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 L95 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 L95 is described by L95=37.9 log(v)+42.5, where v is the wind velocity. The standard deviation with respect to the data points is 2.4 dB. For the A-weighted L95 we found a similar relation [LA95=22.6 log(v)+22.7]. The relation between wind speed and natural ambient sound level for each 13-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 13-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.