Threshold, as it applies to detectors with an adjustable threshold, is a tone (usually in all metal mode), that can be set by the operator. Generally, it is best to set the tone to where it is just audible. At this setting when the detector passes over either a very deep target or very small target that wouldn't ordinarily give enough signal to beep or activate the TID screen, a sometimes very slight bump in the volume of the threshold tone will notify the operator of the target's presence. This is one of the ways that the experienced diggers find the deep, rare targets and why savvy detectorists always wear headphones.
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