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So I was doing a question about "Doppler Ultrasound"

It states that a Doppler ultrasound uses soundwaves at a frequency much higher than what humans can hear. They can me used to measure the speed of fluid in underground pipes by reflecting high frequency sound waves (from 2-18 MHz) off moving liquid and analyzing the reflected waves.

The question is

One of the limitations of the Doppler Ultrasound is that the beam needs to be 'looking' as directly down the pipe as possible. Explain why:

I was thinking this is due to the Doppler machine which emits waves, which reflect of a liquid down the pipe. If the beam is misaligned, then the emitted waves from the machine could hit the walls of the pipe rather than a straight reflection from the machine to the liquid and back but I am unsure...

I was also thinking the change in frequency observed is dependent on the relative motion between he source and the observer. Therefore, only the component of the velocity directly towards or away from the machine will cause a Doppler shift hence it beam needs to be looking down to observe the doppler effect.

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