Equalization of Crackle Sounds to Compensate Thorax Attenuation

Lung sounds recorded on the chest are attenuated by the thorax and thorax-microphone interface. In order to recover waveforms similar to the ones generated within the lungs, this paper equalizes crackle sounds recorded on the chest. From an experiment in which eight subjects (seven men) took part, an estimation of the channel (thorax and thorax-microphone interface) attenuation was obtained to design the equalizer. For that, multiple tones between 100 and 1200 Hz were applied to each subjects' mouth where they were acquired by an electret microphone. These tones were also recorded on the chest using another electret microphone housed into an acoustic coupler. The attenuation of each tone was calculated by the power ratio between the one measured on the chest and that measured at the mouth. After obtaining the average attenuation curve from the volunteers, a discrete-time equalizer was designed and applied to crackles acquired from 30 patients with fibrosis, congestive heart failure, and pneumonia. The maximum frequency and two cycle duration indices measured from these crackles were modified by the equalization. The obtained results show that the equalizer improves the extraction of features from the crackles sounds. Equalization of crackles may contribute to better characterize the different diseases.