First of all I'm going to seal up that door. No, just sealing it won't do. I will need to do something to reduce the amount of noise going back and forth through that door. This kind of doors are usually hollow in the middle giving them a lot of empty space for the noise to go in, vibrates and reflected to the other side. This almost guaranteed lack of privacy for both side of the house.
I got an acoustic chalk for this purpose. Sealing up the surrounding gap of the door. What's that? Acoustic chalk is another type of sealant that remains permanently flexible rather than hardened up like the regular chalk and reflects noises to every direction. This acoustic chalk is able to absorbs noises ( never knew I could be talking about types of sealant chalk).
Then I bought a queen size quilt from IKEA for RM9.90 only (this item is out of production for now) and hang it over the door for further noise absorption. With another layer of foam tape at the gaps with cardboard and I'm done.
Noise reduction works done but it's a total eyesore as can be seen here. I've got one BILLY bookcase up and I need one more to complete it.
And TAHDA!! Here it is as if the door never existed in the first place