Digital Foetal Heart Beat Monitor works on principle of Doppler's effect. A very low intensity ultrasonic frequency of 2 MHZ beamed into the body from a probe and it is reflected back from the organs and blood particles. The frequency of reflected Signal is changed slightly due to movement of foetal heart blood particles and other moving organs. Reflected signal received by the instrument is amplified and processed electronically to get audible signal through built in speaker or external earphone.