Non-contacting sonar system for respiratory air flow detection

A new system for non-contacting respiratory air flow detection is presented. Airborne ultrasound is being used to detect variations in the velocity of sound caused by air flow. Two opposing ultrasonic waves are reflected by the face of the subject to be investigated, and variations in the differential transit times, or phase shifts, are recorded. Due to the non-reciprocity of flow, it is possible to obtain specificity against other influences, such as movements of the subject. Experimentally, an operating frequency in the range 40-200 kHz has been found feasible. In the prototype system, continuous waves emitted by standard piezoelectric transducers operating at 40 kHz and of slightly different frequencies are used for identification of the opposing waves. A linear flow dependence has been verified, with adequate resolution. The function has also been verified on human subjects. A wide range of high-priority clinical applications can be foreseen for the system.