An active respiratory therapeutic device and method for actively loosening and breaking up mucus plugs and phlegm in a user's trachea and bronchial passages and lungs while a user is breathing normally through the device comprises a housing containing a base portion and a cap portion forming at least a portion of a sonic coupling chamber adapted to be held in a patient's mouth during normal breathing activity. The cap portion also includes breathing passages out of the chamber so that the user can breath normally through device during operation. The base portion contains an acoustic generator behind a sanitary flexible diaphragm separating the portions. The acoustic generator produces an audio waveform which may be controlled by the user or automatically controlled by a feedback signal to optimize a sensed parameter indicative of the user's breathing efficiency such as the sound of the user's exhaled breath, blood oxygen levels, or ultrasonic signature changes of mucus plugs. This feedback signal senses a patient parameter and automatically adjust the operating frequency spectrum and/or pulse rate of the device to optimally affect the monitored parameter such as the user's breathing efficiency. The method of the invention basically comprises determining a resonant frequency of a mucus plug, generating an acoustic waveform including the resonant frequency and directing the waveform into a user's airways to loosen the plug while the user breathes through his or her airways.