A Dual-mode Noise-immune Stethoscope for Use in Noisy Vehicles

In combat casualty and civilian environments, an unmet need exists for a stethoscope that can hear heart and especially breathing sounds while inside helicopters, fixed-wing aircraft, or ambulances where noise levels preclude auscultation with standard stethoscopes. Without this capability, patients can suffer from unidentified collapsed lungs or loss of intubation integrity with the threat of loss of life. A conventional acoustic stethoscope will not function in background noise levels beyond 80-85 dB. Electronic stethoscopes, in combination with mechanical impedance-matched transducer designs, can extend this range to about 90 dB. This is, unfortunately, not enough for helicopter noise levels that can reach 110 dB. The use of an ultrasound transmitter and receiver, however, provides an essentially noise-free auscultation channel since transportation vehicles do not produce acoustic energy at ultrasound carrier frequencies of 2-3 MHz. Clean and noise-free heart and breath sounds have been obtained in broadband noise fields of intensities as high as 120 dB. A hybrid stethoscope has been developed that allows auscultation by ultrasound Doppler as well as by electro-mechanical means. Pros and cons of making Doppler sounds subjectively similar to conventional sounds by nonlinear signal processing will be discussed, as well as potentially functional and meaningful aspects of Doppler signals that are not found in conventional stethoscope sounds.