# Homework Help: Doppler effect and Ultrasound waves

1. Dec 12, 2007

### mathcrzy

1. The problem statement, all variables and given/known data

Ultrasound waves are used to measure blood-flow speeds. Suppose a device emits sound at 450 kHz, and the speed of sound in human tissue is taken to be 1540 m/s. What is the expected beat frequency if blood is flowing normally in large leg arteries at 3.0 cm/s directly away from the sound source?
_____Hz

2. Relevant equations

F2 = F1 / ( 1 + ( Vs / V ) )

3. The attempt at a solution

F2 = 450000 / (1 + ( 1540 / 343 ) )
F2 = 81970.26 Hz

2. Dec 13, 2007

### rl.bhat

F2 = F1 / ( 1 + ( Vs / V ) )
Your formula is wrongIt should be F2 = F1/[1 + v/vs] and what is 343?

3. Dec 13, 2007

### Shooting Star

I am a bit confused. The OP has used the correct formula for the Doppler Effect for moving source. However, the formula given by you is the correct formula for the beat frequency.

To help the OP, I'll just give a short deduction.

f1 = f/(1 + vs/v) = fv/(v+vs), where v = speed of sound = 1540 m/s, f = 450 kHz, vs = 0.03 m/s.

Frequency of beats = f –f1 = f(1 – v/(v+vs) = f*vs/(v+vs) ~ f*vs/v, since v>>vs.

Now plug in the values.