Improved data representation in NAH applications by means of zero-padding

Near-field Acoustic Holography (NAH) is a method to identify sound sources. To obtain the radiated sound field in a plane close to the sound source, the sound field is experimentally determined at an array of measurement positions close to the source. This array has a finite number of measurement points because of measurement restrictions in time and spatial domain. With respect to the transformation from the spatial domain to the wavenumber domain, it is common to use windowing techniques in combination with zero-padding. A spatial window such as the Tukey window is used to reduce leakage. Zero-padding in the spatial domain increases the detail of the spectrum in the wavenumber domain through a kind of interpolation. It will be shown that the interpolation due to zero-padding corresponds very well to the actual physical data, enhancing the measurement data in NAH applications.The first part of this paper will focus upon the theory behind NAH signal processing. By means of a basic sine wave all operations concerning the NAH signal processing will be illustrated and explained. Extra care has been taken to explain the data representation improvements by means of zero-padding and the NAH specific imaging improvements by means of a newly introduced method called border-padding. Following the analysis, the application and improvements of windowing techniques as well as zero- and border-padding in both spatial and wavenumber domain for the NAH method will be shown. The results show a clear improvement in the level of detail of the planar acoustic data by optimal interpolation and by minimization of windowing errors