Question

A police car siren emits a sound wave with a frequency fs = 400 Hz and it is moving toward the wall at 25 /m/s. What frequency reflected from the wall does the driver of the police car hear ?
The speed of sound is 340 m/s.

Answer 1

first figure out what higher frequency the wall hears

that frequency emmitted from the wall toward the car at that frequency

then get the still higher frequency back at the car

Answer 2

fi=f(v)/(v-vs)
fi=400(340m/s)/(340m/s-25)
fi=431.7Hz