Where is the sound concentrated after reflection from the back wall of the niche?

Question

A large church has a niche in one wall. On the floor plan it appears as a semicircular indentation of radius 2.91 m. A worshiper stands on the center line of the niche, 2 m out from its deepest point, and whispers a prayer. Answer in units of meters.

Answer 1

3,82 meter.

From the center of the semicircle, if the source moves forward, the 'knot' of the result will move backwards.