[SOLVED] Moving Loudspeakers on a Railroad Car 1. The problem statement, all variables and given/known data Two loudspeakers are at opposite ends of a railroad car as it moves past a stationary observer at u meters per second. If they have identical sound frequencies of f, what is the beat frequency heard by the observer when (a) he listens from the position A, in front of the car, (b) he is between the speakers, at B and (c) he hears the speakers after they have passed him, at C? Note: There is a figure accompanying this problem showing that the observer is besides the railroad tracks. 2. Relevant equations I will need the equations for calculating the frequency heard by an observer of a moving sound source. Also, beat frequency = difference in the two wave frequencies. 3. The attempt at a solution Let's concentrate on just (a) as the others are probably similar in nature. The observer measures a beat frequency fb = f1 - f2, where f1 is the largest frequency the observer hears from one of the two speakers and f2 is the heard frequency of the other speaker. What I don't understand is why is one frequency larger than the other. It would make sense if the observer is a non-negligible distance away from the tracks. However, the observer is close to the tracks so the heard frequency from both speakers should be the same. Also, the distance between the speakers is not mentioned. I would imagine that this plays an important role in the resultant frequency of the sound wave heard by the observer.