Its made of steel but hollow (though I'm not sure if completely hollow or if it has divisions like cells) with zero sound absorption.
I already weatherproofed it with foam strips, but even then every time someone walks or talks in the hallway outside I can hear everything, can even hear noises from other apartments that escape through their doors (which are the same) and enter mine.
Any way I can soundproof my door? I don't care about aesthetics but note that I can't modify anything on the outside-side of the door because its against the building rules. So whatever I do has to be inside the actual door or on the interior-side.
Best Answer
You could try an acoustic curtain.
You could build an interior door to make your exterior door more like an air lock and then sound proof that door/wall to your hearts content.