Question

The sound emitted from the siren of an ambulance has frequency of 1500 Hz. The speed of sound is 340 m/s. Calculate the difference in frequencies heard by a stationary observer if the ambulance initially travels towards and then away from the observer at a speed of 30 m/s.

Answer 1

Given : v_s = 30 m/s, n₀ = 1500 Hz, v = 340 m/s 

To find : Difference in apparent frequencies (n_A – n’_A)

Formulae : 

i. When the ambulance moves towards he stationary observer then n_A = n₀\((\frac{v}{v - v_s})\)

ii. When the ambulance moves away from the stationary observer then, n’_A = n₀\((\frac{v}{v + v_s})\)

Calculation : 

From formula (i), icon’ 340

Difference between n_A and n’_A 

= n_A – n’_A = 1645 – 1378 = 267 Hz