it depends on the machine, the instruction, and the problem solving abilities of the operator. without air induction, it can run 5-10, with air induction 2-7, on a 1-10 scale. i had no instruction and was unable to get many tips from either the Internet or other leak detectors so i made some sounding dummies. these were different size pipes with drilled holes, saw cuts, loose fittings ect that were buried a couple feet deep and had a riser on them for water and air induction, then i practiced bunches. i don't ever plan on being big enough to do correlation and survey work but that's relatively easy, the equipment is placed at strategic spots in the system and the noise is basically triangulated. by the way, inside, outside, it's the way you approach the detection more than the equipment or the location.