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 mad is standing at a height 12m 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