The same remark is valid for the measurement device. The instruments must be calibrated and interchangeable without inducing measurement variation. Ideally, the input, receiving the sensor signal, has to be under control, not necessarily the heterodyne audio output.
Listening is the primary defense line.
The first functionality of an ultrasound device is to transform high frequency to audible sound. The operation is called heterodyning. People not familiar with ultrasound instruments think this is an old-fashioned and outdated method. In fact, it is not, and this is especially the case when inspecting low-speed machinery. An operator without deep knowledge can distinguish a healthy bearing, producing a quiet steady signal from a defective bearing, causing an intermittent or specific repetitive ringing or crackling sound.
However, listening is not enough. Reliable measurements are required to build a solid PdM program. Otherwise, your instrument is no more than a stethoscope.
Static measurements for failure detection
Static measurements (overall values) are simple to implement as the result is just numbers. They are easy to manage for storing, trending curves and triggering alarms. For low-speed bearings, two indicators provided by static measurements are used: the RMS, which characterizes the signal energy and the Peak, which characterizes the signal amplitude. Combining these 2 indicators, early failure detection is easy to find.