Sound-Based Coin Authentication: Looking Lor Beta-Testers

Thats a fantastic idea! Unfortunately, a pauper like me cant afford gold.

Do you reckon an algorithm is possible? Someone with a set of calipres could measure their coin accurately, input the numbers (diameter, thickness) and purity (hopefully known for the coin and what the gold is alloyed with) and let the algorithm work out the recquired tone. Check it via the microphone to get a yes/no on authenticity.

Or, even developing it further, work out the purity of a coin based on its tone (perhaps not possible due to different alloying). A bit more work would allow someone to get a set of purity ratings - content of gold when alloyed with different metals.

But I know nothing of this, perhaps its not possible to do such a thing. I'm sure the shape of the coins surface has some effect, no matter how small, on the tone.

@BenByfield: That would be possible. The problem is, however, that for small variations in alloys the material parameters that govern the acoustic fingerprint can vary. So the estimation would work best for alloys/materials that are well characterized, such as pure gold/silver.

What seems more logical, in my mind, would be having you create your own database of pings for whatever coin you have, and compare that with others. That way you could compare the composition of your coin with other people, if you both take the time to ping your coin. That's currently not a feature, but would that be useful?

I forgot to add this link to a previous discussion...


https://goccf.com/t/112924

Hopefully he will see this thread. I posted a link to this in the other one to get his attention (if he subscribed to it, he will get an e-mail).

I'd really be interested in hearing how you did this. Do you FFT the recording and search for harmonics at expected locations (within a minor threshold of frequency of course), and match that against some known profiles?

Do you feel comfortable sharing your code?

This begs the question, "How do the 'Cash for Coins' shops determine the composition of a coin?" Do they use a device that determines the gold/silver content? Would your idea be able to spot counterfeits?

I use the acoustic method all the time for coins submitted to The Black Cabinet. Freeware programs like Audacity and a few others currently support the waveform analysis charts necessary to track harmonics and resonance, but when dealing with coins that have a certain amount of wear or patina on them, things need to be adjusted on the fly.

If this is something that can be distilled down to a smartphone, that would be useful.

The link to The Black Cabinet's in my signature. It's a growing collection and database of counterfeit American coins that are currently in circulation (either among collectors and dealers or as current money).

I don't have any frequencies and maps recorded for posterity. I more often than not test genuine pieces of similar wear side-by-side with submissions and compare fingerprints when other details are inconclusive. The bulk of what crosses my desk are silver dollars, but over the course of an average year I tend to see fake gold pieces, pennies, and halves, too. Just last month I had a fairly convincing St. Gaudens $20 fake, but the weight was so light that I didn't need to check it for resonance to confirm. :-)