I understand what you guys are saying - and maybe I made it seem too simple or easy. As you guys already know this is not fool proof and it is also a good point that different pull tabs bang differently. That's why I was saying if you tune it with the prevalent tab on your site - you should get close on the tab end. The nickle should be right on. If you have about 5 diffrent types of tabs - you might have to thumb your disc a bit and put a couple of stickies on your dial to find a "tab" range. It takes work, practice, and imagination, but honestly I feel like with the dual disc on the Tejon and some tinkering - you can be EXTREMELY accurate in identifying targets. I've got black marker all over my disc dials . . .
I can only say that this method works pretty darn good - oh yes, you will still dig some tabs(broken, mangled, or odd ones) , but you should save yourself 70 percent of them when you get used to it.
Oh yeah, another thing I do sometimes is dig about 10 tabs and get a sort of rough average of their depth and if it's like 4-5 inches - make a mental note of the tone it made (sound and shape). Then, when you get a similar tone - and combine this with the thumbing of the disc - you can fairly accurately identify a tab. If it's a softer / smaller(deeper) signal, it's likely an older coin.
I feel confident that I can tell the sound of a tab, though. After digging thousands I think it's safe to say that the sound has distinct characteristics. It's a sort of edgy sound - like sort of a "square" sound while a coin to me is honestly quite a "round" smooth sound. I know that's extremely vague - but I think that's the best way to describe it.
The only foolproof way

? Go to a site was not occupied after 1960. It's a shame people are such slobs with their pop tabs, isn't it?
Good luck,
Ian