When you read this, you may wonder why I support the theory of using a "barely audible Threshold". If I had not witnessed what happened during the hunt when I dug the silver 3-cent piece, I wouldn't have believed that Threshold could play the role that it seemed to have played. But I saw what I saw, and made every effort to find another explanation. Since that day, I've ran the subject of Threshold through my mind hundreds of times. I've tried to duplicate the scenerio, without success. At this time, I simply haven't come up with another rational explanation.
With that said, I don't interpret the functionality (usefulness) of the Threshold sound the same as what some might, including the passage you posted from the manual. I do agree that Threshold and audio Volume should work "hand in hand". In other words, we shouldn't mistakenly raise our Threshold level to compensate for a lower Volume level. Or vice-versa. But I believe that the Volume control simply increases the target response by a specific amount of amplification. I do not believe it is a consistent "flat line" sound level. When a larger or more shallow target produces more "target signal strength" to the software, it's audio response will be "louder" than a smaller or deeper target because the small, deep targets provide less initial "target signal strength" to the software. That is why I believe that amplification is gradiant, not consistent or "flat line". While on the subject of target audio responses, I also believe that smaller, deeper targets are not necessarily "fainter" signals. I believe many of them are actually more "narrow" signals. This is based on the "field of detection" for the coil designs used on the X-TERRA. In support of my thoughts, more times than not, when I hear what I first perceive to be a "faint signal", I can slow down my sweep, X over the target, and get a much more distinct audio target response than initially heard. Not always louder. But definitely more distinct. It is no longer "faint". But it typically remains more "narrow". I won't argue the differences between "faint" and "narrow" as I feel it is most likely a matter of symantics.
In response to one of your questions, if you have your Threshold set so you can hear it, the only "change" you will hear is when it is replaced with either the nulling of a rejected target or the audio response of an accepted target.
Over all, here is my take on it...... Threshold level and Volume level are two separate controls with two different purposes. The Threshold setting is nothing more than a volume control for the Theshold tone. It has a 31 segment range that can be lowered to 5 "points" below zero (silent search) to 25 "points" above zero. When you increase the Threshold setting, you are simply increasing the audio volume of that 230 Hz "background" tone. Volume settings start at 0 and increase to a max of 30. Volume is the "loudness" of the target's audio response. In other words, the target response is amplified to the factor represented by the Volume setting you have incorporated. If you set your Volume to 30, all targets will be amplified to the full extent of the X-TERRA. If you have it at 1, you will basically hear the same unamplified signal that is being processed through the receive circuitry. Simply a matter of amplification of the signal strength, not a flat line audio level. With Volume settings at or near the max, larger targets will be "louder" than smaller targets. And more shallow targets will be "louder" than deep targets. In the Coin / Treasure mode, the Threhsold operates at 230 Hz. Even when operating in multiple tone mode, NONE of the target tones duplicate the frequency of the Threshold. When you pass over a target, regardless of whether you have rejected that particular notch segment or accepted it, the Threshold "goes away" for a split second before being replaced by the "null" of a rejected target or the target tone of an accepted target. When you consider Threshold and target response to be two separate and distinct tones, and the Threshold "goes away" before being replaced by either nulling or the target tone, one could argue that Threshold volume plays no role in hearing individual targets. And to a certain degree, with specific operational settings, I would concur. (e.g. zero discrimination and multiple tones) However, even if you are monitoring all notch segments in zero discrimination, not all targets produce the same volume level of audio response. As I said, larger, more shallow targets will produce a tone that is "louder" than smaller, deep targets. If you agree so far, consider this scenerio.....
Let's say that you are hunting with a degree of discrimination. As such, you want to use a Threshold tone so you can be aware of any target blanking. When I hunt with my 705 and Gray Ghost NDT's, I set my Threshold at 5. This Threshold volume setting of 5 is required for me to hear that "just barely" audible 230 Hz Threshold tone. In other words, if it were set lower, I couldn't hear any tone at all. If I set it higher, it becomes annoying. I have my audio Volume set at the max of 30. Your settings for both may vary, depending on your hearing ability. With my settings as described, when I pass the coil over a large or shallow target that is within an accepted notch segment, it provides an audio response that is quite loud. This is due to the large target having it's target signal strength amplified to the fullest extent of the X-TERRA. However, what if I pass the coil over a small, deeply buried target that wouldn't provide the X-TERRA with as much "target signal strength"? The answer is, it won't provide as much target signal strength to the X-TERRA, resulting in a target audio response that is not as "loud" as the larger target provided. Remember, Volume is applied amplification, not a flat line audio level. If you agree with that, what if that target audio response was so "faint" or "narrow" that it only provided 10% as much initial "target signal strength" as the larger target mentioned earlier? In that instance, the Threshold of 5 (that was required for me to hear any tone) is now replaced by a target audio response of 3. (+/- 10% of the 30 max volume setting). If it takes a sound level of 5 for me to hear any tone, I won't be able to hear the "3" level audio tone being produced by the small, deep target. Even worse, if I had set my Threshold at a level higher than actually required to hear it, say 14 as in the example I gave earlier, that Threshold becomes a "baseline" for my "listening level". If I have difficulties separating the pitches of the multiple frequencies, or I don't hear the Threshold being replaced by a target response tone of a different pitch, or I simply like to "dig when it beeps" and don't pay attention to any tone "louder" than the "14" that I have set as the volume level for my Threshold, I'll certainly miss a lot of targets. Those are 3 of the reasons I hunt in multiple tone, minimal discrimination and a barely audible Threshold.
At this time, I have not been able to duplicate my 3-cent silver "experience" for the sake of making a video. But I would be of the opinion that a dime would be much too large to get results similar to those I encountered. JMHO HH Randy
Barnacle Bill pointed out in a prior post (Bill was also one of the original Field Testers), he might run with a negative Threshold when he wants to "ignore" surface targets. http://www.findmall.com/read.php?55,757051,757619#msg-757619. He also pointed out in another post that setting your Threshold high, to compensate for having a low volume, can be a recipe for missing goodies. http://www.findmall.com/read.php?55,757051,757278#msg-757278
. There was no noticeable difference in the detector's ability to sound off on the target regardless of threshold. Foil ball #4 (made from a foil rectangle 15mmx25mm with a final diameter of 4 mm), again reliably detected with an ID of -9 with the rare 48. Threshold having absolutely no noticeable impact on the detector's ability to detect the target. Just for fun I raised the coil to get an idea of the maximum distance foil ball 4 could be detected and the detector lost it at an estimated 1.5 inches. Note that at these settings the detector is running as "hot" as it can get (with the 9in MF concentric on my ground). I dropped sensitivity stepwise and at 16 foil ball #4 could no longer be detected at any distance.