Coherence of inspiratory and expiratory breath sounds as a function of inter-microphone distance

The similarity of breath sounds recorded simultaneously at different sites on the chest was studied as a function of frequency and inter-microphone distance. The coherence between two signals was greatest at low frequencies and small distances. In expiration, the coherence decreases faster as a function of frequency yet slower with inter-microphone distance, relative to inspiration. These findings are consistent with the inherently lower signal-to-noise ratio (SNR) of expiratory breath sounds and the hypothesis that expiratory sounds are produced more centrally than inspiratory sounds, Assuming that 50% of the variance of signals from adjacent microphones need be coherent to extract spatial information, it was found that microphone distances of 6.8 to 85 cm at frequencies between 200 to 400 Hz should be maintained