Abstract
An acoustic technique for evaluating soil physical properties, such as the air content and air permeability, uses sound waves reflected from the soil surface or penetrating sound waves in soil. It is difficult to set the measurement range in the soil depth because the propagation distance of a sound wave in soil depends on the soil physical properties to be measured. In this study, we compared the results of an acoustic resonance method for a sample whose bottom was open to the atmosphere with those for a sample whose bottom was closed. The propagation distance of the sound wave in soil was estimated from the length of the sample whose bottom condition had no effect on the results of the acoustic resonance method. We set the sample length smaller than the estimated propagation distance of a sound wave. The precision of the acoustic resonance method was then examined. We found that the propagation distance of a 5 -- 100 Hz sound wave in a decomposed granite soil sample was 3 -- 7.5 cm. The propagation distance was a maximum at volumetric air content of 20 %. Setting the sample depth to 1.3 cm and closing the sample bottom, the volumetric air content was determined with +0.9 % precision in a measurement range of 10 % --36 % whereas air permeability was determined with precision of + 0.05 cm s-1 in a measurement range of 0.05--0.65 cm s-1.
