Authenticating euro coins - with sound

In years gone by, it was not uncommon for coins to be tested for authenticity by striking them and listening to the tone produced.

That rudimentary technique has now been replaced with tests based on electromagnetic properties of the alloys used in coins and sophisticated imaging, but researchers in Estonia believe sound could still have a role to play in detecting counterfeits.

Writing in the journal Elektronica ir Elektrotechnika, the team from Tallinn University describe how acoustic spectrum analysis, which plays on the different vibration patterns produced by coins in impact, can be used to distinguish genuine coins from fake and also tell one coin denomination from another.

The use of acoustic spectra in this way actually goes back many years, with a device patented as early as 1991, but the researchers describe a new device in which a coin is dropped into a slot and strikes a metal plate linked to a microphone. The acoustic signal is processed using an algorithm called Fast Fourier Transform (FFT) to give a frequency.

By passing multiple coins through the device in various denominations they were able to generate representative spectra for each type and distinguish between genuine and counterfeit coins with "efficiency and high accuracy."

They suggest the accuracy of the device could be further improved by combining the acoustic sensor with imaging technology.

While it might be considered that counterfeiters would target high-value banknotes rather than coins, recent events show that this is not the case - at the end of last year, €500,000-worth of fake euro coins made in China were seized by Italian police.

A search on an online marketplace today revealed several listings for 'commemorative' or 'custom' euro coins from China, one of which claimed to be able to provide a million units per month with a minimum order of 5,000 coins.


Image courtesy of Shutterstock / Marian Weyo