Experimental study on outdoor sound propagation over rice fields

In rice fields, which cover large areas in Japan, the ground condition changes with the season depending on the cultivation condition. To investigate the variation in sound propagation characteristics in rice fields, a series of sound propagation experiments were carried out in a flat rice field area in different seasons. The impulse responses were measured at receiving points arranged in two opposing directions along the straight line in a range of 600 m from sound sources. The acoustic property of the ground surface was evaluated by estimating the effective flow resistivity of the ground by curve fitting between the excess attenuation characteristics determined from the measured impulse responses at short distant receiving points and the calculation results applying Miki's impedance model. It was estimated to be about 20,000 kPa s/m2 in the condition immediately after rice planting and less than 300 kPa s/m2 after rice harvesting, and it was confirmed that the acoustic property greatly changed by the season. It was also confirmed that excess attenuation at distant points varies by more than 20 dB at the same propagation distance by the difference in the sound propagation direction even in the same season due to wind effects.