A drone fitted with siren in flying directly away from the drone operator and towards a distant building at a speed of 15 m/s. The siren produces sound of frequency 780 Hz. What is the frequency that the operator hears in the echo reflected that the operator hears in the echo reflected from the building . [ Speed of sound is 340 m/s. ]

Options

A. 766 Hz
B. 800 Hz
C. 816 Hz
D. 840 Hz

Answer

C

Explanation

A drone fitted with siren as shown in the figure,

Given, speed of drones, `v_(s)=15m//s`, actual
frequency, `n_(0)=780` Hz and speed of sound, `v=340m//s`
As source is moving towards the building, so the apparent frequency reflected by the building is
`n=(n_(0)v)/(v-v_(s))=(780xx340)/(340-15)=816 Hz`
As we know, there is no change in the apparent frequency by the reflection from the building.
Hence,
the echo heard by observer is 816 Hz.
So, the correct option is (c).