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An objective and accurate measurement and characterization of breath sounds was carried out by a fast-Fourier-transform frequency-domain analysis. Normal vesicular breath sounds, picked up over the chest wall of 10 healthy subjects showed a characteristic pattern: the power of the signal decreased exponentially as frequency increased. Since the log amplitude vs. log frequency relationships were linear, they could be characterized by the values of the slope and the maximal frequency. The average slope of the power spectrum curves was found to be (in dB/oct +/- SD) 13.0 +/- 1.4 over the base of the right lung, 12.6 +/- 2.4 over the base of the left lung, 9.8 +/- 1.4 over the interscapular region, and 14.4 +/- 4.3 over the right anterior chest. The maximal frequencies of inspiratory and expiratory breath sounds, picked up over the base of the right lung, were (in Hz +/- SD) 446 +/- 143 and 286 +/- 53 (P less than 0.01), over the base of the left lung 475 +/- 115 and 284 +/- 47 (P less than 0.01), over the interscapular region 434 +/- 130 and 338 +/- 77 (P less than 0.05), and over the right anterior chest 604 +/- 302 and 406 +/- 205 (P less than 0.05). Breath sounds picked up over the trachea were characterized by power spectra typical to a broad spectrum sound with a sharp decrease of power at a cut-off frequency that varied between 850 and 1,600 Hz among the 10 healthy subjects studied.

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We measured the time and frequency domain characteristics of breath sounds in seven asthmatic and three nonasthmatic wheezing patients. The power spectra of the wheezes were evaluated for frequency, amplitude, and timing of peaks of power and for the presence of an exponential decay of power with increasing frequency. Such decay is typical of normal vesicular breath sounds. Two patients who had the most severe asthma had no exponential decay pattern in their spectra. Other asthmatic patients had exponential patterns in some of their analyzed sound segments, with a range of slopes of the log power vs. log frequency curves from 5.7 to 17.3 dB/oct (normal range, 9.8-15.7 dB/oct). The nonasthmatic wheezing patients had normal exponential patterns in most of their analyzed sound segments. All patients had sharp peaks of power in many of the spectra of their expiratory and inspiratory lung sounds. The frequency range of the spectral peaks was 80-1,600 Hz, with some presenting constant frequency peaks throughout numerous inspiratory or expiratory sound segments recorded from one or more pickup locations. We compared the spectral shape, mode of appearance, and frequency range of wheezes with specific predictions of five theories of wheeze production: 1) turbulence-induced wall resonator, 2) turbulence-induced Helmholtz resonator, 3) acoustically stimulated vortex sound (whistle), 4) vortex-induced wall resonator, and 5) fluid dynamic flutter. We conclude that the predictions by 4 and 5 match the experimental observations better than the previously suggested mechanisms. Alterations in the exponential pattern are discussed in view of the mechanisms proposed as underlying the generation and transmission of normal lung sounds. The observed changes may reflect modified sound production in the airways or alterations in their attenuation when transmitted to the chest wall through the hyperinflated lung.