Applying grease by means of sonic or ultra-sonic frequency is a great method to reduce over greasing or assure that the bearing has proper amount.
BUT don't you have to have a baseline prior to implementing the use of the lubrication by frequency? Bearing manufacturers should supply the alarm frequency info but not the baseline for normal operations.
I have been contemplating the use of applying grease by means of frequency. From what I have been told/discovered, first a baseline (frequency) for all greased bearings must be established & entered into a data base. Then the use of ultra-sonic frequency is used to apply the proper amount of grease per application by set alarm limits. This can turn into a major project in any large industrial site. But the end result is worth the effort.