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Perception of Sounds

  1. May 3, 2010 #1
    1. The problem statement, all variables and given/known data

    A person on a bicycle is traveling away from a parked car. If the horn of the car is sounded, how will the bicycle rider perceive the sound?
    A)
    The pitch will be higher than normal.
    B)
    The pitch will be lower than normal.
    C)
    The pitch will appear to steadily increase.
    D)
    The pitch will appear to steadily decrease.


    2. Relevant equations

    none needed



    3. The attempt at a solution

    i think it may be b but i was hoping to get some guidance, thanks.
     
  2. jcsd
  3. May 3, 2010 #2
    You are correct in that the answer is b).
    The effect is called the Doppler Effect. Have you studied this?
     
  4. May 3, 2010 #3
    Think about the sound waves like longitudinal waves, with different concentrated points a certain distance from each other. The distance between these waves is what determines the pitch, closer = higher and farther = lower. Now if these waves are moving toward a person who's moving away from them, there's going to be a little extra time between each concentrated area for them to reach the person, creating the illusion of, like you said a lower pitch than it actually is.

    Hope this helps you understand it better. ^-^
     
  5. May 3, 2010 #4
    This link shows a very easy-to-use, visual simulation that shows the Doppler effect:
    http://lectureonline.cl.msu.edu/~mmp/applist/doppler/d.htm

    click somewhere in the middle of the screen, and a dot will appear. "Sound" waves will begine to be emitted from the dot (it will look like ripples moving out from where rocks have been dropped in a pond). Imagine your bike-rider moving away from the dot. Does the time in between the peaks that reach him increase or decrease? This is the Period, = 1/f. what happens to the period (and to the frequency) if the bike rider moves towards the source?

    Now Click-and-hold anywhere on the gray area, then drag your mouse before releasing it. This will cause the 'source' to move as it emits waves. Compare the wavelength (distance between waves) in front of the moving source to the wavelength behind the moving source. Which is bigger? If the wavelength is biggger, what does this tell you about frequency?
     
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