Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed 1.0 ms − 1 and the man behind walks at a speed 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 __________.