Sound Absorption Measurements of Various Types of Grain

Insects destroy significant amounts of stored grain. To detect the sounds produced by insect activity inside the grain it is necessary to understand the absorption properties of sound in various types of grain. This paper describes some experimental investigations of absorption properties of sound in various types and heaped thickness of grain samples using the method of standing wave tube for the first time. The following conclusions can be drawn: (1) Grain is a highly absorbing acoustical medium. Sound is transmitted mainly in the passageways between the grain kernels. (2) Different types of grain have different absorption characteristics. For the same heaped thickness the larger grains have a smaller sound absorption coefficient than the smaller grains. Spherical kernels have a smaller sound absorption coefficient compared to non-spherical ones. The properties of sound absorption coefficient of grain depend mainly on the geometry and the size of the kernels and not on the solid matrix of the kernels. The average sound absorption coefficients of larger grains are smaller than the small grains in general. (3) The heaped thickness of the grain sample is an important factor affecting the sound absorption coefficient. The sound absorption coefficients at low frequencies increase when the heaped thickness of grain sample increases. The numbers of absorption peaks increase and the frequencies corresponding to the peaks decrease to the low frequencies when the heaped thickness of grain sample increases.