Fetal Doppler work on the principle of listening to reflections of small, high frequency sound waves (ultrasound). These ultrasound waves are generated by microscopic vibrations of piezoelectric crystals. When the waves are reflected from moving objects, such as a fetal heart the frequency changes slightly. It is this change that is analysed by the electronics of the Doppler and converted into a sound that you can hear or a digital display of the heart rate. In a word, it is a device of fetal heart-beat measuring.