S _ 1 and S _ 2 are two identical sound sources of frequency 656Hz...

${S}_{1}$ and ${S}_{2}$ are two identical sound sources of frequency $656Hz$. 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 $324m{s}^{- 1}$]

When only ${S}_{2}$ is emitting sound and it is at $Q$, the frequency of sound measured by the detector in $Hz$ is _____.