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Homework Help: Doppler Effect + Wavelength problem! Help Please

  1. Dec 4, 2011 #1
    1. The problem statement, all variables and given/known data

    A sound source emits sounds of frequency 210 Hz that travel through still air at 340 m/s. The listener moves at 85 m/s relative to still air toward the stationary source. What is the wavelength of the sound between the source and the listener?

    2. Relevant equations

    f' = f[1+ Vlistener/Vsound]

    3. The attempt at a solution
    well using the doppler effect since the listener is moving towards the stationary source i know it will be f'=f[1+Vlistener/Vsound]

    i find the f' right... but when using v=wavelength*frequency, which frequency do i actually use. because i know although im solving for f' , im just wondering if its a trick or something because i'm a little confused on the concept.

    so for f' i get 262.5 Hz

    then i plug it in to the v=wavelength*freq , but which velocity do i use? the speed of sound or the speed of the listener moving?

    using the speed of sound i get 340 m/s = wavelength * 262.5 and end up with 1.295m as the wavelength.
  2. jcsd
  3. Dec 4, 2011 #2
    If the source is not moving the waves are unchanged and their wavelength is simply given by v/f where v is their speed in still air and f the original frequency.
    The frequency of the sound heard by the moving listener is higher because he passes through the approaching waves at a higher rate than if he were stationary. The Doppler formula will give this new frequency.
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