Patent US 4,649,743

An abnormal noise detector for use in evaluating the quality of automotive gear units or the like is disclosed. The present invention employs a microphone which picks up the noise produced by a gear unit being tested and an adjustable bandpass filter connected to the microphone which passes only those signals from the microphone having a desired frequency, the desired frequency being a function of the rate of rotation of the gear unit. The bandpass filter is controlled by a proportional amplifier responsive to the rate of rotation of the gear unit which at any rate of rotation of the gear unit causes the bandpass filter to pass only the desired frequency. The filtered output from the bandpass filter is applied to an X-Y recorder which automatically plots the sound pressure of the noise having the desired frequency as a function of the rate of rotation of the gear unit or as a function of the desired frequency.