Blood

[j2dye]

Ultrasound reflected from an oncoming bloodstream that is moving at 31 cm/s is mixed with the original frequency of 1.3 MHz to produce beats. What is the beat frequency? (Velocity of sound in blood = 1540 m/s.)

I know that there are two doppler shifts; one where the source is fixed and the other where the observer is fixed.

So i'm supposed to use the doppler shift equation to solve for this:

f1=f((V + Vo) / (V - Vs))

but I dont know what numbers to plug into which variables.

After I find the frequency of the doppler shifted echo all I have to do is subtract it from the original frequency of 1.3 MHz, right?

[Andrew Mason]

So i'm supposed to use the doppler shift equation to solve for this:

f1=f((V + Vo) / (V - Vs))

but I dont know what numbers to plug into which variables.

You have used the doppler expression where the source and observer are both moving relative to the medium. You also have to keep in mind that there is a reflection so there are two doppler shifts.

Use the moving observer form:
f_{blood} = f_{source}(\frac{v_{sound}+ v_{blood}}{v_{sound}})

to find the apparent frequency that the blood receives. The blood then acts as a moving source and reflects this back to the observer so used the moving source form:

f_{observer} = f_{blood}(\frac{v_{sound}}{v_{sound} - v_{blood}})




After I find the frequency of the doppler shifted echo all I have to do is subtract it from the original frequency of 1.3 MHz, right?You are correct that the beat frequency is the difference between the original and the reflected sound.

AM