Acoustic holography

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Dr. Fiala Péter
Department of Networked Systems and Services

In 1947 Dennis Gabor (in Hungarian: Gábor Dénes) invented the optical holography, which made it possible to return more information in relation to the previous imaging procedures. In general, each wave is inherently suitable for making holograms, for example there is electron holography and acoustic holography. Acoustic holography appeared in the mid-1960s involves reconstruction of the sound field that arises due to radiation of sound at a boundary. This radiating object can be an instrument, a fuselage of an aircraft, or a surface of a submarine.

Two distinct forms of acoustic holography exist: the farfield acoustical holography (FAH) and the nearfield acoustical holography (NAH) developed by E. G. Williams and J. D. Maynard in 1980. The topic of my thesis is to present two realization methods and a further development technique of NAH.

The recorded pressure of a two-dimensional surface called hologram represents details of radiated sound wave. When evanescent waves (exponentially vanishing waves) are present in nearfield, an essential requirement for the solution of NAH is the measurement of the sound field very close to the source. This latter fact explains the term “nearfield” used in NAH. The acoustic holographic methods backpropagate the measured pressure on the plane of sources as pressure, normal surface velocity or sound intensity.

In the first part of the thesis I present the acoustic theoretical basis of NAH. I demonstrate the derivation of initial equations of the methods. I detail the techniques respectively and I emphasize the practically essential filtering as well.

To investigate the methods are studied by me, I needed a simulation enviroment that I implemented in MATLAB. I could test the techniques in a measurement of top of a classical guitar. In the second part of the thesis, first I describe the results of methods based on simulated measurements and compare them. Later I detail the history and the success of practical measurements. The processing of measurements is done by a boundary element toolbox developed in MATLAB and my previously programmed functions.


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