Criteria for the onset of vascular murmurs.

A general "sound boundary" curve was developed which establishes the combinations of minimum flow Reynolds numbers and percent stenosis required for the onset of vascular murmurs. The curve was derived by a combination of engineering analysis and animal experiments in which thin circular orifice plates were implanted in the descending aortas of anesthetized dogs. The results indicate that the sounds are produced by jet instability and the associated "free turbulence" in the flow. Such turbulence may occur at Reynolds numbers far below that for transition in a straight pipe, depending upon the degree of stenosis. However, below this critical Reynolds number for any given degree of stenosis, a murmur would not be detected even with perfect hearing or an ideal microphone, since the flow remains steady and the jet (if any) is laminar and silent. In pulsating flow, sounds can be produced only when this critical Reynolds number is exceeded.