INTENSITY FLUCTUATIONS OF SPHERICAL ACOUSTIC-WAVES PROPAGATING THROUGH THERMAL TURBULENCE

The intensity fluctuations of acoustic waves that propagate through th ermal turbulence are investigated under well controlled laboratory con ditions. Two heated grids in air are placed horizontally in a large an echoic room and the mixing of the free convection plumes above them ge nerates a homogeneous isotropic random thermal field. The spectrum of refractive index fluctuations is accurately described by a modified vo n Karman model which takes into account the entire spectrum of turbule nce. Experimental data are obtained by varying both the frequency of t he spherical wave and the distance of propagation. In this paper we co ncentrate on the variance of the normalized intensity fluctuations and on their probability distributions. These measurements cover all the regimes from weak scattering to strong scattering including the peak o f the intensity variance. Experimental values of the scintillation ind ex are compared with classical theoretical predictions and also with t he results of recent numerical simulations. The classical probability density functions (log-normal, exponential, I-K) are tested against th e measured probability distributions. The generalized gamma distributi on, which varies smoothly from log-normal to exponential as a function of two parameters, appears to represent the experimental data for a v ery large range of scattering conditions.