Propagation of sound in the presence of gradients and turbulence near the ground

Abstract

Sound-pressure levels of pure tones between 1 and 16 kHz, generated by a point source on hard ground, were measured as a function of height at horizontal distances up to 21 m. In another experiment, measurements were made above hard ground and ground of finite impedance at horizontal distances up to 250 m using frequencies between 250 and 4000 Hz. Five frequencies were generated simultaneously and measured at four (and sometimes five) heights. The sound-pressure levels below the refractive shadow boundary are compared with a simple theory that assumes a linear change of sound speed with height. This vertical gradient was obtained by approximating the temperature and wind velocity profiles measured during the acoustical experiment. Good agreement between theory and experiment was obtained at all distances and frequencies. However, at the largest distances and highest frequencies, there was evidence of additional energy penetrating the refractive shadow from scattering by atmospheric turbulence. Therefore, in addition to their vertical gradients, the temperature and wind velocity fluctuations were also recorded. These were used to obtain the strength and scale of turbulence, and the analysis of these results is summarized. The amplitude and phase fluctuations predicted from a simple theory that approximates the source region of the turbulence by a Gaussian spectrum were compared with measurements. In particular, transverse coherence length decreases deep within the refractive shadow compared with its predicted values and with values measured above the shadow boundary.