Traditional interior doors are hollow and do not create much of a sound barrier. Conversations are often carried through the door, or around gaps in the frame, into the next room. If you find yourself longing for a little privacy, or you simply want to shut out the sounds of your teenagers music, there are ways to soundproof doors that don't have to cost a small fortune.
Check for gaps under the door or around the door frame. Even a 1/4 of an inch can channel a lot of sound. If you find gaps seal them with closed cell tape. This can be purchased from your local hardware store for around £6.
Install a door sweep if the bottom of the door has a gap between the door and the floor. This is a metal strip with a rubber sweep on it. It is screwed into the base of the door so that the rubber portion just brushes the floor.
Check to see if there is any insulation between the door frame and the interior wall. You can check by removing the moulding around the door. This section is often left bare and simply covered by the moulding. Insulate the area with caulking and then cover with lead tape.
Cover the door with a sound barrier. This can be as simple as quilt that covers the door. Attach Velcro around the outside of the door frame and on the edge of the quilt. Secure the quilt over the door whenever you wish to cushion sound.
Consider replacing the interior door with an outside door if these measures do not provide the soundproofing you desire. Exterior doors are solid and will block more sound than hollow interior doors.
Do not seal cracks or gaps with foam insulation. This will conduct sound and create a bigger problem.