The calibration of stethoscopes and phonocardiographic microphones

The response of stethoscopes and chest microphones depends on the impedance of the sound source, which must therefore have the same impedance as the body, and must emit a signal related to the sound intensity in the body when no instrument is applied. Body impedance depends on the geometry of the instrument and on the force of application, etc. The source impedance must, therefore, be variable to suit test conditions. Unexplained discrepancies in published impedance measurements hinder standardisation of source impedance. The impedance of an electromagnetic vibrator can be modified electronically to meet these requirements.