Sound refraction characteristics near the ground based on turbulence similarity theory

Abstract

Sound refraction in the near-ground atmosphere over uniform terrain is systematically addressed with the Monin–Obukhov turbulence similarity theory. The gradient of the effective sound speed (actual sound speed plus the component of the wind vector in the propagation direction) is parametrized with two nondimensional ratios, one proportional to the height and the other representing the magnitude of sound-speed fluctuations relative to wind-speed fluctuations. Based on these parameters, three distinct propagation regimes (with transitions) can be identified: (1) strongly unstable atmospheric stratification where refraction is upward at all propagation angles relative to the mean wind, (2) strongly stable atmospheric stratification where refraction is downward for all angles, and (3) neutral atmospheric stratification where refraction is determined by the wind direction for small values of the nondimensional height ratio and is upward for greater heights. The relative contributions of temperature and humidity to the sound-speed gradient are found to depend on the Bowen ratio, defined as the ratio of the sensible and latent heat fluxes at the surface. Weather conditions appropriate for measuring sound-pressure levels are discussed in connection with the specifications in ANSI S12.18-1994.