Doppler Effect: Principles, Formulas, and Medical Applications

Ultrasound Physics Exam 1 Review

Who first described the Doppler Effect?

Christian Andreas Doppler in 1842.

What phenomenon does the Doppler Effect describe?

The change in frequency of sound, light, or other waves caused by the motion of the source or the observer.

What happens to the frequency of sound as the source approaches an observer?

The frequency increases (waves are compressed).

What happens to the frequency of sound as the source moves away from an observer?

The frequency decreases (waves are stretched out).

What was the purpose of Hendrik Diedriek Buys Ballot's experiment?

To test and confirm Doppler's theory using musicians and a moving train.

What is the Doppler shift?

The difference between the reflected frequency and the transmitted frequency.

What is the relationship between the speed of the reflector and the Doppler shift?

The greater the speed of the relative motion, the greater the Doppler shift.

What is the formula for the Doppler frequency shift?

D f = 2 f0 v cos θ / c.

What does the variable 'c' represent in the Doppler equation?

The velocity of sound in the medium.

How does the Doppler angle affect frequency shift?

The frequency shift is greatest at 0° and decreases as the angle approaches 90°.

What is the significance of the Doppler Effect in clinical applications?

It is used to assess blood flow, including presence, direction, speed, and character of flow.

What is the audible range for Doppler shifts?

20 Hz to 20 kHz.

What is the effect of using a lower transmitted frequency in Doppler applications?

Lower frequencies can detect higher velocities.

What is the Doppler frequency if the transmitted frequency is 5 MHz and the reflector is moving at 1 m/s at a 0° angle?

The Doppler frequency shift is 6.5 kHz.

What is the impact of estimating the Doppler angle incorrectly?

It introduces large errors in estimating reflector velocities, especially as the angle approaches 90°.

How is the velocity of a reflector calculated using Doppler frequency?

v = Df c / (2 f0 cos θ).

What is the result of a Doppler calculation with a 2000 Hz signal using a 5 MHz transducer at a 60° angle?

The reflector is moving at 0.6 m/s (60 cm/s).

What does the term 'Rayleigh scatterers' refer to in Doppler applications?

Blood cells acting as small scatterers of ultrasound waves.

What did Doppler predict about his theory in 1846?

He believed it would help astronomers probe the happenings of the universe.

What is the relationship between reflector velocity and Doppler frequency?

Doppler frequency is directly proportional to reflector velocity.

What is the effect of increasing the ultrasound frequency on Doppler frequency?

Increasing ultrasound frequency increases the Doppler frequency.

What is the Doppler angle when the sound beam is perpendicular to the flow?

The Doppler frequency shift is theoretically zero.

What is the relationship between the angle θ and the accuracy of velocity computation?

Accurate velocity computation requires the input of the Doppler angle θ.