The absolute sensitivity characteristics of lung-sound transducers coupled to chest wall

The authors studied the absolute sensitivity characteristics of an air-coupled microphone and an accelerometer when they were coupled to the chest wall. The frequency responses of these transducers were theoretically predicted based on an equivalent circuit model and verified by model experiment. The experimental results were well agreed with the theoretical predictions. Due to the capacitive impedance of the air-chamber the pressure response of air-coupled microphone shows low-pass characteristics. It is important to set the air-chamber dimension as small as possible, however, practically it is difficult to have the act-off frequency above 1 kHz. An accelerometer acts, on the chest wall, as a pressure sensor rather than an acceleration sensor, because the mechanical impedance of the accelerometer is much higher than that of the chest wall. The authors can measure stop-surface sound pressure on the chest wall either by air-coupled microphone or by accelerometer. Accordingly they can quantitatively compare the data measured by air-coupled microphones with the data measured by accelerometers.