Holographic resolution of complex sound fields

Abstract

A system for determining the spatial structure or composition of complex sound fields within a duct or other structural enclosure that is excited by acoustic energy is disclosed wherein at least two holograms are recorded and analyzed to provide both the amplitude and phase of the various modal components that comprise the sound field. In situations in which the sound field of interest is periodic, the desired amplitude and phase information is determined from a pair of holograms in which each hologram is formed by photographically recording the diffraction pattern resulting when an array of elastomeric regions, positioned on the wall of the acoustically excited structure, is illuminated by two pulses of coherent monochromatic light that are spaced apart in time by nT/2, where T is the period of the dominant acoustic excitation signal and n is an odd integer. In situations in which the sound field is aperiodic, the elastomeric array is sequentially illuminated by a series of monochromatic light pulses with the reflected energy of each pulse being utilized to form separate holographic records. The holographic records resulting from consecutive pulses are then combined to form a number of interferograms. In either case, each of the interferograms are read with a video scanner to supply electrical signals representative of the deflection of each elastomeric region that occurs during the time between those light pulses which caused the interferogram. A computer-implemented Fourier analysis is then performed on the signal record of each interferogram to determine the displacement of each modal component.