Disclosed is a method of control for a respiratory pressure treatment device. The method comprises generating a flow of breathable gas at a patient interface. The flow of breathable gas comprising inspiratory portions and expiratory portions wherein the breathable gas during an expiratory portion is at a pressure generally lower than that during an inspiratory portion and wherein each of the expiratory portions comprises a pressure rise. The method also comprises controlling the pressure rise of the expiratory portions with a polynomial function of at least a degree of two. The polynomial function for controlling the pressure rise has a range between a start of expiration and an end of expiration.
