Question

A wall is moving with velocity u and a source of sound moves with velocity u/2 in the same direction as shown in the figure. Assuming that the sound travels with velocity 10u. The ratio of incident sound wavelength on the wall to the reflected sound wavelength by the wall, is equal to –

• A. 9 : 11
• B. 11 : 9
• C. 4 : 5
• D. 5 : 4

Answer 1

Answer: (A)

9 : 11

Answer 2

l_i = wavelength of the incident sound

= $\frac{10u - \frac{u}{2}}{f} = \frac{19u}{2f}$

f_i = frequency of the incident sound

= $\frac{10u - u}{10u - \frac{u}{2}}$f =$\frac{18}{19}$f = f = f_r = frequency of the reflected sound

l_r = wavelength of the reflected sound = $\frac{10u + u}{f_{r}}$ = × 19 = $\frac { 11 \times 19 } { 18 }$ . $\frac { \mathrm { u } } { \mathrm { f } }$

$\frac { \lambda _ { i } } { \lambda _ { r } }$ = × $\frac { 18 \mathrm { f } } { 11 \times 19 \mathrm { u } }$ = $\frac { 9 } { 11 }$