Just found an expensive high end gadget that might be handy in the field for someone with big pockets. I was looking at industrial devices wondering if the big boys have tools that I had not heard of. Fluke Sonic Imager popped up on a search. The $20,000 camera takes a high res picture and overlays sound received on the picture. BF walking, wood knocking, or using infrasound would not only be detected but the location of the sound would be superimposed on the optic picture. The specs list the sound detection frequencies at 2 to 52Khtz. The device looks like an IPad and it says a series of microphones in the device fix the location of the sound relative to the optical picture. An air leak would be a red blob on a pipe. Perhaps it could even pick up the breathing of a nearby BF. It would be pretty handy knowing exactly where a noise is coming from. Fluke also produces high resolution IR cameras but they start at $20,000 and go up from there. Think of what could be thrown at BF research if the money was there.