A comparison of two patch NAH methods

The spatial FFT processing used in Near-field Acoustical Holography (NAH) makes the method computationally efficient, but it introduces severe spatial windowing effects, unless the measurement area is significantly larger than the source. To avoid the need for these very extended measurement areas, Patch Holography techniques have been developed to provide acceptable accuracy based on measurements that only partially cover the source area. Calculation and mapping is then performed over a corresponding concentric patch, typically on the source surface. One such method performs an iterative analytic continuation of the measured pressure beyond the measurement area, followed by the use of standard NAH procedures. Two other methods completely avoid the use of spatial FFT, and instead they perform a least squares fit of a local sound field model to the measured sound pressure data. Statistically Optimized Near-field Acoustical Holography (SONAH) uses a local plane wave model, whereas the Helmholtz Equation Least Squares (HELS) method applies a local spherical wave model. The present paper gives first a short introduction to the HELS and the SONAH methods, with the aim of highlighting similarities and differences between the two methods. Then, the accuracy of the two methods is compared through a set of simulated measurements.