Noise Cancellation Technol., Inc., 1015 W. Nursery Rd., Linthicum, MD 21090
J. S. Bolton
Purdue Univ., West Lafayette, IN 47907
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.