Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Determing frequency shift of an ultrasound

  1. Mar 19, 2010 #1
    1. The problem statement, all variables and given/known data
    A Doppler blood flow unit emits ultrasound at 5.0 MHz.

    What is the frequency shift of the ultrasound reflected from blood moving in an artery at a speed of 0.20 m/s?

    Express your answer using two significant figures.

    2. Relevant equations
    f' = f0(v/v-Vs)
    frequency shift = f' - f0

    f0 = 5*10^6 Hz
    v = 1500 m/s (assumed)
    Vs = 0.20 m/s

    3. The attempt at a solution
    Plugging in the numbers we get the following equation for frequency shift:


    Which comes out to approx 666 Hz. or 6.7 *10^2 Hz using 2 significant figures. However, this problem comes from an online hw set and according to that 6.7 *10^2 is not the correct solution.

    Does anyone have an idea of what I am doing wrong? Thanks in advance for any and all help!
    Last edited: Mar 19, 2010
  2. jcsd
  3. Mar 19, 2010 #2


    User Avatar
    Homework Helper

    Hi grigri9, welcome to PF.
    We have here a case of the "listener" (the blood corpuscles) moving away from a stationary source, and thus the frequency, f corpuscle that will be reflected by the corpuscles will be
    f corpuscle = f(transmitted)[(Vs - Vb)/Vs

    For the reflected waves, reflection, the corpuscles are the source and the "listener" is the receiver. The frequency at the receiver will be
    f(receiver) = f(corpuscles)*Vs/(Vs + Vb)

    from which we get
    f(receiver) = f(transmitted)*(Vs-Vb)/(Vs+Vb)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook