Pistachio nuts can be sorted using the difference in impact noise between a nut with an open and closed shell. It was found that upon impact with a steel plate, nuts with closed shells emit sound with higher signal magnitudes than do nuts with open shells.
The classification accuracy of this system is approximately 97%, and the cost of the system is about $7k per channel, which is less than conventional sorters that use color of the nuts. Increased sorting accuracy of the acoustic sorter, coupled with low cost, enables a payback period of less than one year.
Reference: “Detection of pistachio nuts with closed shells using impact acoustics”, Pearson, T.C., USDA ARS GMPRC, Manhattan, KS 66502, United States, Applied Engineering in Agriculture v 17 n 2 March 2001. p 249-253 , 2001.