Crackle Sounds Analysis By Empirical Mode Decomposition

After the first qualitative description of normal and abnormal thoracic sounds made by Laennec in 1819 [1], formal quantitative analysis was suggested and pursued by Forgacs in 1969 [2]. Such analysis was mainly based on time and frequency domains, where the quantitative description was limited to intensity and spectral content of the thoracic sound signals. In this sense, further studies resulted in a general classification scheme of respiratory sounds, as proposed by the American Thoracic Society (ATS) in 1980 [3]. Respiratory sounds were classified as normal, abnormal, and adventitious sounds, where adventitious sounds were divided as continuous and discontinuous. Continuous adventitious sounds were characterized by a fundamental frequency, which is higher than 400 Hz for wheezes and lower than 200 Hz for rhonchi; they both last 250 ms or more [3]. Discontinuous adventitious sounds, including crackles and squawks, showed a wide frequency band between 50 and 2,000 Hz and were of shorter duration [3].