Acoustic locators are one of several instruments that can be used to trace out poly water pipes that do not have a tracer. The common instrument is connected to a fire hydrant and then produces a repetitive tapping that can be audibly traced down the pipe through the water.
The receiver includes a very sensitive microphone rod and headphones. The modern instruments have improved audio so the sound can be detected at a long distance. However, the sound detection is now improved to the point that even a passing automobile can cause an incredible noise through the headphones. Although the instruments come with an automated shut off for loud noises, you do need to be careful around any possibility of sudden noises.