JEE Advanced 2023 Physics question on Oscillations and Waves

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$S_1$ and $S_2$ are two identical sound sources of frequency 656 Hz. The source $S_1$ is located at $O$ and $S_2$ moves anti-clockwise with a uniform speed $4\sqrt{2}$ 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 $S_1$ can move along direction $OP$.

[Given: The speed of sound in air is 324 m s$^{-1}$]

When only $S_2$ is emitting sound and it is at $Q$, the frequency of sound measured by the detector in Hz is __________.