Respiratory therapy apparatus includes device (100) with a rocker arm (12) supporting a valve (11) that opens and closes an expiration opening (10) so that an oscillating resistance to flow is produced accompanied by an alternating sound at the frequency of oscillation. The apparatus also includes a sensor (20) with a microphone (21) that detects sound from the device (100) transmitted through air. The sensor (20) computes the frequency of the detected sound and represents this on a display (23).
