Nocturnal boundary layer profiles and measured frequency dependent influence on sound propagation

During four evenings including one night the sound propagation up to a distance of 200 m south and north of a railway line was measured and the sound exposure level difference between the distances 25 and 200 m from a railway line was calculated. At the same time the meteorological situation was measured with a tethered balloon up to a height of 100 m. These measurements were compared with calculated wind and temperature profiles and a reasonable fit of the measured with the theoretical profiles were found. The A-weighted sound exposure level differences were correlated with the effective sound speed gradient with respect to atmospheric stability. In the case of upwind sound propagation a difference of the A-weighted sound level in the range of 10 dB between stable and not stable (adiabatic and instable) meteorological situation was found (7 dB in the frequency range between 315 and 3150 Hz) and in the case of downwind sound propagation in the range of 3 dB(A). The stability of the nocturnal layer is expressed with the Obukhov length L which includes (in the bulk definition) a relation of the wind speed gradient to the temperature gradient. The effective sound speed gradient includes the sum of the wind speed gradient and temperature gradient. The main result is that the description of sound propagation should be improved by use of the Obukhov length to include the stability effects of the atmosphere which is especially necessary in the case of upwind sound propagation during the nocturnal boundary layer.