Asymptotic Analyses and Experiments of Sound Propagation in a Surface Duct

In this paper, we propose the asymptotic representations for calculating the sound field in the surface duct with the exponentially increasing sound velocity profile. We show that the problem of sound propagation in the surface duct is equivalent to that of sound propagation in the concave cylindrical boundary. Thus, utilizing the property of this equivalence, we experimented on sound propagation in the surface duct using a cylindrical water tank. In comparison with the experimental results, we have shown that ray representation is valid in the region sufficiently away from the caustic, while the uniform asymptotic representation is useful in the regions near and away from the caustic. By experiments, we have clarified that the use of the cylindrical water tank is efficient for investigating sound propagation in the surface duct.