Acoustic Holographic Techniques for Nondestructive Testing

Abstract

The first part of this paper reports progress to date on an experimental investigation of the feasibility of applying holographic techniques to acoustical imaging to obtain three-dimensional images of opaque objects as encountered in nondestructive testing. Equipment was fabricated to make acoustical holograms of objects in water. Acoustical holograms were recorded by mechanically scanning an acoustical point transducer over the water’s surface in a TV-like raster. The acoustic frequency was 5 MHz. The resulting holograms were then used to make visible images. Holograms of objects examined in water, where the ultrasonic wavelength was 0.3 mm, show image detail smaller than 1 mm. A variety of acoustical holograms and their corresponding images (including one of a hole in an aluminum block) are presented and discussed. The second part of this paper presents a simplified theoretical analysis of these acoustical holograms. The effects of nonlinearities, sampling rates and pulse duration, coherent sound, and wavelength scaling on the acoustical image are discussed. The problem of three-dimensional visualization of the acoustical image is considered in detail. The third part of this paper discusses some potential uses of acoustical holographic techniques in nondestructive testing.