Y. Okudaira et al., SOUND-ABSORPTION MEASUREMENTS FOR EVALUATING DYNAMIC PHYSICAL-PROPERTIES OF A POWDER BED, Powder technology, 77(1), 1993, pp. 39-48
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.