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Doppler Effect

  • Doppler Effect states that there is a change in the frequency of sound because of the relative motion between the observer or the listener and the source of the object.
  • In simpler words when the source and the listener are in a relative motion there is a change in the frequency of sound and this effect of a change in frequency is known as the Doppler's Effect of sound.

  • When the objects move apart the frequency decreases.
  • When the objects move towards each other the frequency increases.
  • The change in the frequency of the sound is given by the formula:

n = n' (V±Vo / V±Vs)

Where, n = actual frequency of sound
             n' = Apparent Frequency
             v = Speed of sound
             Vo = Spped of the observer
             Vs = Speed of the source


1. When the observer is at rest i.e. Vo = 0
     In this case, the formula becomes:

     n = n' (V / V±Vs)
     a) If the source is moving away from the observer then Vs will be positive as n decreases with increase in Distance.
     b)If the source is coming towards the observer then Vs will be negative.

2.When the source is at rest i.e. Vs = 0
     In this case, the formula becomes:

     n = n' (V±Vo / V)
     a)If the observer is moving away from the source then Vo will be negative.
     b)If the source is coming towards the observer then Vo will be positive.

3.Both are moving
   In this case, the formula becomes:

   n = n' (V±Vo / V±Vs)
   a)If both are moving away then Vo will be negative and Vs will be positive.
   b)If both are moving closer then Vo will be positive and Vs will be negative.

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