what are you going to be looking for, and in what type environment?
Eg.: individual coins and fumble-finger losses? Or perhaps you're on a clue for a large item?
whatever it is you're expecting to find, take sample targets like that, put them out in a clean stretch of ground (like a sandbox where you're sure there's no underlying rebar, or chicken screen mesh, etc... and try the targets at all different depths, swing speeds, machine settings, etc... Make note of the sounds with the variations of settings and speeds. Also do the same with junk target: Nails of various sizes, coins, large iron objects like a RR spike or wrench, cans of various sizes and compositions, your wedding ring, tabs, foil wads of diff. sizes, etc... And even put them in close proximety so you can get the sense of what a cluttered mix target area would be like, etc....
