STUDIES OF SOUND-TRANSMISSION IN VARIOUS TYPES OF STORED GRAIN FOR ACOUSTIC DETECTION OF INSECTS

In developing passive acoustic systems for detecting insect infestatio ns in bulk-stored grain, it is advantageous to understand the transmis sion of sound in the grain between the insects and the sensors. in the work presented here grain is shown to be a strong acoustical absorber , the attenuation coefficient ino easing roughly as the square root of the frequency. Tests with soft wheat immersed in three different gase s. air, argon and carbon dioxide, support an earlier conclusion that s ound is transmitted principally through the gas in the passageways bet ween the gr ain kernels. The speed of sound and the attenuation coeffi cient were measured as a function of frequency for six different types of grain: hard and soft wheat, brown rice, soybeans, corn and sorghum . It was determined that sound is transmitted over longer distances in grains with a larger inter-kernel spacing, such as corn and soybeans. Grain depth, up to several meters, appears to have little effect on s ound transmission. (C) 1997 Elsevier Science Ltd.