Date of Award
Doctor of Engineering Science in Electrical Engineering
W. H. Warren Ball
Raj Pratap Misra
The ability to "see" with sound has long been an intriguing concept. It is apparent that ultrasonic energy can give an image of an object not obtainable with light or even with x-rays.
In this dissertation, a novel "three-step acoustical holographic imaging system" is described and several means for its implementation are analyzed. A novel holographic acoustic image converter, which constitutes a fundamental link in the three step imaging method was also developed. The use of optical holographic techniques to convert an arbitrary acoustic image to a visible image is conducted and resulted in the introduction of the new technique of "Step-biased holographic interferometry", which is shown to permit increasing the sensitivity of conventional holographic interferometry methods by one order of magnitude.
A theoretical and experimental study of some possible "couplers", for augmenting and amplifying the displacement amplitude of an acoustical diffraction pattern as it is transferred from a surface bounded by water to a surface bounded by air, is conducted. A particularly detailed study is conducted on the two-quarter-wave acoustic impedance transformer, and on a mosaic of step horn velocity amplifiers. The theoretical relations describing the behavior of both the acoustic impedance transformer and the velocity amplifiers are developed and experimentally verified. As a result of using beryllium to construct the first quarter-wave matching plate in the acoustic impedance transformer and the addition of the microhorn structure in the output stage of the image coupler, interelement crosscoupling has been brought down to a minimum value of -25 db, and the coupler sensitivity has been improved. A mechanical advantage (gain) of 40 at 410 KHz across the water air interface with beryllium-epoxy structure has been achieved.
As a result of the increased sensitivity of both, the optical system by using the step-biased holographic interferometry, and of the image coupler by using the microhorn velocity amplifiers as well as the partial impedance matching of water to air, a "holographic sound image converter" having a threshold intensity of 1 mW/cm2 at 410 KHz appears feasible. During the course of this experimentation, useful acoustical and optical methods for tuning and testing the image coupler materials is devised.
Finally, the ability of the system to cast real time shadowgrams of the insonified object appears useful for many practical applications requiring real time operation, and by utilizing computer processing, quasi-real time operation can be achieved for the three-dimensional acoustic imaging.
Hanafy, Amin Mohamed, "Acoustical holography by holographic interferometry" (1977). Dissertations. 1312.