Abstract: An apparatus for detecting resonant frequency of bones has an electro-mechanical transducer arranged for application to the external skin surface covering a bone, the transducer being in a feed-back circuit with an amplifier and phase shifter having a unity gain and a loop phase of zero. The feed-back circuit is connected to a display comprising an A-D converter and a digital display. The apparatus displays the lowest order transverse mode set up in a bone.In a further embodiment the transducer is arranged to pierce the skin and to be in direct contact with a bone. The transducer has a coupling rod and a detector which may be both positioned at the same station on a bone or at spaced stations along a bone.