This instrument work on the principle of Doppler effect.
Very low intensity ultrasonic frequency 2MHZ is beamed in to body
from a probe. It is reflected back from organs & blood particles.
Frequency of reflected signal is changed slightly due to movement
of fetal 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.