The term “Doppler effect” honors Austrian physicist Christian Doppler. Doppler discovered that the distance between waves, such as sound or light, changes as an observer of the waves and the source of the waves move relative to each other.
Dopplers operate on the principle of listening to reflections of small, high frequency sound waves. These sound waves, called ultrasound, are generated by microscopic vibrations of a ceramic crystal. When ultrasound waves reflect off moving objects such as the fetal heart or blood flow, the waves are altered slightly in frequency. The Doppler then picks up these signals, processing and amplifying them so they are audible.
When choosing Doppler probes, lower frequency probes will penetrate the body deeper and will have a wider beam. Likewise, the higher frequency probes will not penetrate as deeply and will have a narrower beam. So, for listening to deeper objects such as a fetal heartbeat, a lower frequency 2 or 3 MHz probe is needed. For listening to superficial objects such as blood vessels, a higher frequency 5 or 8 MHz probe is appropriate.
Cephalon carries a lot of dopplers and their accessories, some can be found here: