My first thought was that you have to sweep your coil over it. I wouldn't say that to be a wise-guy. But it is true. I've dug modern coins at depths of 7 or 8 inches. And the next hole I dug was a Barber quarter at less than 2 inches. So, honestly, deeper coins don't necessarily mean older coins. In fact I honestly believe there is more old silver still out there due to target masking, opposed to simply being too deep.
But I think I understand what it is you are asking. I know that opinions vary on this matter. But, I'll give my two cents worth............I've never heard a coin "whisper". In fact, I am not convinced that the tones on "deep silver" are fainter than the tones on silver that is not as deep. I prefer to think of the deeper coins giving off a more "narrow" target response. Maybe I analyze things too much. But in my mind, the detector is designed to send an electromagnetic field, via the transmit coil. That electromagnetic field spreads out the farther it gets away from the coil. I suppose you could say that it "weakens" as it penetrates deeper into the ground. It will eventually reach a point that it simply won't go any deeper. Anyway, if a target is "within range", the transmit's electromagnetic field is induced into the target, which in turn sends back it's own electromagnetic field, opposite in polarity to the transmit signal. The receive coil of the detector senses that signal and processes it. Part of that process is amplifying the signal and producing an audible response. I believe that (for the most part) the audio tone is just as "loud" on say a 6 inch dime as it is a 3 inch dime. But the electromagnetic field being produced by the coil at 6 inches is "of shorter duration" (narrower) than the electromagnetic field produced by a coin that is 3 inches deep. To your ears, it may sound fainter. But I am convinced that the audio volume is the same. The sound you hear is not the sound made by the target. It is an audio tone being generated by the detector, simply to let you know that your coill has passed over a target. In my mind, the deeper target signals are just of shorter duration and will not "jump out at you" like one that is not as deep. For some, those deeper signals just don't seem as loud. But again, I think the difference is how long that audio signal "lasts", as you are sweeping over the target. You will find that, once you get the target pinpointed, the vast majority of deep targets will lock on loud and clear. The reason it may have sounded "faint" to begin with is because you did not pass over it with the exact center of the coil. Never say never! And I will assert that exceptions could be coins on edge, adjacent targets, improper GB or improper NC channel. But for the most part, if the receive coil is notified that it has passed over a target, the electronics will initiate a synthesized tone to alert you. And it will be as clear, sharp and as "loud" on a coin at 6 inches as it is on one at 3 inches. It is just "narrower". On the other hand, maybe I do think too much!
As to the display not giving an indication when you hear a "keeper" tone..... that is not unusual. The TID display on most single frequency VLF detectors give a target indication based on common coins. And when the electromagnetic response is so weak that it can't be identified, the X-Terra will still give you an audible tone. But it might not be getting back a strong enough signal to provide a solid visual TID. Not to get too technical, but the display on an X-Terra works off the same information outlined above. When you pass the coil over a target, and the electromagnetic field is induced into that target, the target produces it's own electromagnetic field. The transmit coil produces a signal with a given frequency. The target will produce a responsive signal with a different frequency. That frequency is dependent on the metallic content of the target and the diference is called the phase shift. Higher conductive targets produce signals with less phase shift than targets with lower conductivity (more ferrous content). The TID is based on that phase shift information. If the target is too deep to determine the amount of phase shift, the TID will not be accurate. But again, you will get an audio tone. Phase shift is how a detector implements discrimination characteristics. But keep in mind that target discrimination is based on the phase shift of that target. Not on the actual identity of the target itself. That is why you can find a gold ring when you think you are digging a nickel. Or worse yet, you may have passed over a real beauty of a ring by passing that last pulltab! To sum it up, TID is based on generalities. Not specifics. In fact, even what appear to be two identical coins may not read the same. If you take a look at the coin chart (FAQFAQ section) I built a couple years ago, you will see that there is a wide variance in some US coins. Quarters generally read 40 or 42. But IH cents can read anywhere from 24 to 32. It simply depends on the metallic content and the wear of the coin. If you are in an actual hunt situation, the composition of the soil, angle of the coin, adjacent targets and the depth of the targets may also influence the TID. JMHO HH Randy
