How Fetal Dopplers Work
Fetal doppler technology is based on the Doppler shift principle. This theory was first discovered by Christian Doppler an Austrian physicist in 1842. Doppler discovered that sound waves from a moving source would be compressed or expanded, or that the frequency would change.
Dopplers work on the principle of listening to reflections of small, high frequency sound waves (ultrasound). These ultrasound waves are generated by microscopic vibrations of pizeoelectric crystals. When the waves are reflected from moving objects, such as a foetal 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.