Doppler shift of an approaching source with interference problem

[Solidearth]

Homework Statement

P15. The police car with its 300 Hz siren is moving toward a warehouse at 30 m/s, intending to crash through the door. What frequency does the driver of the police car hear reflected from the warehouse? (5 marks)

Homework Equations

doppler shift for an approaching source. (f+) = fo / [1-vs/v]

The Attempt at a Solution

the doppler shift of the siren at the door is (300Hz)/ (1- [(30m/s)/(343m/s)]) =328.75Hz

now assume that the warehouse/ warehouse door reflects 100% of the incident waves.

I really have no idea where to go from here.

 

[rl.bhat]

For reflected waves, treat source at rest and the observer is moving towards the source with the velocity equal to the velocity of the police car.

 

[Simon Bridge]

The warehouse reflects each wave that hits it, and becomes a source.
The car is driving through the reflected waves.

 

[Solidearth]

hey guys thanks for the reply I tried it and it worked.
cheers