MEASUREMENT OF RESPIRATORY ACOUSTIC-SIGNALS - EFFECT OF MICROPHONE AIR CAVITY DEPTH

The use of electret microphones to measure lung sounds is widespread b ecause of their small size, high fidelity, and low cost. Typically, an air cavity is placed between the skin surf ace and the microphone to convert the chest wail vibrations into a measurable sound pressure. Th e importance of air cavity depth on this transduction process was inve stigated in this study. An acoustic model of chest wall-air cavity-mic rophone interface was developed and the predicted effects of depth wer e compared with measurements performed using an artificial chest wall and lung sounds from a healthy subject. Model predictions are in gener al agreement with both in vitro and in situ measurements and indicate that the overall high-frequency response of the transduction diminishe s with increasing cavity depth. This finding suggests that smaller cav ity depths are more appropriate for detection of lung sounds over a wi de band width and stresses the importance of coupler size on microphon e measurements.