Effect of rib-cage structure on acoustic chest impedance

When a stethoscope is placed on the surface of the chest, the coupler picks up sound from heart and lungs transmitted through the tissues of the ribcage and from the surface of the skin. If the acoustic impedance of the chest surface is known, it is possible to optimize the coupler for picking up even weak sounds originating from e.g. the heart. The acoustic impedance is influenced by the structure of the ribcage; hence the acoustic impedance will change depending on if the coupler has been placed on a top of a rib or between the ribs (the intercostal).

The impedance of the chest is measured in the frequency range from 40 Hz to 5 kHz using an acoustic impedance tube made specifically for the purpose. The measurements are carried out in a grid pattern on the surface of the chest. The grid is aligned according to the ribs; hence the measurements are either on a top of the ribs or between the ribs. The measurements reveal the structure of the ribcage from an acoustic point of view in addition to describing the variation of the impedance depending on the position of the coupler. The measurements are carried out on a small number of subjects.

The assessment of the ribcage structure based on impedance measurements is a part of a larger study which aims at optimizing an acoustic coupler for picking up weak murmur sounds from the coronary arteries of the heart.