Question

A source of sound which is emitting a sound of frequency 600 Hz is moving towards a wall with a velocity of 30 m/s. Three observes A, B, C are taken into consideration. A and B are moving with velocity 20 m/s, 'A' away from the wall and 'B' towards the wall. Here, C is inside the source. Velocity of sound in air is 330 m/s. Then,

• A. the frequency of direct sound observed by observer A will be $620Hz$.
• B. the frequency of reflected sound observed by observer B is will be $700Hz$.
• C. the frequency of direct sound observed by observer C will be $600Hz$.
• D. observer A will not hear any of reflected sound.

Answer 1

Answer: (A), (B), (C)

The correct options are
A the frequency of direct sound observed by observer A will be $620Hz$.
B the frequency of reflected sound observed by observer B is will be $700Hz$.
C the frequency of direct sound observed by observer C will be $600Hz$.

Frequency of direct sound observed by observer A is
$f=f_0\left(\frac{v\pm v_{om}}{v\pm v_{sm}}\right)f_A=600\left(\frac{330-20}{330+30}\right)=620Hz$

Reflected frequency from the wall is
$fwall=600\left(\frac{300}{330-30}\right)=600\frac{330}{300}\mathrm{The}\mathrm{frequency}\mathrm{observed}\mathrm{by}\mathrm{observer}B\mathrm{is}f=fwall\left(\frac{330+20}{330}\right)=600\frac{330}{300}\times \frac{350}{330}=700Hz$

Frequency of direct sound observed by observer C is $600Hz$.