ABSTRACT
Many endogenous sources of acoustic energy have diagnostic significance. These include:
• Heart sounds (valves, blood turbulence, pericardial friction rub) • Lungs (rales, rhonchi, squeaks, crepitations, gurgling, pleural friction rub,
silence) • Arteries (
bruit
aka turbulence sounds caused by a
stenosis
) • Stomach and intestines (sounds of digestion) • Joints (arthritic friction rub, tendon snap, etc.) • Inner ear (
otoacoustic emissions
)
Most of these sounds have acoustic spectral energy in the lowest range of human hearing, as well as at audible low frequencies. Some have origin in the elastic vibrations of dense tissues, or vibrations induced in arteries by blood turbulence, or vibrations induced by blood passing through small apertures. All such sound vibrations propagate through the body’s tissues with losses, reflections and refractions to the skin, in which perpendicular vibrations are introduced. Stethoscopes and microphones respond to the sound waves that the vibrating skin radiates into the air. Surface vibrations can be measured directly by accelerometers on the skin surface, or laser Doppler sensors in which the Doppler shift in a laser beam reflected from the skin is detected. The Doppler frequency shift is proportional to skin velocity, so this signal must be integrated to obtain a signal proportional to vibration amplitude.
