knock detection: earphones vs sensors

Just wondering if any of the guys that have had their cars on the dyno could chime in here. I'm curious as to how sensitive our knock sensors are. My mech/engineer/tuner guy has said that he's seen both situations (on other cars) where knock sensors were doing nothing yet he could hear audible pinging, but also had situations where an aftermarket knock sensing unit was picking up pings that he was struggling to hear on the earphones. Just wondering if anyone out there knows what kind of correlation our knock sensors/timing pull have with audible/earphone detection.