A man beside a train at rest detects a sound with frequency $400\ \text{Hz}$ produced by the train’s horn. The train then travels directly away from the man and sounds its horn while moving at $50\ \text{m s}^{-1}$. The speed of sound in air is $340\ \text{m s}^{-1}$. What is the difference between the frequencies of the sound heard by the man in the two situations?
- A$51\ \text{Hz}$
- B$69\ \text{Hz}$
- C$350\ \text{Hz}$
- D$470\ \text{Hz}$