Jack,
I'll do my best to answer this question. It's not an easy one to answer as describing sounds, etc is less than ideal in text. There's nothing too technical about it, just something that has developed over considerable hours of use.
Fistly, I'm only really hunting gold/platinum jewellery (rings/chains), so targets that have a large physical signature are often discounted. I'm fortunate that I don't have much iron junk to deal with (except bottle caps). The audio from hair pins, irregular pieces of foil/aluminium and some iron objects will give the usual double blip or "broken" signal and to me just don't have that nice softer/mellower and symetric audio signature. Most iron targets (to me) also have a brash and louder audio. What I look for in a signal is a combination of;
1. Physical target audio "shape". What size target am I dealing with ??
2. How symetric the audio is from different angles.
3. How smooth/mellow the signal is (ie gold).
4. How satisfying the signal is overall.
I know it sounds like "yeah, fine in theory but in the field it all sounds diggable". Well you're right when first starting out, but after many hours digging every target known to mankind, your brain will soon begin to "log" what you have dug up against what sound it initially heard. I always try to guess what I am about to dig up based on what I have heard and I don't always get it right.
I'd also like to say that on some days I will DIG EVERY TARGET regardless of what I think the target may be and even knowing 99% that it is junk. This process allows me to test my theories and to see if I am making correct assumptions during my normal detecting. I would guess that normally, of ALL targets that I hear, I would only physically dig about 70%. I am now confident that the 30% (as checked on days when I dig everything) I don't bother to investigate any further as it has a very high chance of being junk. Having said that, there is still plenty of junk in the 70% that I choose to dig up but that is the limits of what I can discriminate out. I would also say that of the 30% that I choose not to dig, it is impossible for it to be 100% accurate, however, I am happy with the results of my "dig everything" tests.
I also find that certain beaches are limited in what type of junk might be found there. I still dig ALL pulltabs as they signal exactly like gold rings. My main junk item is bottlecaps. These give a larger signal proportional to their size and it becomes larger the more they rust. The signal is symetric but sound larger than rings I have found/tested and the audio is "brash" and not that softer, more pleasing sound (rings,coins). I generally dismiss big objects that often turn out to be large pieces of metal or beer cans, and yes, I may miss the odd watch. If I am still digging past 18" or so, then the chances of it being the typical ring sized object is diminishing all the time. When first starting out, I was constantly digging holes past 2 feet that resulted in a beer can or a steel tent peg
being unearthed. Assuming that the target isn't in the side walls of the hole, I never dig to a depth that exceeds the limits of detection for a typical gold ring/chain.
I hope some of this makes sense to you. It's all about time spent swinging the coil, concentrating by listening to the signal signature (and not just hearing it), "guessing" the target and logging it inside your head, whether you are right or wrong.
All the best,
Tony.
PS How's that Welsh flag going ??
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