Sound speed profile structure and variability measured over flat terrain

This paper describes a fundamental data base and data-variability guide for outdoor sound propagation models requiring sound speed profiles. The vertical sound speed profile has been measured directly over an extended period at a flat terrain site and to a height of 150m using a RASS (radio-acoustic sounding instrument). Additionally, vector wind profiles were available at 10m height intervals from a SODAR (an acoustic radar), and carefully calibrated wind and temperature data recorded at a number of fixed sites on a 120m mast. Combinations of these data sources are used to evaluate a number of influences, which include: applicability of the log-linear approximation; vertical variability of sound speed; change of wind direction with height; vector sound speed variability; effects of averaging intervals; short-term gust effects; longer-term diurnal effects; and fetch. These results demonstrate how variations in temperature, wind speed, and wind direction propagate through to sound speed profile changes, and into fitted parameter changes, thereby providing guidance on interpretation of comparisons between model output and propagation measurements