Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed  1.0m/s and the man behind walks at a speed  2.0m/s. A third man 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  1430Hz. The speed of sound in air is  330m/s. At the instant, when the moving men are 10m  apart, the stationary man is equidistant from them. The frequency of beats in Hz, heard by the stationary man at this instant, is ____________  .