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] Consider both sources emitting sound. When S₂ is at R and S₁ approaches the detector with a speed 4 m s⁻¹, the beat frequency measured by the detector is _______ Hz.