Daytime sound levels and coherence as a function of height: Experiment and theory

Abstract

Daytime ground-to-ground acoustic propagation has been measured by a number of researchers over the years. Measurements of daytime sound levels as a function of height, however, have been made only in a limited number of near-ground experiments and apparently never at a considerable height. This paper reports measurements using a tethered balloon to measure the mean sound level and coherence from the ground up to 210 m at three separate ranges (300 m, 600 m, and 900 m) and at three frequencies (210 Hz, 380 Hz, and 600 Hz). A meteorological instrument package on the balloon measured the mean temperature, wind and humidity as a function of height. Fluctuation spectra were estimated using a stationary sonic anemometer at 5-m height which sampled the temperature and vector wind at 10 Hz. Acoustical inputs computed from the meteorological data were used with a three-dimensional parabolic equation model to predict the mean sound level and coherence as a function of height and range. The predictions and measurements agree reasonably well, both showing a clear shadow-zone boundary that is a function of the meteorological conditions. Descending across the shadow boundary, the predicted and measured mean sound levels and coherence decrease dramatically.