Find the beat frequency detected by O ($$\in \text{Hz}$$).The source of sound and observer move with velocities of $$20\text{ m/s}$$ and $$40\text{ m/s}$$, respectively in the same line. The frequency of the source emitting the sound is $$1000\text{ Hz}$$.

Note that a wall is placed in front of the observer.

Take velocity of sound in air $$340\text{ m/s}$$.

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