This invention relates to apparatus for the detection of accustic impact frequencies, characteristic of bearing assembly failure during operation, which modulate an acoustic carrier frequency band. This apparatus allows for on-site or field application with high-level of ambient noise such as are present in railroad yards. This apparatus allows for early detection of such failures, so that a repair may be made before damaging heat build-up or catastrophic bearing failure occurs. Additionally, with sufficient early warning in railroad applications, a train may continue running to a railyard for convenient shop repair rather than expensive field repair.