A method and apparatus for processing electrical signals

An apparatus (10) for processing acoustic emissions or stress waves in order to recognise features indicative of variations in the process producing the acoustic emissions comprises a transducer (12) acoustically coupled to a machine or industrial process and arranged to detect the acoustic emissions and produce an electrical signal. The electrical signal is amplified by an amplifier (14) and the mean level of the electrical signal is measured by a mean level detector (18) and the minimum level of the electrical signal is measured by a minimum level detector (20). A ratio measurer (22) measures the ratio of the mean level of the electrical signal to the minimum level of the electrical signal. This method is self compensating for variations in the operating conditions of the machine or industrial process, is not significantly affected by electromagnetic switching transients, and can cope with very high numbers of transient excursions.