1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sound problem: Beat frequency

  1. Apr 30, 2004 #1
    I have been trying this problem for a couple of days:

    Ultrasound reflected from an oncoming bloodstream that is moving at 30 cm/s is mixed with the original frequency of 2.5 MHz to produce beats. What is the beat frequency? (Velocity of sound in blood = 1540 m/s.)

    I tried this as a doppler effect problem where the frequency is
    (original frequency)*{v/(v-velocity of source)}. I then subtracted that from the original frequency to find the beat frequency, and came up with 487.1 Hz, but that was not the right answer. For v, I used 1540 m/s, and velocity of source was 0.3 m/s. Is there something I am missing in my approach to the problem?
     
  2. jcsd
  3. Apr 30, 2004 #2
    I think you have to solve this with two shifts.

    Consider a second observer riding along with the oncoming bloodstream. This observer is moving toward the original source & therefore perceives an increase in frequency. So the reflected wave starts out with a shifted frequency, which is then shifted again because it's source (the oncoming bloodstream) is moving toward the original observer.
     
  4. Apr 30, 2004 #3
    Thank you for the help, gnome. You were right about the second shift, and I finally got the answer, which was 974.2 Hz.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Sound problem: Beat frequency
  1. Beat Frequency problem (Replies: 2)

  2. Beat Frequency Problem (Replies: 6)

  3. Beat frequency problem (Replies: 7)

Loading...