You're not confused Tom, you're dead on from what I'm seeing so far. Some big iron and tin sheets will still sound off, I guess it depends on many factors.
What's really cool about the GT is that when running in silent search/threshold mode the SOUND of the threshold itself will also change to whatever caused it to null out.
<span style="background-color:#ffff00;">Quote from the GT manual below:
Example: If you are searching in disc mode and have just detected a coin, the threshold will remain at the tone of the coin signal, but when you pass over a junk target such as a nail, the audio will NULL ( go silent) over the nail, and the threshold, when it returns, will drop to a lower tone. This can be a usefull feature, as unlike other detectors using Silent Search discrimination modes, you still ignore junk targets, but you know when you have passed over them and can estimate the conductivity they have. End of quote:</span>
I found a 1933 merc this morning doing exactly what's explained above and it wasn't any further than a foot away from a large piece of iron.
The merc was around 10" down and the iron 5 or 6" down and off to the side of the hole. I didn't get a signal per say on the dime, just a rising threshold.
I walked past the iron and over it first but then it hit me that the threshold sound had raised up high like a coin would so I dug and found the merc.
I really like this GT, it's full of surprises <img src="/metal/html/wink.gif" border=0 width=15 height=15 alt="

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Good Hunting
Mike