Abstract: A method of determining respiratory states, comprising measuring an unfiltered sound waveform emanating from an airflow through a mammalian trachea and applying time-averages to each of a plurality of respiratory phases of the unfiltered sound waveform to create normalized and unnormalized autocorrelation function (ACF) curves. Determining from the normalized and unnormalized ACF curves at least one feature from a first group of features consisting of (a) a first minimum value of the normalized ACF curve; (b) a second maximum value of the normalized ACF curve; (c) a value of the unnormalized ACF curve at zero lag; (d) variance after the normalized ACF curve second maximum value; (e) slope after the normalized ACF curve second maximum value; and (f) sum of the squares of the difference between successive normalized ACF curve maximum and minimum values. Applying a classifier to the at least one feature from the group of features.