I had 2 doors and its was ok with silencing outside, but recently i removed one of them and it was terrible noice from corridor.
So i decided to find out what the heck is going on. After removing sheath a was sunned - its EMPTY INSIDE!
Like huge Drum...
They forgot to fill it with sound absorbing materials!
So i started to fill it with several kind of it - mastic and others
Three layers of sound absorbing maths
As result - door weight increase for 15 kilograms but much more sound absorbing!