teleman --
Remember, though, if you want to hear those "quieter" sounds, you have to have your gain set at less than 10. If I understand correctly, the gain is an "amplifier" of sorts -- it amplifies the sound, such that the higher the gain you run, the smaller/deeper of a target will be progressively set to the "high" volume setting. If your gain is set at 10, I think ALL targets are "amplified" to the "loud" tonal volume level. Imagine a hypothetical scenario with a 2" coin, a 4" coin, and a 6" coin, and an 8" coin. With gain set at 1, I would expect the 2" coin to sound loud, the 4" to sound "medium," and the 6" to sound "quiet," and then maybe not hear the 8" coin at all. With gain set at, say 5, I would expect the 2" coin to be loud, the 4" coin to be loud, the 6" coin to sound "medium," and then, if your sensivity is set high enough to allow it, the 8" coin might be detected, and then would sound "quiet." With gain set at, say, 8, I would expect the 2", the 4", and the 6" coin to be "loud," and the 8" coin -- again, assuming you had enough sensitivity to detect it, would probably sound "medium," or "quiet." Finally, with gain at 10, ALL detected coins should sound loud. You have maxed out the gain, with virtually all targets now set to max tonal volume (as I understand it).
What I was experiencing, with gain set at 8, but running auto-sensivity, was that I was NOT HEARING any of the "quiet" targets. I SHOULD have heard a deeper target sound quiet, but I DIDN'T, until I set it in manual sensitivity. So, what that tells me is, I was only DETECTING the 2", 4", and 6" coins -- which, with gain at 8 per my hypothetical example, were all set to sound "loud." An 8" coin WOULD HAVE sounded quiet at that gain setting, but with sensitivity in semi auto, I was apparently not DETECTING those 8" coins. When I switched to manual, VOILA -- I was able to detect those smaller, deeper targets, and they then BECAME AUDIBLE in my headphones, and -- due to the 8 gain setting -- the tones sounded "quiet" on these targets.
Keep in mind these depths are just made up, but I am trying to illustrate a point. This is what I discovered. Gain set at 8 SHOULD have allowed me to hear the deeper, smaller targets as being "quiet," but I was NOT HEARING those deeper, smaller targets AT ALL -- as running semi-auto sensitivity WAS APPARENTLY KEEPING MY DETECTOR FROM SEEING THEM.
Hope this makes sense; just sharing my experiences as food for thought. Remember, this whole post is not based on any knowledge of the internal operations of the detector and its software. I have no such knowledge. This is just what I've deduced based on how my machine seems to perform with different settings.
Steve