Abstract:

The statistical properties of an ultrasonic wave that traverses a medium exhibiting temperature fluctuations are investigated under well-controlled laboratory conditions. A heated grid in air is placed horizontally in a large anechoic room and the mixing of the free convection plumes above the grid generates a homogeneous isotropic random thermal field. The spectrum of refractrive index fluctuations is accurately described by a modified von Karman model which takes into account the entire spectrum of turbulence. Experimental data are obtained by varying both the frequency of the acoustic source and the distance of propagation. Measurements will be presented for a rigid boundary and for a finite impedance boundary. In this paper, attention is concentrated on the mean sound pressure levels and the intensity fluctuations. Experimental values will be compared with results of numerical simulations based on a wide angle parabolic code. [P. Chevret et al., J. Acoust. Soc. Am. 100, 3587--3599 (1996)]. [Work supported by DGA-DRET under Contract No. 94-083.]