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Old 06-25-2008, 08:10 PM
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Rudy Rudy is offline
Join Date: Jan 2006
Location: Beaumont, CA
Posts: 25,142
Default Confused about Threshold vs Volume?

Many newbies go out and buy a detector that has an adjustable, audible threshold and are confused by its function or proper use. The situation gets even more confusing if the detector also has a volume control. Oh wait, you also bought a nice set of earphones and they also have a volume control. What to do, what to do.

Ok, first, lets look at what is this so called threshold and what purpose an adjustable threshold serves. Then we can tackle what to do with the volume control. Setting both properly maximizes your ability to get as much depth as you can from your detector.

Not all detectors sold now a days have an adjustable threshold that you can hear. These latter machines are termed "silent search" detectors. Basically, they are quiet running until you sweep over a target and then they beep at you.

You can think of the threshold as a type of squelch control as found on a CB or short wave radio. Without a squelch control circuit on a radio, the speaker would be putting out a constant stream of hizzing noise when you are not tuned in to a station. This can be annoying. By dialing up on the squelch control, we are basically telling the radio that we don't want to hear a signal until it reaches a certain strength. The hizzing noise coming from the speakers is suppressed because it is low energy background noise and the radio is quiet until you tune in an actual signal. By the way, on some detectors the manufacturer calls the knob that adjusts the threshold a "tuning" knob. Now you know why.

If you dial in even more squelch, you'll start to miss some of the weaker signals from far away stations.

This is exactly how the threshold control works in a metal detector!

With a silent search machine, the "squelch" control is factory set to suppress the small signals and allow signals with a strength above the threshold setting to get through and make a "beep". If you bought a detector with an adjustable threshold (good for you!), you want to set it such that you can barely hear the threshold sound on your earphones. The sound is hard to describe, but it sounds like bees buzzing your ears. By having the threshold at this setting, right on the edge, you are guaranteeing that very weak signals (like from deep targets) will be able to get through the threshold control. This gives a machine with adjustable threshold a slight depth edge over a silent search machine, everything else being equal. This is because a very weak signal may not get through the "squelch" setting of the silent search machine.

You don't want to make the threshold any louder than what you can barely hear. If the threshold hum gets too loud, you'll miss the real small signals again. This time because they are drowned by the louder threshold buzz.

If like some other folks you can't stand that constant low level buzzing sound of the threshold, you can always back it down a hair until it is not audible anymore. Essentially, you'll be running the detector as if it was a silent mode machine. But you pay a price for doing so. First, you'll lose some of that depth advantage you had over a silent search machine. If you back off on the threshold to just make it inaudible and no more, then you haven't lost much depth. Second, you'll lose some potentially valuable information. You see, when you go over a ferrous item, the threshold sound will usually null (will go quiet) momentarily. When this happens, your detector may not detect a valid target for some time after that. This elapsed time is called the recovery time and varies by the detector model used.

When you hear the threshold null out, you know you went over a ferrous item and you need to slow down and or change your sweep direction lest you miss a valuable target buried nearby the ferrous item. With a silent search machine you won't know it's happening.

Ok, now that we understand threshold, lets look at volume. This is again, just like a radio. Your detector may or may not have a volume control (separate from the threshold or tuner). If it doesn't have one, you definitely would want earphones with volume control. If you have volume control in both the detector and the earphones, then set one at max volume and set the other one to actual use conditions.

My MXT does not have a volume control but it has a very powerful (read that loud) audio amp. So, I set the volume control on my earphones to produce as loud a beep as I can without it being painful. I do this by just dropping my digger on the ground and sweeping over it and advancing the volume control until it as loud as I want, without hurting my ears.

Then I set the threshold control until I can barely hear the buzz. As a side note, if the threshold is erratic instead of stable, the Gain is too high for the ground you are sweeping. Back off on the gain until the threshold is stable.

That's it! Hope you found this useful. For information on understanding the sounds your metal detector makes you might want to look at
Understanding the sounds from your detector.

HH, Rudy

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