To investigate the function of a direct-contact type phonocardiographic microphone in actual use, the mechanical impedances of the contact microphone and that of the living tissue were measured. The mechanical impedance of the chest wall, measured by the piston method, showed a resonance at about 100 c/s. The impedance becomes elastic below resonance and mass reactive above resonance. The equivalent mechanical circuit of the tissue is added to that of the contact microphone to get the equivalent model of the microphone actually set on the chest wall. The equivalent model showed a twin-hump characteristic with two resonant frequencies, which we defined as a performance characteristic. In order to obtain more systematic analysis, a simulator—using electrical elements—was constructed to simulate the performance characteristic. A contact microphone was experimentally constructed, based upon these studies, and its superior characteristic proved by the “arm method”. It gave good results in clinical use, being both efficient and free from disturbance from ambient noise.