Calibration of an electronic phonocardiograph.

Calibration of a phonocardiograph is more difficult than calibration of an electrocardiograph and the clinical value of the procedure is less obvious. The loudness of the heart sounds and murmurs at the front of the chest does not depend exclusively on cardiac activity but is influenced also by the position of the heart in the chest, by the nature and thickness of the chest wall, and by extraneous factors related to disease of the lungs, pleura, or pericardium. For a valid comparison of phonocardiographic with auscultatory findings it is, however, essential to have some indication on the phonocardiogram of the absolute loudness of heart sounds and murmurs at the chest wall. In logarithmic phonocardiography (Rappaport and Sprague, 1942) the low-frequency vibrations are selectively attenuated as they are in auscultation by the human auditory mechanism: the amplitude of the deflections on a logarithmic phonocardiogram are therefore directly related to the loudness of the audible sounds. In linear phonocardiography, without this low-frequency attenuation, the audio-frequency heart sound vibrations are masked on the record by lower-frequency vibrations of much greater amplitude: the deflections on a linear phonocardiogram thus correspond to palpable pulsation or thrills rather than to auditory phenomena. In stethoscopic phonocardiography an attempt is made to include the distortion of sound normally introduced by a stethoscope, without the distortion due in auscultation to the human ear. Because the stethoscopic phonocardiogram is an arbitrary recording, not corresponding to any sensory phenomenon, we do not favour it.