What Is Ultrasonic Doppler?

Paul Reed

Ultrasonic doppler is a technique used to measure flow of liquids by reflecting sound waves. Astronomer Christian Doppler first suggested the Doppler effect in the 1840s, when he found that certain stars had different colors than expected. He proposed that the color differences were due to the fact that stars were moving toward or away from the observer, changing their visible colors. Although Doppler was studying starlight, scientists believed the effect also occurred with sound.

Ultrasonic Doppler, which uses high frequency sound waves to monitor the movement of fluids, is often employed in medical applications.
Ultrasonic Doppler, which uses high frequency sound waves to monitor the movement of fluids, is often employed in medical applications.

A later experiment using musicians on a moving train and observers standing on a train platform confirmed Doppler's theory using sound. As an object moves toward a stationary or non-moving observer, the sound waves are compressed slightly, resulting in a higher pitch than the actual sound. After the object reaches the observer and moves away, the apparent sound becomes lower in pitch, because the sound waves are stretched slightly.

Longer sound waves are lower in pitch, and the result is a sound that appears lower than the actual. The experiment is easy to reproduce, by listening to a vehicle coming toward an observer, then passing and moving away. If the vehicle sounds its horn, the approaching vehicle horn has a higher pitch, which will change to a lower pitch as the vehicle passes and moves away.

This Doppler effect can be used in an ultrasonic flow-measuring device. Ultrasonic sounds are very high frequencies above the range of human hearing. They can pass through many liquids and human tissues before being absorbed, which make them useful in medical diagnostics and industrial applications. An ultrasonic doppler velocity measurement takes advantage of the frequency shift when sound waves are reflected off moving liquids.

The best results are obtained when an ultrasonic doppler unit measures a liquid containing bubbles or solid particles. Ultrasonic sounds do not reflect well from clear or very thick liquids, because some reflection of sound back to a receiver is needed to measure the speed of the liquid stream. The device sends out short pulses of high-frequency sound, and compares the returning signal with the outgoing one. Any frequency difference can be calculated to obtain the speed of the liquid.

Early applications of ultrasonic doppler measuring were in the medical field, where sound measurements were used to check blood flow in arteries and veins without having to perform surgery. Applications were also developed to check a fetus during pregnancy by looking at the beating heart and blood vessels. Devices developed in the late-20th century could show visible movement of heart valves to diagnose defects and blockages.

In an industrial application, ultrasonic doppler measurement works best if the sound is sent into the liquid at an angle other than 90°. The particles or bubbles in the liquid need to be moving toward or away from the device for it to accurately measure the speed. A full pipe is needed for a proper measurement, because a partially filled system will not return a usable sound signal to measure velocity.

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