### ASA 124th Meeting New Orleans 1992 October

## 4pPA4. A discrete implementation of a Hankel transform technique for
predicting multipole sound propagation over plane, absorbing surfaces.

**Z. Hu
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*Noise Cancellation Technol., Inc., 1015 W. Nursery Rd., Linthicum, MD
21090
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**J. S. Bolton
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*Purdue Univ., West Lafayette, IN 47907
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Many theories are available for predicting sound propagation outdoors;
they are usually expressed in terms of sound radiation from a source (e.g., a
monopole) that generates a cylindrically symmetric sound field that propagates
over a homogeneous, finite impedance plane. However, not all practical noise
sources are monopolar in character nor do they necessarily generate azimuthally
symmetric sound fields. In this paper, a discrete implementation of a
two-dimensional finite Hankel transform technique is described that makes it
possible to predict sound propagation from higher-order sources that generate
noncylindrically symmetric sound fields: e.g., arbitrarily oriented dipole and
quadrupoles. As a result, the proposed technique may prove useful for
predicting sound propagation from aerodynamic noise sources place near plane
outdoor surfaces. The prediction procedure is based on representing the field
of a source as a two-dimensional wave-number spectrum. The wave-number spectrum
of the source field is then combined with the reflection coefficient of the
impedance plane to yield the wave-number spectrum of the reflected field. The
latter component is then added to the direct wave-number spectrum and the
result is inverse transformed to give the sound pressure as a function of
radius and azimuth angle. Predictions made using the new technique will be
compared with experimental results.