My current setup is the Echologics correlator from Mueller. You have 3 boxes, they all have antennas to transmit or receive information, 2 have sensors. You put a sensor on either side of where the leak is, enter some basic info (main size, material, distance) and let it do it's thing. It measures noise level in the pipe. It gives you a graph that shows footage across the bottom and essentially noise level on the side, you get a "spike" on the graph and you look at the footage and it'll tell you it's x amount of feet from one box or the other. In theory this works great, in practice there's a lot of things that go wrong.

We're also currently using the Gutterman Aquascope as a listening device. It's essentially an electronic version of a doctor's stethoscope but it's tuned into listen to frequencies carried in water mains. I think we're changing up one of both of these pieces of equipment in the near future though. We use the listening device if there's no water surfacing but say you can see it coming into the sewer or something, we can listen to hydrants, valves and services to narrow down the area to correlate.

It's all a bit more complicated than that but that's the short "I have no idea what you're doing" version lmao.