Abstract: A noise-free active stethoscope was introduced, which can be used not only as conventional passive stethoscope, but also as ultrasound-based active stethoscope for use in noisy environments. It allows a physician to switch on and off between passive and active modes during auscultation of a patient. In active mode, a high-frequency (2.3 MHz) sound signal is generated, transmitted from the stethoscope head into the patient's body, and reflections from moving body tissue are picked up by a receiver also located in the stethoscope's head. Since these reflections have a slightly different frequency than the transmitted signal caused by the Doppler effect, a difference-frequency can easily be computed and made into an audible sound. The experimental results show that for conventional stethoscope, the maximum noise level that still allows successful auscultation is between 80 dB and 85 dB Sound Pressure Level (SPL); for modern electronic stethoscopes the maximum noise level is to about 90N95 dB; but even in a noise level of 120 dB SPL, the ultrasound stethoscope's output remains noise-immune.