Fig. (2) Shannon Entropy of the tracheal sounds acquired using a smartphone. Top: Segment of tracheal sound and corresponding airflow from spirometer (positive lobes are inspirations and negative lobes are expirations); Middle: Volume signal obtained with the spirometer as the integral of the flow; Bottom: Shannon entropy of tracheal sound. Observe that local minima of the Shannon entropy are obtained around the onset of each respiratory phase. Image and figure legend from Reyes BA, Reljin N, Chon KH. Tracheal sounds acquisition using smartphones. Sensors (Basel). 2014;14(8):13830-50. Published 2014 Jul 30. doi:10.3390/s140813830 and used under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).