A student is at some point between two sound reflecting walls. He's holding a tuning fork and runs towards one of the walls at 5m/s. The pulsating frequency he hears is 8Hz. ¿What's the sound frequency emitted by the tuning fork?
Answer: 137.17 Hz.
I know I'm supposed to use doppler effect but I guess I have to consider wave superposition since there are two walls, but I don't know how to unify both.