Gerard P. Carroll
Structural and Hydroacoust. Res. Branch, Carderock Division, Naval Surface Warfare Ctr. CDNSWC), Bethesda, MD 20084-5000
Near-field acoustic holography (NAH) has been validated in precisely controlled laboratory conditions for small scale models. The practical validity of the approach in nonlaboratory settings for larger scale models, where increased background noise, inexact sensor positioning, and other environmental contaminants are likely, needs to be established. In this paper, the effect of these error mechanisms on the accuracy of cylindrical near-field acoustic holography are investigated numerically. This is accomplished using simulated hologram plane pressure data obtained from surface pressure and velocity results. The surface pressure and velocity are obtained using FEM/BEM calculations (SARA) for a ribbed cylindrical model. The simulated hologram plane data are obtained from the SARA results using a numerical implementation (Gaussian quadrature) of the Helmholtz integral. The errors in the holographic reconstructions associated with background noise, sensor placement, and other error mechanisms are revealed by comparing the holographically reconstructed results with the original SARA results. Since the ribbed cylindrical model under investigation is quite detailed, this approach is a good test of the ability of NAH to appropriately reproduce the complex surface behavior. [Work supported by ONR.]
Harvey Fletcher is both ``The father of stereophonic sound'' and the ``father of psychoacoustics.'' He was the first one to show during his Ph.D. work (with Robert Millikan) that the electron had a fixed quantized charge, and the first to accurately measure the threshold of hearing. He provided A. Edison with a shoe-box sized electronic hearing aid, manufactured by Western Electric, and personally demonstrated stereo to A. G. Bell. He produced the first electronic hearing aid, the first electronic (2-A) audiometer, discovered the critical bands of the cochlea, and is well known for ``the Fletcher--Munson curves.'' This film presents some amazing footage from the 1950 celebration of the 25th Anniversary of the Acoustical Society in New York City that might bring tears to your eyes (54 mins, 16-mm color film).