Like any job, there is a right way and a wrong way to do things. Simply listening to a bearing with an ultrasound device that gives no quantitative feedback is a recipe for disaster. The audible feedback is too subjective to draw any comparative conclusions. No two people hear the same and there is no way to remember what the bearing sounded like a month ago.
The third mistake is depending solely on subjective ultrasound data when precise quantifiable data is available.
Always use an ultrasound instrument with digital decibel metering. Better still, use a device that provides multiple condition indicators. Max and Peak RMS decibel measurements indicate alarm levels and greasing intervals while Ultrasonic Crest Factor provides insight about the bearing condition in relation to its lubricant. Crest factors help us differentiate between bearings that need grease and bearings that need to be replaced.
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Lubrication, Ultrasound by Allan Rienstra - SDT Ultrasound Solutions