We measured the acoustic transmission of the respiratory system in five healthy subjects and low-pass filtering action of the normal thorax was evaluated. Sinusoidal sound waves of 100Hz to 1, 000Hz were introduced through the mouth and transmitted sounds were picked up with microphones at four locations; at the mouth, over the trachea at the anterior neck, and at the apical and basal position of the chest wall. The signal of ventilatory movement was simultaneously recorded.
The sound intensity over the trachea and the chest was significantly influenced by the opening of the glottis accompanying respiration.
The sound intensity in the chest, which was normalised by the intensity over the trachea, showed rapid attenuation with increasing frequency. The cut-off frequency was about 300Hz at the apex and 150Hz at the base, and the attenuation rate was 12dB/octave in both the apex and the base. Below the cut-off frequency, the sounds were evenly transmitted and were attenuated at about 15dB in both the apex and the base. There were no significant differences in acoustic transmission according to lung volumes.
This method may be applied as a noninvasive examination in pulmonary diseases.
