A condition monitoring apparatus (300) for a system comprising rotating or reciprocating machinery and a seal is disclosed herein. The apparatus comprises a first acoustic sensor (301) coupled to the seal for providing a first acoustic emission signal, a second acoustic sensor (302) coupled to the rotating or reciprocating machinery for providing a second acoustic emission signal, and a filter module (310). The filter module is configured to perform active noise cancellation based on a comparison of the first acoustic emission signal and the second acoustic emission signal.