1. The problem statement, all variables and given/known data When a train running at a speed of 72 km/h approaches a crossing signal, a passenger in the train hears the siren at 720 Hz. What frequency does the passenger detect after the train passes the crossing signal? Take the speed of sound in air to be 340 m/s. 2. Relevant equations Doppler effect 3. The attempt at a solution First I convert the speed of train from 72 km/h into 20 m/s. Speed of detector = 20 m/s Speed of source = 0 Speed of sound = 340 m/s Frequency of source = 720 Hz Frequency of detector = ? Using Doppler effect formula: Fd = (340 - 20) / 340 x 720 (Since the detector is moving away from source, we want to make the denominator greater) Fd = 677.64 Hz But this isn't the answer. (The answer is 640 Hz) May I know which part of my attempt goes wrong?