Phonocardiography, the graphic recording of heart sounds and murmurs, has become an indispensable tool in many clinics and teaching centers. For proper understanding of this technique, a few fundamental concepts of sound and its recording first must be reviewed.1
Cardiovascular sound, like all other sound, is a vibratory phenomenon in which a suitable medium (such as air or body tissues) is caused to vibrate back and forth. The number of complete vibrations per unit time is the so-called frequency and is usually expressed in cycles per second (cps) (Fig 1). These vibrations are propagated through the various media, and, if allowed to reach the tympanic membrane and set this structure into motion, the vibrations are appreciated by the brain as sound. Only those vibrations having frequencies of 20 to 20,000 cps are so appreciated by human audition, and frequencies below or above this 20 to 20,000 range are termed
Tavel ME. Clinical Phonocardiography: I. General Principles. JAMA. 1967;202(4):309–310. doi:10.1001/jama.1967.03130170109020
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