Nearfield acoustic holography and its evolution

This paper reviews the evolution of nearfield acoustic holography (NAH) over the past two decades. The original NAH (Williams and Maynard, 1980) could only reconstruct acoustic radiation from a surface containing a level of constant coordinate in an exterior region. Soon a generalized NAH based on the Helmholtz integral theory and boundary element method (BEM) was developed to reconstruct acoustic radiation from arbitrarily shaped surfaces. This BEM-based NAH is advantageous in many aspects, but has several inherent shortcomings. An alternative is to expand the acoustic field in terms of the spherical wave functions, the most prominent one being the Helmholtz equation–least squares (HELS) method. This HELS method offers a great flexibility and versatility in reconstructing the acoustic fields in both exterior and interior regions, and allows reconstruction of transient acoustic radiation from impulsively accelerated objects. Meanwhile, we have seen the extension of NAH to characterize acoustic sources of a moving object and variations of NAH based on measurements of acoustic intensity rather than pressures. Many techniques are developed along the way to tackle the ill-posedness difficulties inherent in this inverse acoustic problem. Progression of able researchers is transforming NAH into an ever more powerful diagnostic tool. [Work supported by NSF.]