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Various probes are described that will amplify the sound of contact with any hard object in the body and thus signify its presence. One of the most useful forms consists of a sound transducer that clamps to the handle of any pair of forceps so that the surgeon knows when he is grasping a kidney stone or gall stone or other hard body. Also mentioned are active probes in which loading of a bilateral sound transducer, which is being driven by an external electrical circuit, causes an indication of contact as an apparent change in impedance. These forms seem especially sensitive when fitted into the end of a catheter.