The guy is on the right track with the mini-vibe motors. Everyone knows how strong the vibe is on a cell phone or game controller. IF some company wanted to, they could design a vibe receiver using one of those mini motors. It would be based on the latest Bluetooth transmitter/receiver tech. The vibe motor would be driven by a controller that makes vibration strength (motor speed) correspond with target conductivety. The higher the vid, the stronger the vibration. The vibe receiver could be strapped around the wrist or ankle. I am not an EE but I know this is a cake walk for the designers at the major detector companies, as well as other hobby related electronics companies. The cost would be comparable to good Bluetooth headphone technology. This idea paired with tid display would allow any hearing impaired person to be very good at metal detecting.