Gearboxes can be inspected with ultrasound, but some recommendation should be followed for the best results. In order to minimize the signal lost for attenuation, do a mapping of the gearbox. Put your magnetic sensor close to the gear mesh and record the Dynamic signal for analysis. For a better diagnosis you must know speeds and number of teeth for each gear.
Tip Provided By: Allan Rienstra, SDT Ultrasound Solutions