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\( S_1 \) and \( S_2 \) are two identical sound sources of frequency \( 656 \, \text{Hz} \). The source \( S_1 \) is located at \( O \) and \( S_2 \) moves anti-clockwise with a uniform speed \( 4\sqrt{2} \, \text{m s}^{-1} \) 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 \( 324 \, \text{m s}^{-1} \) ]
When only \( S_2 \) is emitting sound and it is at \( Q \), the frequency of sound measured by the detector in \( {H_2} \) is_________.
