A Moiré Fringe Analog of Sound Propagation in Shallow Water

A moiré fringe analog of the normal modes of sound propagation in shallow water is described, in which interesting patterns are obtained both by direct visual observation and by photographic recording. These are analogs for the profiles of sound pressure as a function of range and depth in the underwater field due to a single-frequency source. Since phase is correctly represented, the interaction patterns due to the presence of considerable numbers of interfering modes may be investigated experimentally. This is important since mathematical analysis becomes difficult when there are more than two modes. There is a discussion of the characteristics of these patterns (including the formation of “loops”) and also of those obtained by A. B. Wood in his sound propagation model experiments.