Question

A vehicle with horn of frequency ‘n’ is moving with a velocity of 30m|s in a direction perpendicular to the straight line joining the observer and the vehicle. If the observer perceives the sound to have a frequency of n+n₁. Calculate n₁? 

Answer 1

By Doppler effect, the apparent change in frequency of wave due to the relative motion between the source of waves and observer. 

If v₂ = velocity of the listener 

v_s = velocity of the source 

v = velocity of sound 

γ = frequency of sound reaching from the source to the listener

γ¹ = Apparent frequency (i.e. Changed frequency due to movement of source and listener)

But in our case, the source and observer move at right angles to each other. The Doppler Effect is not observed when the source of the sound and the observer are moving at right argyles to each other. 

So, if n = original frequency of sound the observer will perceive the sound with a frequency of n (because of no Doppler effect). Hence the n₁ = charge frequency = 0.