Damping of Sound on the Chest Surface

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Abstract
The damping of sound during its transmission over the chest wall was measured in humans in order to establish whether the heart sounds spread in two-dimensions over the chest wall from their auscultatory areas, or in three-dimensions directly from their origins deep in the body. The damping of frequencies from 50 to 400 cycles/sec in soft tissues and ribs can be predicted by an equation which consists of a geometrical term and a term due to viscous damping. The term which represents the viscous damping in the soft tissues and ribs of the chest wall is proportional to the square root of the frequency. The value of the geometrical term suggested that the intensity of the sounds was inversely proportional to the distance travelled, rather than inversely proportional to the square of the distance travelled. The former relationship would apply if the sound travels in a two-dimensional plane of material rather than in a three-dimensional volume. This is consistent with our earlier finding that the heart sounds travel from the heart to their auscultatory areas on the chest and from there over the chest surface. For frequencies under 100 cycles/sec, the chest wall appears to be homogenous. For frequencies above 100 cycles/sec, the damping is much less for transmission over the sternum than for transmission elsewhere on the precordium.

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