Question

Two point sources emit sound waves of 1 m wavelength. The sources are pointed at each other. The sources, 2 m apart, emit waves which are in phase with each other at the instant of emission.
a) Where, along the line between the sources are the waves out of phase with each other by pi radians?
b) If the emitted wavelength changes to 0.5 m, where are the waves out of phase with each other by pi/2 radians.