S and S are two identical sound sources of frequency 656 Hz. The source S, is located at O and S moves anti-clockwise with a uniform speed 42 ms on a circular path around O, as shown in the figure. There are three points P, Q and R on this path such that P and R are diametrically opposite while Q is equidistant from them. A sound detector is placed at point P. The source S, can move along direction OP. [Given: The speed of sound in air is 324 m s] R S S 15. Consider both sources emitting sound. When S2 is at R and S, approaches the detector with a speed 4 m s, the beat frequency measured by the detector is Hz.