Measurement of respiratory acoustic signals. Effect of microphone air cavity width, shape, and venting

We have previously investigated the effects of microphone 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.

We used a single electret microphone with a variety of plastic couplers. The couplers were identical except for the diameter and shape of the air chamber. We used cylindrical chambers of 5, 10, and 15 mm in diameter at the skin and conical chambers of 8, 10, and 15 mm in diameter. We compared the inspiratory lung sound spectra obtained using each of the couplers. We also examined the tendency of various needle vents to transmit ambient noise into the microphone chamber.

Anechoic chamber.

The shape and diameter had little important effect on the lung sound spectrum below 500 Hz. From approximately 500 to 1,500 Hz, the 5-mm diameter couplers showed slightly less sensitivity than the 10- and 15-mm diameter couplers. All conical couplers provided approximately 5 to 10 decibel more sensitivity than the cylindrical couplers. All vents allowed some ambient noise to enter the chamber but the amount was trivial using the narrowest, longest vent.

These data suggest that the optimal electret microphone coupler chamber for lung sound acquisition should be conical in shape, between 10 and 15 mm in diameter at the skin, and either not vented or vented with a tube no wider than 23-g or shorter than 20 mm.