This is phantom powered tone generator in the shape of a long XLR plug. It produces tone, clicks, or white noise as selected by a rotary switch on the far end. I've found it works well and is ideal for setting up equipment in sound systems. I was once doing live sound at an event that was being filmed for TV and the cameraman (from a major broadcaster) needed some tone to line up his camera. I plugged this into a spare mic input on the mixer, problem solved, and the camerman was well impressed. It is solidly made with a metal body and has survived several years in a bag of adaptors and connectors. It's simple, effective, and very cheaply priced. I'd say that every sound engineer should have one!