MEASUREMENT OF RESPIRATORY ACOUSTIC-SIGNALS - EFFECT OF MICROPHONE AIR CAVITY WIDTH, SHAPE, AND VENTING

Study objective: We have previously investigated the effects of microp hone type and coupler air chamber depth on lung sound characteristics. We now report the results of experiments exploring the effects of air chamber width, shape, and venting on lung sounds. Design: We used a s ingle electret microphone with a variety of plastic couplers. The coup lers were identical except for the diameter and shape of the air chamb er. We used cylindrical chambers of 5, 10, and 15 mm in diameter at th e skin and conical chambers of 8, 10, and 15 mm in diameter. We compar ed the inspiratory lung sound spectra obtained using each of the coupl ers, We also examined the tendency of various needle vents to transmit ambient noise into the microphone chamber. Setting: Anechoic chamber. Measurements and results: The shape and diameter had little important effect on the lung sound spectrum below 500 Hz. From approximately 50 0 to 1,500 Hz, the 5-mm diameter couplers showed slightly less sensiti vity than the 10- and 15-mm diameter couplers. All conical couplers pr ovided approximately 5 to 10 decibel more sensitivity than the cylindr ical couplers. All vents allowed some ambient noise to enter the chamb er but the amount was trivial using the narrowest, longest vent. Concl usions: These data suggest that the optimal electret microphone couple r chamber for lung sound acquisition should be conical in shape, betwe en 10 and 15 mm in diameter at the skin, and either not vented or rent ed with a tube no wider than 23-g or shorter than 20 mm.