How can the Dobbler Effect be used to monitor fetal heartbeats?

[vinny380]

Dobbler Effect Question !

Question:
The Dobbler Effect, using ultrasonic waves of frequency of 2.25 *10^6 Hz is used to monitor the heartbeat of a fetus. A beat frequency of 500Hz is observed. Assuming the speed of sound is 1.54 *10^3 m/s, calculate the maximum velocity of the surface of the beating heart.

Equations:
Only one relevant equation:
f'=f((Vsound (+ or -) Vobserver)/(Vsound (- or t) Vsource))

Attempt
I know that the answer is .171m/s. I also know that in this questions, you have to use the above formula two times (double dobbler effect) to get the answer. If anyone can help me, I would really appreciate. I have pretty much tried plugging every number into the formula above, but everytime i do it, i never get .171m/s as my final answer! Maybe I am confusing whether the heart is the source of the observer. Please help! Thanks

 

[dextercioby]

I think you might mean "Doppler". Yes, i know I'm picky, but you wouldn't like your name to misspelled either.

Daniel.

 

[OlderDan]

Question:
The Dobbler Effect, using ultrasonic waves of frequency of 2.25 *10^6 Hz is used to monitor the heartbeat of a fetus. A beat frequency of 500Hz is observed. Assuming the speed of sound is 1.54 *10^3 m/s, calculate the maximum velocity of the surface of the beating heart.

Equations:
Only one relevant equation:
f'=f((Vsound (+ or -) Vobserver)/(Vsound (- or t) Vsource))

Attempt
I know that the answer is .171m/s. I also know that in this questions, you have to use the above formula two times (double dobbler effect) to get the answer. If anyone can help me, I would really appreciate. I have pretty much tried plugging every number into the formula above, but everytime i do it, i never get .171m/s as my final answer! Maybe I am confusing whether the heart is the source of the observer. Please help! Thanks

Beat frequency is the frequency difference between the transmitted wave and the reflected wave, yes? The "doubling" effect is due to the fact that the heart is both the observer and the source in this problem. It is a moving observer as it encounters the wave sent out by the original source, and a moving source as it reflects that wave back to the original source.

The best way to understand the combined effect is to draw a diagram representing the wave fronts from the original source and the reflected wave fronts, taking into consideration the motion of the reflector. See how the reflected wavelength is shortened by an approaching reflector.