# Homework Help: Finding the beat frequency

1. Nov 13, 2012

### alwayslearnin

Doppler ultrasound is used to measure the speed of blood flow. If the speed of the red blood cells is v, the speed of sound in blood is u, the ultrasound source emits waves of frequency f, and we assume that the blood cells are moving directly toward the ultrasound source, then the frequency fr of reflected waves detected by the apparatus is given by the formula below.
fr =
f
x
(1 + v/u) / (1 − v/u)

The reflected sound interferes with the emitted sound, producing beats. If the speed of red blood cells is 0.12 m/s, the ultrasound frequency used is 4.3 MHz, and the speed of sound in blood is 1570 m/s, what is the beat frequency?

____ Hz

I try to put it in the equation I got it wrong and my previous answers were wrong too:

2.45 4.34 and 329
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 14, 2012

### Simon Bridge

Putting the numbers into the equation you wrote above will tell you the reflected frequency. So you have two waves with frequency f and fr interfering with each other ... you have an equation for that situation in your course notes (or just google for "beat frequency"). What is it?