The present invention provides a sound activated detection system disposed within the conveyor and/or hammer roll area of a reduction mill for detecting unshredable materials fed into the machine. More specifically, an embodiment of this invention comprises a unshredable debris detector disposed in operative relationship in the material input path and includes a transducer, preferably a piezoelectric crystal, acoustically coupled to a sensing surface disposed transversely across a portion of the input path. Alternative embodiments may include one or more accelerometers, microphones, or other vibration or acoustic sensors either alone or in conjunction with the transducer for detecting the unshredable material.