Figure 2

Time domain sound characteristics and spectrograms of breathing sounds. (A) The acoustic energy was relatively small and concentrated mainly below 2000 Hz in normal breathing sounds. (B) A detailed image of normal breathing sounds under 3 kHz. (C) The acoustic energy was scattered over a large area, from 500 to 12,000 Hz in abnormal breathing sounds. In addition to the preceding reason, characteristic multiple vertical lines created by the movable obstacle repeatedly blocking the transmission of sounds were shown in vibrant breathing sounds. (D) In sharp breathing sounds, characteristic multiple horizontal lines were shown because fixed obstacles, which induced a wide range of high-frequency breathing sounds, narrowed the airway continuously.