

Random dynamie excitation of plates in contact with fluids gener-ally extends over a finite region of the piąte. Consequently both excita-tion and response fields are spatially inhomogeneous. Two-point corre-lations of such fields are here described as functions of an averagc and a distance coordinate. The latter is subseąuently Fourier transformed and the resulting wave-vector spectrum r(x,k;<u) (often called the Wigner distribution function), defined simultaneously in physical and wavc-vector space, namely a phase-space, loosely describcs energy flow at x associated with the length scalę and dircction of k. U is shown that the Wigner functions for the response for the excitation are related through a convolution with a kemel function G_t (x — r,k;o>) that de-pends onły on a smali number of nondimensional system parameters. The naturę of this kemel function and the resulting energy flow dircc-tivity are discussed and interpreted. In addition, results are presented for a specific example of an infinite plale in contact with a heavy fluid and excited by a finite region of turbulence in the fluid.

2:05

5SA2. Observabłe-based hybrid ray-mode-resonance system parametrization of acoustic scattering by a finite submerged Steel piąte. L. B. Felsen*^> and T. K. Kapoor (Dept. of Ocean Eng., MIT, Cambridge, MA 02139)

Numerical finite diffcrence data for pulsc scattering by a submerged Steel piąte of finite width, when processed in the slowness-time domain [J. R. Fricke and A. B. Baggeroer, J. Acoust. Soc. Am. Suppl. 1 88, S51 (1990)], reveals distinct features related to the longitudinal and flexural modes in the piąte, their excitation by, and conversion to, edge-

1932 J. Acoust. Soc. Am., Vol. 89, No. 4. Pt. 2. April 1991 diffracted fields in the fluid, and other wave-oricnted observables. By an observable-based parametrization (OBP), the problem is phrased here systematically and phenomenologically in terms of ray fields, traveling modę fields, edge coupling matrices, resonances, etc., to build up a self-consistent OBP system format for quantitative prcdiction. Determi-nation of the elements in the coupling matrices poses a set of canonical problems that can be addressed analytically under highly simplified conditions but must generally be done numerically or by experiment. For the very specialized case of a thin piąte under heavy fluid loading, the piąte supports only one subsonic wave, and singlc-edge diffraction has been evaluated analytically (D. C. Crighton and D. Innes, Philos. Trans. R. Soc. London Ser. A 312, 291-341 (1984)]. This solution has been employed in the OBP algorithm to synthesize and computc the far-field acoustic response to on-plate linę force excitation of a finite strip. The result is compared with a difTcrently synthesized solution by Crighton. (Work supported by ONR.] ^a)Visiting.

2:20

5SA3. Acoustic radiatiun from fluid-loaded struć tu res with discontinuities. Charles Thompson, Vineet Mehta, and Rahul Sen (Dcpi. of Elcc. Eng., Lab. for Advanccd Computation, Univ. of Lowell, Lowell, MA 01854)

The presence of discontinuities, such as joints, in a fluid-loaded structure results in far-field acoustic radiation even when subsonic wave numbers prevail in the main body of the structure. The object of this study is to develop a rational model of this phenomenon using singular perturbation methods. When a flexural wave in a thin structure im-pinges on a joint, evanescent structural displacement fields arc set up in the vicinity of the joint. It is proposed to show that these fields are associated with rotation-dominated modes that are supported by shear-corrected piąte theories of the Timoshenko-Mindlin-type flexural modę. The method of matched asymptotic expansions will be used to obtain

121st Meeting: Acoustical Sodety of America 1932