dani --
I am glad to help, and glad you feel that what I posted may have helped you understand a bit better how your machine works.
I think you will have fun with it, and Europe certainly has LOTS of old stuff in the ground!
I will try to answer your four questions...
1.) I'm not sure what you are asking about threshold being "audio only." If you are asking if the threshold simply refers to that audible "hum," and if adjusting the threshold knob simply increases or decreases the volume of that hum, then yes, that's basically what threshold does. Yes, knowing your detector and how it behaves will help you discern when it is just electromagnetic interference making your threshold increase or decrease in volume or tone, and in contrast, when it might instead be a small, deep target. For instance, if you have a fairly stable threshold, and you are swinging your coil, and you hear a little "blip" in the threshold hum, it COULD be a small pulse of background radiation (referred to as EMI, or ElectroMagnetic Interference), OR it could be a small and/or deep target. By running your coil back and forth over that spot, and seeing if that "blip" is repeatable, is how you can discern whether the tone you heard was a sporadic "blip" of EMI, or instead may have been an indication of a metal target. But remember, this "splitting of hairs" and "listening for the subtle changes in threshold" is only necessary for VERY small or VERY deep targets. Most larger, coin- or jewelry- or relic-sized targets will give you a very CLEAR response -- MUCH louder than your threshold; there will be NO question. These subtle little "blips" we are talking about are difficult to work with for a new detectorist; focusing on the clearer, unquestionable responses from your machine will be a better way for you to learn your machine. I might suggest putting some metal targets on the ground and running your coil over them, to get used to how the detector responds; you might also, before your trip, go to your local playground, where shallow coins should be plentiful, and get used to hearing, and digging, targets in the ground. Don't make this too difficult at first; it takes some time and experience to find and dig those more "complex" targets. Focus on the simpler, louder, clearer, repeatable signals at first.
2.) Yes, "hearing" signal/tone variations will, over time, become a VERY important piece of information for you, in determining what your machine is "seeing" underground. However, the only way to TEACH your brain and ears what those signals mean is to find a target, LISTEN to the machine's response, THINK about what it might be (size, depth, type of metal), and then DIG IT, to find out. At first, you may have NO CLUE how to "guess" what the machine sees, until you dig it. Over time, though, you will learn -- but ONLY after hearing, and digging, many many metal targets. PRACTICE! As for "numbers on the display," they can help, but please understand they are NOT foolproof. A quarter may give you an "86" number, if it sits at 4" deep and there are no other metal targets around. A nail may give you a "20." Now, put the nail and the coin on top of each other, and run the machine over them, and it may sound like ONE target to your machine, and give you a "75" number. Reason? The machine is "blending" the two targets and thus coming up with a number that is NOT representative of EITHER the quarter OR the nail, but something else. See what I mean? This is just ONE way the numbers can fool you. Another way? A smashed aluminum screw cap from a wine bottle may give you an "86" number -- just like a quarter. A coin laying on its side, instead of flat, may give you an ID number much lower than it otherwise would. Do you see what I'm saying? The numbers can help, but they are NOT foolproof. If you went out on a given day and said "I'm going to dig every 86 signal I get," you would dig some quarters, you would dig some smashed screw caps, you would probably dig a few bent rusty nails, you may dig a brass washer, possibly a "tarp grommet," possibly a "parking token" or two. Further, you would MISS many quarters -- quarters that were on edge and thus mis-ID'd, quarters that were next to nails or other trash and thus mis-ID'd, etc. My point? Numbers are a guide, but EXPERIENCE will help you dig less trash, and more good targets, as you get more used to your detector.
As for "numbers on the display," this is part of why I said it is easier for a beginner to use disc. mode. Because, in disc. mode, that big number in the middle of your screen is the VDI number, or the "machine's attempt at telling you what kind of target it is detecting, based on algorithms in the detector's software." Again, the numbers are not foolproof by any means, but they can be quite helpful. HOWEVER, in all-metals mode, that number in the middle of the screen is NOT target ID, but instead switches to "ground phase" (i.e., a readout of the "mineralization" of the soil you are hunting in). I will say that while the large number in all-metals mode is NOT target ID, you still have some numerical ID info -- it's at the top of the screen on that 0 to 100 "dial." Place a dime on the ground, and with your machine in all-metals mode, run the coil over the dime, and watch the "dial" at the top of the screen. You will see a few black bars show up within the 80 to 85 range, at the same time you hear the tone representing the dime. This is telling you that, similar to what that large number in the center of the screen WOULD show if you were in disc. mode, that your target (the dime) is being ID'd by the machine with a number in the low 80s. So, to answer your second question directly, you do NOT have a digital number in all-metals mode that helps ID a target, but you DO have a small "range" of numbers indicated on the top of the screen, along that 0 to 100 "dial."
3.) I'm not sure what you are asking in your third question, but if you are talking about that 0 to 100 "scale" or "dial" on top of the screen, YES, you CAN use it, just as I explained above -- in all-metals mode, that "dial" is your "probable target ID" information.
4.) As far as the numbers on the display and what they show, I have talked alot about that in some of what I wrote above. Again, in disc. mode, the big number is "probable target ID", also called "VDI." In all-metals mode, that number now represents "ground phase," or the type of mineralization in the soil you are hunting in. As far as what the numbers tell you, again -- the ground phase tells you the type of mineralization in your soil (this information is most important to gold prospectors, and not AS important to coin, jewelry, or relic hunters); in disc. mode, the numbers tell you, again, VDI or "target ID." Like I mentioned in an earlier email, numbers below 40 are iron; 40-50 will generally be tiny pieces of aluminum or lead -- and also small gold jewelry; 50-60 is where pull tabs and such often ID, as well as nickels, and also somewhat larger gold jewelry; the 60 to 70 range contains various types of metal junk, as well as many gold rings; the 70 to 80 range will include various types of metal junk, as well as large gold rings, and also zinc pennies. The 80 to 90 range is where a large number of U.S. coins will fall, along with a good bit of the silver jewelry (and also a large amount of junk such as smashed aluminum cans, screw caps, etc.); in the 90s range, some large silver will be found, as well as rusted iron objects such as nails, some rusty screw caps, some very large pieces of iron, etc.
Again, keep in mind the limitations with respect to ID numbers. Over time, you will learn to use a wide array of information when making your dig vs. no dig decisions; ID numbers help, but you will also learn to use the different sounds/tones (which in time, will help you to determine likely size and shape of the object, depth, etc.); the depth readout helps (it can indicate whether the target may be older and thus deeper, or possibly newer and shallower); finally, your general EXPERIENCE, both with your machine, and with the area you are hunting, will help you decide what you want to dig, and what you don't. Experience cannot be understated -- and the only way to gain it is to get out, swing the coil, listen and pay attention, and DIG -- dig nearly everything at first, until you learn how different objects (both trash AND treasure) are reported by your machine.
Hope this helps.
Have fun!
Steve
Hopefully, that was semi-clear and not too confusing! People who know me, would quickly recognize all that wordiness, and just roll their eyes!