S₁ and S₂ are two identical sound sources of frequency 656 Hz. The source S₁ is located at O and S₂ moves anti-clockwise with a uniform speed 4√2 m s⁻¹ 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 S₁ can move along direction OP. [Given: The speed of sound in air is 324 m s⁻¹] When only S₂ is emitting sound and it is at Q, the frequency of sound measured by the detector in Hz is _________.