Listening sticks

I was on the streets of Skipton, on holiday, last week and I saw something I had never seen before. Men from the waterboard were putting rods down drains and listening in to the wooden diabolo shapes at the end. I asked what they were doing, having never seen such a thing, and they kindly explained that they were listening for leaks in the pipeline. People often use electronic ones these days but the old fashioned acoustic ones are obviously still being used by some.