Here are some suggestions in the order of being effective (and most likely the most costly first)
1. Change the door and frame to an outside door with a wooden threshold and insure there is good weather stripping.
2. Change the door to solid core, weather strip the entire door, add a threshold at the bottom and use one of the rubber gaskets on the bottom of the door to seal the bottom.
3. Change the door to a solid core door and weather strip,
4. Add layers of sound absorbing material to both sides of the door (this can be done to any, but will not have much benefit on solid core doors).
The other aspect to consider is that having mass and an airgap makes the best sound barrier. Studios use 2 doors with an air gap in between in critical areas. If you have a location or way of adding another door with an airgap (need only be 6 inches), you can make great strides.
1. Change the door and frame to an outside door with a wooden threshold and insure there is good weather stripping.
2. Change the door to solid core, weather strip the entire door, add a threshold at the bottom and use one of the rubber gaskets on the bottom of the door to seal the bottom.
3. Change the door to a solid core door and weather strip,
4. Add layers of sound absorbing material to both sides of the door (this can be done to any, but will not have much benefit on solid core doors).
The other aspect to consider is that having mass and an airgap makes the best sound barrier. Studios use 2 doors with an air gap in between in critical areas. If you have a location or way of adding another door with an airgap (need only be 6 inches), you can make great strides.