Date of Award
Doctor of Engineering Science in Electrical Engineering
Raj Pratap Misra
W. H. Warren Ball
Werner J. Wenisch
In this dissertation, a novel "four-step acoustical holographic imaging system" is described and several means for its implementation are analyzed both theoretically and experimentally. This system is compared with other methods of acoustical holography, (ie. permitting to obtain a visible, 3-D image of an object insonified with supersonic waves) and some of its advantages are indicated.
The use of optical holographic techniques to convert an arbitrary acoustic image to a visible image is investigated and a new method - the "holographic sound image converter" - is introduced and analyzed. This converter consists of the "holographic interferometer" and an appropriate "coupler". Unlike present acoustic detectors, it exhibits the ability to simultaneously detect a quantity related to the vibration amplitude at each point of the acoustical diffraction pattern. Other advantages of this technique are frequency selectivity and the possibility of amplification and "reference wave simulation." This converter is a non-scanned device; yet it features all of the desirable characteristics normally associated only with scanned, linear detectors.
The new technique of "shifted reference holographic interferometry" is presented. This technique, as shown in the analysis, permits increasing the sensitivity of conventional time-averaged holographic interferometry by approximately one order of magnitude. Experimental results confirm this prediction.
Holographic techniques are also applied to the measurement of acoustic parameters and several ways of implementing this application are proposed and investigated. In a number of experiments, the advantages of the holographic method in the field of sonics and ultrasonics are demonstrated, and a theoretical relation comparing this method to the Schlieren method is developed and experimentally confirmed.
A theoretical and experimental study of some possible "couplers", each capable of augmenting the displacement amplitude of an acoustical diffraction pattern as it is transferred from a surface bounded by water to one bounded by air, is also conducted. The "couplers" investigated range from a simple, acoustic impedance transformer to a mosaic of velocity amplifiers.
A detailed study is conducted on the development and testing of a mechanical velocity transformer consisting of two lossy, nearly quarter-wave plates. The theoretical relations describing the behavior of such plates are developed and experimentally verified. An advantage of 9.5 (19.6 dB) at 916 KHz has been obtained across the water/air interface - primarily due to the construction of tuned aluminum-epoxy plates.
A useful method for "tuning" quarter-wave metal-epoxy plates is introduced and demonstrated. Other methods suitable for the measurement of the speed of sound and the attenuation coefficient of epoxy are also discussed.
"Shifted reference holographic interferometry" and the partial impedance matching of water to air, afforded by the tuned velocity transformer, result in a high sensitivity of this optical detector of acoustic vibrations. It is shown that, by these methods, a "holographic sound image converter" having a threshold intensity of 2.8 mw/ cm2 at 1 MHz appears feasible. Higher sensitivities (1.6 x 10-11 w/cm2) and full realization of all of the advantages of the "holographic converter" are expected with the use of an "active coupler", ie. one which makes use of electronic amplification.
Fischer, Wolfgang, "Methods for acoustic holography and acoustic measurements" (1972). Dissertations. 1282.