Coupon Accepted Successfully!


Doppler Effect

Whenever there is a relative motion between a source of sound and the listener, the apparent frequency of sound heard by the listener is different from the actual frequency of sound emitted by the source.
When the distance between the source and listener is decreases, the apparent frequency increases. It means the apparent frequency is more than the actual frequency of sound. The reverse is also true.
General expression for apparent frequency:
n’ = 30404.png
Fig. 15
Here n = actual frequency; vL = velocity of listener, vS = velocity of source, vm = Velocity of medium and v = Velocity of sound wave
Sign convention: All velocities along the direction S to L are taken as positive and all velocities along the direction L to S are taken as negative. If the medium is stationary vm = 0, then n’ = 30341.png

Special cases

  • Source and listener are approaching each other,
  • Source and listener moving away from each other,
  • Both moves in the same direction with same velocity n’ = n, i.e., there will be no Doppler effect because relative motion between source and listener is zero.
  • Source and listener moves at right angle to the direction of wave propagation. n’= n
    It means there is no change in frequency of sound heard if there is a small displacement of source and listener at right angle to the direction of wave propagation but for a large displacement the frequency decreases because the distance between source of sound and listener increases.
Some Important Points
  • If the velocity of source and listener is equal to or greater than the sound velocity, then Doppler effect is not seen.
  • Doppler effect gives information regarding the change in frequency only. It does not says about intensity of sound.
  • Doppler effect in sound is asymmetric,but in light it is symmetric.

Some typical features of doppler’s effect in sound

When a source is moving in a direction making an angle θ wrt the listener The apparent frequency heard by listener L at rest, when source is at point A is
Fig. 16
As source moves along AB, value of θ increases, cosθ decreases, n’ goes on decreasing.
At point C, θ = 90°, cos θ = cos 90° = 0, n’ = n.
At point B, the apparent frequency of sound becomes
When a source of sound approaches a high wall or a hill With a constant velocity vs, the reflected sound propagates in a direction opposite to that of direct sound. We can assume that the source and observer are approaching each other with same velocity, i.e., vs= vL.
∴ 30003.png
When a listener moves between two distant sound sources Let vL be the velocity of listener away from S1 and towards S2. Apparent frequency from S1 is 29997.png and apparent frequency heard from SL is 29991.png.
∴ Beat frequency29985.png

Test Your Skills Now!
Take a Quiz now
Reviewer Name