Question
Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed of 1.0 ms−1 and the man behind walks at a speed of 2.0 ms−1. A third man is standing at a height 12 m above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency 1430 Hz. The speed of sound in air is 330 ms−1. At the instant, when the moving men are 10 m apart, the stationary man is equidistant from them. The frequency of beats in Hz. heard by the stationary man at this instant, is ______