Moiré fringes in underwater acoustics
Date
1974
Authors
Noble, Michael Richard
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Abstract
Reverberation (non-specular reflection) of sound from a floating ice sheet was studied by means of models. The ice layer was represented by plane and corrugated sheets of glass and plastic. Data were obtained in the form of charts showing reverberant energy as a function of both angle of incidence and sound frequency. Attempts to interpret these data led to the conclusion that the observed data had undergone a transformation or distortion of some kind.
Simple, parallel-rod targets were studied in an effort to obtain reverberation patterns whose form could be predicted theoretically. The striking differences between the calculated and observed patterns were interpreted as being caused by a moire effect, of a type not previously reported in the literature. An optical superpositon of the calculated reverberation pattern and the raster of plotter lines by which the acoustic pattern is displayed showed many of the features observed in the acoustic data.
The analytical theory of optical moire pattern formation has been adapted to the acoustic case. Theoretically calculated moire patterns for double-rod and multiple-rod targets are shown to agree closely with the observed patterns. A method of extending the work to the more difficult cases of reverberation from plane and corrugated sheets is indicated.