The idea is that the cold, slow surrounding air mixes smoothly with the hot, high speed jet exhaust.
In fact the jet exhaust is also the bypass air surrounding the core air, and if all these can mix with minimum turbulence that's as quiet as you can get.
Anyone old enough to remenber the lobed jetpipes of 707s - same solution to a much worse problem
illustrations and explanation here:
Noise Control/Supression
andrew