Measurement of respiratory acoustic signals. Effect of microphone air cavity depth.

The use of electret microphones to measure lung sounds is widespread because of their small size, high fidelity, and low cost. Typically, an air cavity is placed between the skin surface and the microphone to convert the chest wall vibrations into a measurable sound pressure. The importance of air cavity depth on this transduction process was investigated in this study. An acoustic model of chest wall--air cavity--microphone interface was developed and the predicted effects of depth were compared with measurements performed using an artificial chest wall and lung sounds from a healthy subject. Model predictions are in general agreement with both in vitro and in situ measurements and indicate that the overall high-frequency response of the transduction diminishes with increasing cavity depth. This finding suggests that smaller cavity depths are more appropriate for detection of lung sounds over a wide band width and stresses the importance of coupler size on microphone measurements.