Abstract
We analyzed the absolute sensitivity characteristics of air-coupled microphones and accelerometers in lung-sound measurements. Studies have revealed that the air-coupled microphone shows low-pass frequency responses because of its air-chamber compliance, but how the coupler affects the microphone's response in the pass band has not been examined. In the pass band, since the mechanical impedance of the air chamber is much higher than that of the chest wall, the vibration of the measured area on the chest-wall surface is stopped by the air chamber and the coupler rim in contact with the chest wall. We analyzed the normal stress distribution on the stopped surface and then assessed the effect of the coupler rim on the microphone response by calculating the ratio of the sound pressure exerted on the air chamber to the stop-surface pressure, i. e., the sound pressure averaged over the stopped surface. The stress concentrates on the outer edge of the rim, and its distribution changes with frequency. The measured sound pressure is therefore lower than the stop-surface pressure, and the measured pressure decreases as the frequency decreases. Although the response of the accelerometer greatly depends on the accelerometer's mass and contact area, it is generally difficult to measure unloaded chest-wall surface acceleration. We considered the accelerometer's response to sound pressure and showed it, using the stop-surface pressure as reference. We also showed that the response to sound pressure can be improved easily by increasing the seismic mass of the accelerometer and that the accelerometer may be used as a pressure sensor. The results of model experiments well agreed with theoretical predictions based on the mechanical properties of the medium used in the experiments. We evaluated the absolute sensitivity of the air-coupled microphone and the accelerometer by using stop-surface pressure as a common reference. This allows, regardless of the type of transducer, measured lung sounds to be treated consistently with the unit of sound pressure.
