Efficient Sampling for Nearfield Acoustic Holography
The purpose of this investigation was to determine an efficient means of sampling and windowing data for the digital signal processing that is required for nearfield acoustic holography (NAH). In particular, a method for choosing the best grid spacing for data in the hologram plane was considered.
The pressure distribution over the surface of a baffled piston calculated by using the back-projected nearfield holography was of particular interest in this research. The effect of two sampling parameters, namely the aperture size and the sampling rate, were studied numerically. The recommended values for the efficient sampling parameters were developed.
A second aspect of this research concerned the location of the grid of the source. A power-of-two FFT algorithm is often used for the transformations, thus necessitating an even number of points in the sample. Centering this grid directly above the circular piston source left no data point on the axis of the piston. The nearfield of a piston vibrating at high frequencies had sharp peaks or depressions on the axis, so a centered grid with a relatively small spacing still missed a substantial portion of the on-axis maximum or minimum value. A routine was used to calculate the pressure on a grid that contained the on-axis value.
KeywordsSpatial Frequency Evanescent Wave Aperture Size Sampling Parameter Pressure Magnitude
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