Long-term experiments on influences of ground surface and meteorological conditions on outdoor sound propagation using an unattended impulse response measurement system

In order to investigate the variation in excess attenuation due to ground surface and meteorological conditions, a series of experiments on outdoor sound propagation has been conducted at a glider airfield in Hokkaido, Japan. The series of experiments started in November 2018 and are still in progress (May, 2020). An unattended impulse response measurement system has been developed and impulse responses at 155 m and 300 m from a sound source have been measured on grass field automatically three times a day. The experimental field is covered with snow in winter. Although the results of each experiment scatter due to weather conditions, the average of the excess attenuation obtained under each wind condition varies with vector wind speed and it tends to converge under strong upwind and strong downwind conditions, respectively. The range of the variation is from about 20 dB (upwind) to about -5 dB (downwind). The convergence value under upwind conditions changes clearly due to ground conditions, whereas it changes little under downwind conditions. The vector wind speed when the excess attenuation becomes the convergence value changes with frequency and propagation distance.