A fighter jet plane ‘F’ and a submarine ‘S’ are directly heading towards free surface of sea as shown in figure. Both the plane and the submarine starts sending high ultra sonic compression pulses towards each other.

Speed of sound is maximum in water, hence water is rarer  medium for sound waves w.r.t. air.Longitudinal pressure waves suffer no change in phase from rigid end i.e. compression pulse reflects as compression pulse. On the other hand if longitudinal pressure wave reflects from free end, it suffer a phase change of π i.e. compression reflects as rarefaction and vice-versa.No phase change happens during transmission.