Rick, I know you asked the question strictly in terms of numerical cross-hair TID readouts. But invariably, this type question always trends over into "sounds" "tone" and "repeatibility" type issues too.
For the first part, the numerical TID readouts: Those of you who have been around long enough will remember this: Back when the very first TID units started to hit the market (Teknetics, etc...), in the early to mid 1980s, this was studied very deeply. Especially when programable units (like the first Whites Eagle) started to hit the market, where ..... not only did you have a bouncing needle or screen, but you could notch in or out infinate categories to your disc. patterns. Of course this was only on the "up/down" axis at that time (and with no left/right 3D axis like we have today). There were persons then, in the mid 1980s, who tested multiples hundreds of random gold rings (they must've had a friend with a large jewelry store! haha). They fed the information in to a computer program statistical chart, to determine the ratios of where the various rings tended to fall, on the Whites 1 to 95 scale. Then they took multiple hundreds of recurring aluminum junk items (foil wads, tabs, etc....) and also tested them, to determine the ratios of where the various junk items tended to fall on the 1 to 95 scale. Then they simply did computer program statistical analysis to tell the md'r the best #'s to dig
So for example, it was determined that rings that fall into the pulltab range, were really only about 10% (for example) of the gold rings out there "in circulation", while the pulltab range accounted for 40% of all aluminum junk you might typically encounter in the field. So it was determined that you should disc. out tabs, for example. And the nickel TID, should be nixed out, as should the smaller foil wads, etc.... And the highest percentage of rings, that fell into the least amount of junk, was the numbers above small foil, but below nickel. And above nickel, but below tabs, and so forth. Theoretically, by programming in these "ring enhancement programs", you would pass the most junk, while digging the most gold rings, per the target ratios encountered in the average park conditions.
I'm sure these charts are out there on the net somewhere. And the numbers can just be converted over to the Minelab numerical scale. But they won't take in to account the left/right axis. I also recall that when machines like the Spectrum, XLT, etc... were introduced, they had not only the up/down scale, but introduced something akin to the left/right scale: the "graph" where you could see bars, smears, etc.... Immediately people thought "aha, finally we can see if there is a tendency for gold, verses aluminum, by incoorporating this new info". But it became immediately clear that this added no help in discerning gold vs aluminum. Items of either metal were still "all over the board" depending on size, shape, karots, angle in the ground, depth, etc.....
As far as the 2nd part of the question: sound, tones, repeatability, etc... It has been thought by many that even though gold and aluminum share the exact same conductivity and target cross-hairs, that gold somehow seems to "sound different". I believe this is really nothing more than selective memory though: There are reams of pulltabs or foil wads that "lock on" and "sound" just perfect! And there are reams of gold rings that "waffle" and "sound" kinda iffy. I think the only reason the human mind wants to think gold "sounds different", is the following psychology: If you go out to a junky park, and dig 100 aluminum junk items, and then FINALLY dig a gold ring, your mind will immediately think "this gold ring 'sounded' different". But all that is happening, is selective memory. It works like this: Every time you go to dig a target, you are subliminally thinking "this sounds different". But when it turns out to be junk, you immediately forget your premonitions, and say to yourself "
yeah, come to think of it, it DID sound kinda junky".
But finally, when you dig a gold ring, only THEN you remember your premonitions, and think "Aha! I KNEW it sounded different!". Kind of like how we think our dreams at night come true, when you hear that song on the radio that you dreamed about, or that friend you dreamed about calls on the phone the very next morning, etc....: We dreams hundreds of dreams per night, that never come true. So we promptly forget them all. But when one randomly does come true, only THEN do we remember the dream, and think "Aha! I'm psychic!".
It's the same psychology happening for those of us that want to think that gold rings "sound different".
Or put it this way: whenever I encounter someone who insists that you can tell the difference between gold and aluminum, by the "sounds" and "tones" and "smoothness" type differences, I challenge them: Tell them to go down to the nearest blighted inner city junky park, and turn them loose. See how many gold rings they can dig, while leaving even a majority of tabs and foil behind. I think you will find that they quickly abandon their theory.
