The sound of a bad penny - Finding counterfeit coins by sound
You might assume that counterfeiters only bother with high-value bank notes, but there is a chance that some of the coins jangling around in your pocket right now are fake. If Mototsugu Suzuki gets his way, it may be that jangling that gives them away.
Suzuki, a researcher at the Tokyo Metropolitan Police Department's Criminal Investigation Laboratory in Japan, has developed a way of examining coins based on the sound they make.
In Suzuki's method, coins slide down a slope and then fall onto a brass block. The sound they make on impact is relayed via a microphone to a computer.
Although the human ear cannot usually tell the difference between real and fake, a computer can. Genuine 500-yen coins showed four distinctive peaks of natural resonance frequencies in the 5-20 kilohertz range. This was not the case for fakes; some fakes produced only three peaks, while others showed four but at different frequencies to genuine coins.
In addition to helping detect counterfeits, the sound data could be used to build up a database of fake coins, suggests Suzuki. This might help law enforcement officers to prove how many coins one counterfeit operation is responsible for.
This would make for an awesome Instructable! Start with these two Instructables and get dropping pennies!