People go out in the park or on a school ground swinging a metal detector. If they have their headphones on, you are not going to hear a thing; if they don't have headphones, there will be a little beeps as they go over something metallic. The modern machines were initially cheap. If you went over anything metallic, it would beep; it didn't matter if it was piece of tinfoil or horseshoe or a coin. It would make the same sound. A modern detector of average or high price will tell you what the machine thinks you have found and how deep it is.
The advantage of this is that you can tune out tinfoil, bottle caps, and all the things you are not interested in digging up. If you are looking for coins, you can just concentrate on that. If you are looking for a specific piece of jewelry that's been lost, you can tune the machine to find just that. As a warning, none of these machines are 100% accurate. There are factors that can cause it to misdiagnose. Most treasure hunters or most people using metal detectors don't want to take tinfoil and broken horseshoes. They want to dig for coins or jewelry. If they are a Civil War battlefield nut, you can't go metal detecting in a national park that is a Civil War battlefield. All national state parks and monuments and historical sites are off limits to treasure hunters. The archeologists want to do all the digging there. If I was visiting in Virginia and the friend of mine had a farm somewhere near a Civil War battlefield, or what have you, we might be able to go out and find all kinds of fired bullets and dropped things from a Civil War battle, as long as I was on private property. The areas of interest of treasure hunters or people using metal detectors are: coins, jewelry, battlefield mementos, and artifacts from early activities like early farming, trading, and hunting. The other general field of interest is gold. In the southwest, people want a detector that will find tiny, tiny little gold nuggets and gold dust and some detectors specialize in that.
