Development of a simple and non-destructive examination for counterfeit coins using acoustic characteristics

The measurement of the sound by the shock wave and the analysis of the natural frequencies were carried out for the authentic and five kinds of counterfeit 500-yen coins for the purpose of the quick and simple discrimination. Four peaks of natural frequencies were observed in the range of 5–20 kHz for authentic coins. On the other hand, only three peaks were observed for some kinds of the counterfeit coins. Four peaks were observed for other counterfeit coins, however the peaks were slightly shifted compared to those for authentic coins. These results show that the discrimination of the counterfeit coins from authentic ones can be realized by the examination of the natural frequencies. This method is quite useful for the simple screening of authentic and counterfeit coins.