1. A system for controlling wheel squealing noises produced by an activated railroad car retarder comprising, transducer means located in the vicinity of the car retarder for producing electrical signals in response to sound waves, filter means coupled to said transducer means for passing a selected range of frequencies of said electrical signals, rectifier-integrator means coupled to said filtering means for sensing the time integral of said filtered electrical signals, comparator means connected to said integrator means for producing an output signal when the time integral exceeds a given value, and output means coupled to comparator means and responsive to said output signal for deactivating the car retarder thereby suppressing wheel squealing noises.

11. A noise control system for braking apparatus comprising, means for converting sound into electrical waves, means for filtering undesirable frequencies from said electrical waves, means for rectifying and integrating said electrical waves, means for sensing the magnitude of said integrated electrical waves and for producing an output signal when the magnitude of said integrated electrical waves exceeds a predetermined value, and means responsive to said output signal to deactivate the braking apparatus to mitigate noise pollution.