An experimental test of the concept of Fresnel zones for modes

Abstract

Analysis of mode amplitude variations due to scattering by large-scaled inhomogeneities in waveguides can be performed by an approach based on using the notion of the Fresnel zones for modes (generalization of the well-known Fresnel zones for rays). This approach can be applied to describe sound scattering by an opaque screen, by inhomogeneities of refractive index, and by those of a rough waveguide boundary. In order to test several theoretical predictions made in the framework of the above approach, a tank experiment on sound diffraction by an opaque screen in a waveguide has been carried out. The waveguide was a water layer over a rubber bottom. The opaque screen crossed two thirds of a waveguide vertical section. The field was excited by a point source and registered behind the screen by a uniformly moving point receiver. The spectrum of the received signal was used to study the modal structure variation due to the screen. The spatial spectrum of the sound field and the field intensity as a function of range were measured behind the screen and compared with the theoretical predictions. The comparison showed a good agreement between theory and experiment. [Work supported by RFBR Grant No. 95-02-04565.]