### ASA 124th Meeting New Orleans 1992 October

## 3pUW5. Reflection and diffraction from a disk: A look in the time domain.

**Guy V. Norton
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**
R. Keiffer
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**
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*Naval Res. Lab., Stennis Space Center, MS 39529-5004
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*
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**Jorge C. Novarini
**

**
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*Planning Systems, Inc., Slidell, LA 70458
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The classical problem of sound scattering from an infinitesimal thin disk
is re-examined in the time domain via Fourier synthesis of solutions
calculated, in the frequency domain, through a T-matrix formalism. Two T
matrices were used. The first was specifically developed for the disk, the
second was more general in that it allowed a sphere to be transposed into an
oblate spheriod, where in the limit that the length of the semi-minor axis goes
to zero produces an infinitesimal thin disk. At difference from the frequency
domain, once the solution is mapped onto the time domain, diffracted and
reflected components of the scattered field are clearly separated. In this work
this feature is used to analyze the relative strength of the different
component of the impulse response, for the case of an acoustically rigid disk
(Neumann boundary condition), and an acoustically soft disk (Dirichlet boundary
condition). A point source is assumed, and the analysis is restricted to normal
incidence. Results are compared with predictions from a Helmholtz--Kirchhoff
solution. The main findings are that, while the reflected components of the
hard and soft disks differ only in their sign (as predicted by Kirchhoff
theory), the relative strengths of diffractions and reflections are drastically
different for the two cases. Furthermore, for the hard disk secondary
diffraction is clearly identified in terms of Huygens contributions, while no
counterpart is observed for the soft disk. In addition, results from the oblate
spheroid approaching the disk will be shown.