A bat flies at steady speed of `4" m s"^(-1)` emitting a sound of `f=90xx10^(3)`. Hz It is flying horizontally towards a vertical wall. The frequency of the reflected sound as detected by the bat will be
(Take velocity of sound in air as `330" m s"^(-1)`)

Options

A. `88.1xx10^(3)` Hz
B. `87.1xx10^(3)` Hz
C. `92.1xx10^(3)` Hz
D. `89.1xx10^(3)Hz`

Answer

C

Explanation

Here, bat is a source of sound and the wall is an observe at rest.
`:.` Frequency of sound reaching the wall is
`f.=(vf)/(v-v_(s))` . . . .(i)
where v is the velocity of sound in air and `v_(s)` is velocity of source.
On reflection, the wall is the source of sound of frequency `f..` at rest and bat is an observer approaching the wall
`:.` frequency heard by the bat is
`f..=(f.(v+v_(0)))/(v)=f((v+v_(0))/(v-v_(s)))" (Using (i))"`
`=90xx10^(3)((330+4)/(330-4))=(90xx10^(3)xx334)/(326)`
`=92.1xx10^(3)Hz`