How to increase the sound insulation of the interior door

Creating privacy in an apartment where several people live is quite difficult.

And if you are still fenced off from unwanted glances behind the door in a separate room, then getting rid of sounds is quite problematic. And, having decided to change the doors, you should think about it in advance, so that later you can’t be in a hopeless position, when all the furniture, including the doors, was purchased and installed, and the conditions were not the same that were expected.