SOUND-ABSORPTION MEASUREMENTS FOR EVALUATING DYNAMIC PHYSICAL-PROPERTIES OF A POWDER BED

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 we re used as samples; the mean particle diameter varied from 32.3 mum to 152.3 mum and the bed thickness varied from 10 mm to 40 mm. The sound absorption curves thus obtained had several distinct peaks; their fre quency ratios were odd multiplications and the peak frequencies change d inversely with bed thickness. These peaks are attributable to the ex citation of the normal vibration of the powder bed by sound wave incid ence. Consequently, the sound absorption peak frequencies correspond t o the natural frequencies of the powder bed. The measurements of vibra tion acceleration in the powder bed have clarified the fact that the p rimary sound absorption peak is related to the fundamental vibration m ode, which is the one-end fixed longitudinal mode excited when one-fou rth of the sound wavelength in the powder bed coincides with the bed t hickness. These results show the possibility of estimating the dynamic physical properties of a powder bed, i.e. sound velocity and longitud inal elastic coefficient, from the primary sound absorption peak frequ ency of the powder bed. The measurements of the sound absorption coeff icient by this method are hardly affected by powder vibration fluidiza tion, etc., because they can be achieved in a non-contact state and in a short time, using a small vibrating force.