Question

A source and a detector move away from each other in absence of wind with a speed of $20\text{m/s}$ with respect to the ground. If the detector detects a frequency of $1800\text{Hz}$ of the sound coming from the source, then the original frequency of source considering speed of sound in air $340\text{m/s}$ will be Hz.

• A. 2025
• B. 2025.0

Answer 1

Answer: (A), (B)

Given :-
$v_o=v_s=20\text{m/s}$
$f^{\prime }=1800\text{Hz}$
$v=340\text{m/s}$

The frequency detected by the detector is :
$f^{\prime }=f_o\left(\frac{v-v_o}{v+v_s}\right)$
$1800=f_o\left(\frac{340-20}{340+20}\right)$
$f_o=2025\text{Hz}$