Even if cheap ultrasound testers are not good enough, then I guess at least the big dealers will be able to buy good ones.
i don't like that solution at all if you expect any meaningful rise in price from here. any real solution has to come from the root of the problem on the production side. if that's even possible. any solutions that are reactive, such as US, are just bandaids as far as i'm concerned. and then that doesn't solve the trust issue with your dealer.
How can it be possible to solve that ? There will always be people making fake coins/bars, like n8rwJeTt8TrrLKPa55eU showed it's not a new problem. History shows it didn't prevented gold/silver to impose itself as money in history.
It's an arms race between people making these bars and people testing them. And people testing are winning. If these bars were not detectable without melting it, that would be a real problem.