1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Calculating the Doppler effect

  1. Jul 29, 2015 #1
    1. The problem statement, all variables and given/known data
    Given the speed of sound in an environment, c = 300 m/s. We have an object moving with a ceratin velocity. If we know that the soundwaves behind the object are lower by 3 octaves than in front of it, then what is the velocity of the object?

    2. Attempt at solving it.
    I arrived at the conclusion that if the wavelength of the soundwaves in front of the object is x, then behind it is eight times of that (eg. x*8), since octaves are on a logaritmic scale. I don't know where to go on from here.
    Last edited: Jul 29, 2015
  2. jcsd
  3. Jul 29, 2015 #2
    Where are your equations relating speed to frequency.

    You know that f1 = 8 f2, where f1 is the frequency in the front and f2 is the frequency behind.

    Plug these into your equations relating speed to frequency, and you should be able to solve for the unknown rest frequency and velocity.
    (Two equations and two unknowns.)
  4. Jul 29, 2015 #3
    I don't have those equations, could you please ellaborate? :)

    Do you mean speed = wavelength * frequency?
    Last edited: Jul 29, 2015
  5. Jul 29, 2015 #4
    You must have Doppler effect equations relating speed and frequency shifts for sources moving toward and away.
  6. Jul 29, 2015 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    I don't think you need to use Doppler effect equations solve this.

    If frequency, fB, is one octave above frequency, fA, then how is fB related to fA mathematically?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted