What is the Doppler effect?

The Doppler effect is a change in frequency of received sound reflected from a moving object compared with the transmitted sound. If an object moves away from the ultrasound transducer, the wavelength increases (sound waves spread apart) and frequency decreases because the speed of the sound (v) is constant (v =f * λ).

If the object moves toward the transducer, wavelength decreases and frequency increases. For example, recall how the pitch of a whistle of an oncoming train is higher than that of a departing train. The Doppler effect is used in vascular studies. This principle is known as Doppler shift.