If you can't ping it, leave it
Recently I've caught myself tapping various things and listening to the sound they make to get an idea of the material: sculptures (solid, hollow? ceramic, metal, wood?), furniture (wood, polymer?), a piano music holder (plastic? lightweight wood?). We naturally use a ping test to test everyday materials.
Don't buy precious metals unless you can apply this essential test (and compare with how a coin should sound). Due to their construction, coins are far easier to ping test than bars. Both are impossible to ping test unless you can hold the coin bare.
EDIT: another slogan idea: If you can't ping it, don't believe it.
No ping? No bling.
Posted Sep 9, 2020, 11:42 AM UTC · edited
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It's not enough to have a ringing tone. Fakes ping, too. The ping fundamental frequency is the diagnostic characteristic, and what matters critically. So you must read the ping frequency, either comparing the ping of a known-good specimen (either one you have, or a recording of one you don't), or with a smartphone audio spectrum analyzer app.
Also, Au .9999 coins don't ping nearly as distinctly as Au/Cu/Ag alloy coins. That's because pure gold is softer, making the ring tone lower frequency and less energetic compared to an alloy coin, but still distinctive.
There's a myth that tungsten fakes don't ping. In fact they do, but faintly at a very high frequency that is hard to hear.
For the scientific background, see the paper The Music of Gold by Arnaud Manas.
The diligent, diagnostic, conclusive test of a standard format comprises three easy readings: measure, weigh, ping; followed by comparison to known, trusted reference values.
I suggest the slogan, Learn to test diligently, and then diligently test.
Yes, unlike metal, plastic, and wood, most of us don't have years of experience ringing coins to learn how different metals sound. I imagine 100 years ago people knew the correct sound of coins since they were still made of precious metals.
That's interesting that the common alloyed gold coins pinged better than pure ones. So the alloy served two useful purposes: added durability, and made them easier to test by ear.