(511k) Measurements on the Acoustic Properties of Powders

In previous research, the feasibility of using Helmholtz Resonance to measure fill volume of the material in a hopper and the discharge rate from it, was investigated. This is done by applying an excitation sound wave to a hopper containing a particulate material and measuring the resonance frequency of the system. It was observed that the resonance quality of such a system greatly depends on the particle properties of the sample material and the resonance frequency range of the hopper. To gain a better understanding of the correlation between the physical properties of a bulk material and the acoustic behavior of a resonator containing it, a vertical impedance tube has been constructed. The impedance tube consists of a sound generator, a tube of adjustable length, and a cylinder that holds a powder sample. The apparatus is used to determine various acoustic parameters including normal incidence sound absorption coefficients. Sound absorption coefficient was measured for different types of particles with different size distributions and bulk densities, at different material depths in the sample holder. Initial experiments have largely focused on readily available foodstuff materials and early indications on sound absorption are consistent with the trends shown in past trials with a laboratory scale resonator discharging mustard seeds and agricultural lime.