Sound absorption measurements for evaluating dynamic physical properties of a powder bed

Abstract

The normal incident sound absorption coefficient of a powder bed on a rigid plate has been measured in the frequency range of 200 Hz to 2000 Hz using an acoustic tube. Six different kinds of powder materials were used as samples; the mean particle diameter varied from 32.3 μm to 152.3 μm and the bed thickness varied from 10 mm to 40 mm. The sound absorption curves thus obtained had several distinct peaks; their frequency ratios were odd multiplications and the peak frequencies changed inversely with bed thickness. These peaks are attributable to the excitation of the normal vibration of the powder bed by sound wave incidence. Consequently, the sound absorption peak frequencies correspond to the natural frequencies of the powder bed. The measurements of vibration acceleration in the powder bed have clarified the fact that the primary sound absorption peak is related to the fundamental vibration mode, which is the one-end fixed longitudinal mode excited when one-fourth of the sound wavelength in the powder bed coincides with the bed thickness. These results show the possibility of estimating the dynamic physical properties of a powder bed, i.e. sound velocity and longitudinal elastic coefficient, from the primary sound absorption peak frequency of the powder bed. The measurements of the sound absorption coefficient by this method are hardly affected by powder vibration fluidization, etc., because they can be achieved in a non-contact state and in a short time, using a small vibrating force.