Published online 22 January 2008 | Nature | doi:10.1038/news.2008.518

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Acoustic method could quickly catch counterfeit coins.

Now hear this: the jingle of a good coin is different from a fake. GETTY

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.

The traditional method of spotting a fake coin is to look at it — very closely. While this requires equipment no more specialist than a reasonable microscope, says Suzuki, it is time consuming and can cause "a lot of trouble" if the coins are heavily worn or when a large number of counterfeit coins are in the system.

This isn't just a theoretical problem. In Japan, so many counterfeit 500-yen coins (worth just under US$5 each) were found in cash dispensers in 2005 that the coin was temporarily removed from use.

In a paper in Forensic Science International1, Suzuki proposes a quick and easy way of sorting the fakes from the genuine article.

The penny drops

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.

Ka'ching

Robert Matthews, who previously worked for the United Kingdom’s Royal Mint and now runs his own coin-authentication business, says that there is a long history of using sound to check coins — but only by ear. Experts at mints could often spot the 'ring' of badly cast coins when they dropped onto surfaces, he says. Poor-quality lead fakes could easily be detected from their dull 'thunk'.

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Suzuki's method provides a hi-tech update on this old method, allowing more sophisticated fakes to be more quickly detected, he says.

"This technique offers the promise of monitoring circulation coins and identifying the counterfeits," says Matthews. It could even be added to automated coin sorting and counting machines to ramp up fraud detection. "Obviously it needs to be proved that the technique will identify all types of counterfeits," he adds.