A method of determining a fitness level of user with an acoustic measurement device configured to measure sound associated with airflow through a mammalian trachea. The acoustic measurement device is in communication with a controller having processing circuitry. The method includes correlating the measured sound into a measurement of the user's respiratory rate and tidal volume; calculating a second respiratory rate value using the measured tidal volume; calculating a breathing efficiency (BE) ratio based on a comparison of the user's measured respiratory rate and the calculated second respiratory rate value; correlating the calculated BE ratio to a predetermined threshold; and assigning a classification to the user based on the calculated BE ratio. The classification is indicative of the user's respiratory function performance.