Key-Idea If source and observer are moving towards a wall, then apparent frequency after the reflection from wall is same as the apparent frequency by the source and observer moving towards each other. Here, speed of sound, vs=320m∕s, actual frequency, v=8000Hz and speed of source vs=20m∕s Since, both the observer and source are moving with same speed. ⇒vs=20m∕s Apparent frequency, v′=(
v+v0
v−vs
)v ⇒v′=(
320+20
320−20
)×8000 =9066.66≈9.1kHz Hence, the frequency of siren heard by car driver is 9.1kHz