DOPPLER EFFECT IN SOUND

The apparent change in frequency due to relative motion between source and the observer is known as Doppler Effect.

This is observed in our day to day life. For example: When a whistling engine approaches the platform, the sound appears sharp to our ears while it recedes away from the platform, it appears grave.

DOPPLER'S EFFECT OF SOUND

We will talk about three situations while discussing Doppler's effect of sound.

• Sound-source is moving and observer is stationary

Let v₀ be the true frequency of the source. 'v' is the velocity of sound in air.

It implies v₀ no of waves will be produced by the waves which will travel with velocity 'v'.

Frequency as heard by the observer is v=v₀ (v/(v-v_s).

This apparent frequency as heard by the observer is greater than the true frequency.

If the source moves away from the observer, relative velocity becomes (V+V_s). Apparent frequency then becomes v=v₀ (v/(v+v_s). This is less than the true frequency.

• Observer is moving and source is stationary

Let the velocity of the observer be V₀ then apparent frequency v=v₀ +(V₀/?)

So, v=v₀(v+v₀)/v

If observer recedes away from source then, v=v₀(v-v₀)/v

• Both source and observer are moving

Suppose, observer moves with velocity V₀ and velocity of source be V_S. then apparent frequency becomes: v=v₀(v-v₀/v-v_s)

Note: in the above case it has been assumed both source and observer move along the direction of sound. If there is a change in direction, sign will be changed.

APPLICATION OF DOPPLER EFFECT:

Doppler effect of sound is used in estimating the sound of aero plane and also the sound of submarine under water.

DOPPLER EFFECT OF LIGHT:

Depending upon the relative velocity between source and observer, the frequency of spectral line differs. This is accounted for Doppler effect of light. The observed frequency increases if source moves towards the observer. This is an important cause of broadening of spectral lines.

It can be shown that of source of light v₀ is moving with relative velocity V along the line of sight, then frequency as observed by an observer is :

Doppler effect in sound is asymmetrical whereas Doppler effect in light is symmetrical.