DE3100045

An illustrative embodiment of the invention provides an acoustic sensor for detecting stones and other hard foreign objects mixed within crop material in a harvesting machine susceptible to damage thereby. The acoustic sensor comprises a sensing surface disposed within an input path in the harvesting machine for impacting engagement with the flow of crop material therein and an acoustic sensing device coupled to the sensing surface for detecting acoustic vibrations induced within the sensing surface as a result of impact therewith by stones and other foreign objects mixed within the crop material. The sensor provides electrical signals in response to the acoustic vibrations indicative of the impact of the crop material and the foreign objects with the sensing surface and includes isolation means for obviating the effects of extraneous acoustical signals of the same character as the foreign object impacting signals to accentuate the foreign object impacting signals and thereby reduce the false alarm signals.