Question

A bat is travelling with uniform velocity 20 ms${}^{-1}$ towards a wall in between two parallel walls. The bat produces ultrasonic sound note and it hears echoes from both walls. If minimum apparent frequency of echo is 32 KHz, the maximum apparent frequency of echo is : (velocity of sound in air = 340 ms${}^{-1}$)

• A. 34kHz
• B. 36kHz
• C. 40.5 KHz
• D. 45k Hz

Answer 1

Answer: (C)

40.5 KHz

The distance between wall and bat is changed by $20\times 2=40m/s$

$f_{min}=32\times {10}^{3}Hz$

Let frequency of source is $f$.

$f_{min}=f\left(\frac{340-(2\times 20)}{340}\right)$

$f_{max}=f\left(\frac{340+\left(20\right)\times 2}{340}\right)$

$\therefore$ dividing them:

$\frac{f_{max}}{32\times {10}^3}=\frac{380}{300}$ $=40.5kHz$