Projecting nearfield acoustical holography (NAH) past intermediate sources

Abstract

With planar nearfield acoustic holography (NAH), the complex acoustic pressure and the three components of the acoustic velocity vector can be estimated on a plane near a complex source surface to identify individual noise sources. However, there are often intermediate sources between the measurement plane and the primary source plane. The NAH technique in its classical form cannot be extended past the intermediate source. A method will be presented to remove the intermediate sources, so that NAH can be extended to the surface of the primary source. The normal velocity at the surf ace of an intermediate source is used to estimate the complex pressure due to the intermediate source on the measurement plane, which is then subtracted from the measured data, providing an estimate of the measured sound pressure, if the intermediate source was not present. NAH is then used to project to the surface of the primary source. The source removal process was tested with simulated and measured data. Results for two methods to estimate the pressure caused by the intermediate source, inverse NAH and a point source approximation, will be presented. Also, a third method of source removal based on approximating the intermediate source as a force distribution function in the wave equation is also presented. The results show the effectiveness and limitations of the source removal process.