Dictionary.net

Graphic registration of the heart sounds picked up as vibrations and transformed by a piezoelectric crystal microphone into a varying electrical output according to the stresses imposed by the sound waves. The electrical output is amplified by a stethograph amplifier and recorded by a device incorporated into the electrocardiograph or by a multichannel recording machine.

Registration of the heart sounds; the sounds, received by a stethoscope and magnified by a resonator, cause vibrations in a tuning-fork; these excite a microphone which sets in action an electromagnetic signal apparatus, the movements of which are registered by means of a pantograph.