S1 and S2 are two identical sound sources of frequency 656 Hz. The source S is located at O and S2 moves anti- clockwise with a uniform speed 42 ms1 on a circular path around 0, 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 s1] S S 14. When only S2 is emitting sound and it is at Q, the frequency of sound measured by the detector in Hz is