Characterization of the natural ambient sound environment: Measurements in open agricultural grassland

The audibility of manmade sound in a natural environment is affected because of masking by ambient sound. In this report results are presented of measurements of the level and spectral composition of natural ambient sound. The statistical L-95 level was determined, i.e., the sound pressure level which is exceeded for 95% of the time, at various wind velocities in open agricultural grassland. The total L-95 is described by L-95=37.9 log(upsilon)+42.5, where upsilon is the wind velocity. The standard deviation with respect to the data points is 2.4 dB. For the A-weighted L-95 we found a similar relation [L-A95=226 log(upsilon)+22.7]. The relation between wind speed and natural ambient sound level for each 1/3-oct band was also determined with nominal midband frequencies ranging from 6.3 Hz to 20 kHz separately. The frequency spectrum of ambient sound shows at low frequencies a behavior typical for turbulent processes. The 1/3-oct band intensity of sound at these frequencies is found to be proportional to f(-2). Dimensionless spectra obtained for low frequencies at wind speeds exceeding 2 m/s collapse into a line almost identical to results of low-turbulence measurements performed by Strasberg. (C) 1997 Acoustical Society of America.