Spectral analysis of cough sounds recorded with and without a nose clip

The voluntary cough sounds of healthy volunteers, patients with chronic rhinitis, chronic rhinitis with bronchial asthma and asthma were recorded with and without a nose clip. A gated series of signals of the first cough sound lasting 200 ms was analysed by a system with a 20 ms delay of the first signal. In the case of filtered cough sounds, low-cut digitally, the mean values of averaged spectra of the healthy volunteers showed a first peak around 350 Hz, similar to the expiratory spectra of respiratory sounds. Second and third harmonics were also identified. The mean values of averaged spectra from the patients with airway disease differed significantly in the range of low frequency components. The upper airways work as narrow-band acoustic filters determining the harmonic contents of speech as well as those of cough sounds. To minimize these effects for acoustic analytical purposes, the application of a nose clip is suggested to find the cough harmonics related to those of pulmonary sounds. The voluntary cough sounds contain diagnostic information, but to build up a quantitative, diagnostic, decision-making system, further investigations as well as standardization of recording and analysis are necessary.