Abstract: The present invention provides a method of measuring respiratory flow rate. The method includes a user exhaling through a flow dependent sound producing device which comprises a mouthpiece and a vortex chamber. The vortex chamber has an axis and an outlet such that exhaled air flows through the mouthpiece into the vortex chamber causing the exhaled air to form a vortex around the axis and then pass out of the chamber via the outlet in an axial direction, thereby producing a sound. The method further includes detecting the sound using a mobile electronic device and analyzing said sound to determine a frequency of said sound and using said frequency to determine the respiratory flow rate.