# A motor car blowing a horn of frequency 124 vib/sec moves with  velocity 72 km/hr towards a tall wall. The frequency of the reflected sound heard by the driver will be (velocity of sound in air is 330 m/s) Option 1) 109 vib/sec Option 2) 132 vib/sec Option 3) 140 vib/sec Option 4) 248 vib/sec

In the condition source and listener are at the same position i.e car

$n' = \frac{c+ v_{car}}{c- v_{car}}v_{0}= \frac{330=20}{330-20}= 140 vib/s$

Doppler Effect in reflected sound -

$\nu _{ap}= \nu _{0}\left ( \frac{C+V_{c}}{C-V_{c}} \right )$

- wherein

$C=$ Speed of sound

$V_{c}=$ Speed of car

Option 1)

109 vib/sec

This is incorrect

Option 2)

132 vib/sec

This is incorrect

Option 3)

140 vib/sec

This is correct

Option 4)

248 vib/sec

This is incorrect

Exams
Articles
Questions