Question

A motor car blowing a horn of frequency $124vib/sec$ moves with a velocity $72km/hr$ towards a tall wall. The frequency of the reflected sound heard by the driver will be (velocity of sound in air is $330m/s$)

• A. $132vib/sec$
• B. $109vib/sec$
• C. $248vib/sec$
• D. $140vib/sec$

Answer 1

The correct option is D $140vib/sec$


Given, frequency of the source (motorcycle), $n=124vib/sec$
velocity of sound $v=330m/s$
velocity of the source $v_s=72\times 5/18=20m/s$

Here,
velocity of the source $v_s$ and
velocity of the observer $v_0$ are same.
$v_0=v_s=20m/s$

We know that
$n^{\prime }=n\left(\frac{v\pm v_0}{v\mp v_s}\right)$

Since, the observer heard the reflected sound, so velocity of the observer has negative sign
$n^{\prime }=n\left(\frac{v+v_0}{v-v_s}\right)$

$n^{\prime }=124\left(\frac{330+20}{330-20}\right)$

$n^{\prime }=124\left(11.290\right)$

$n^{\prime }=140vib/sec$

Final answer: (c)