The best common doors to use are exterior grade, solid-core wood (“slab”) doors that are flat, without moldings. Also common, but more expensive, are commercial and/or exterior grade insulated steel doors.
You can add SheetBlok to one or both sides of either type of door before installing the knob to provide additional transmission loss, then Studiofoam over the SheetBlok.
If you have the inclination, you can make a door sandwich out of two (2) solid-core doors and a couple layers of SheetBlok in the middle (this is the sort of thing Eddie Van Halen did at his 5150 studio). If you desire to have the ability to lock your door, be sure you can find a knob/lock that will work with your thicker-than-normal door.
Double doors (back-to-back) are of some benefit if they are (a) attached to physically separate door jambs that are floated, and (b) are as far apart as possible given the constraints of your framing structure. Build your walls and double doors in such a way as to give you as much dead air space between the doors as possible. shows methods of installing back-to-back doors for single and double framed walls. Alternate your door knobs and hinges left to right. You can add surface moldings to your slab doors if you want to dress them up. Install Studiofoam on your doors – especially the sides that face each other. This absorbs any resonance that might occur between them.
The biggest reason that doors are poor in the area of sound control often has little to do with the physical construction of the doors themselves (if you are using one of the types outlined above). The weakest link in most door systems is that they are not sealed well with the floor below them or with the frame around them. You must use a compressed rubber threshold below your door and you must make sure that wherever the door shuts and would normally contact the door jamb it meets foam weatherstrip tape or a rubber gasket. Magnetic seals can also be used, like you would find on a refrigerator door.
For those requiring the ultimate in door seals, you might contact Zero International. They specialize in door seals that do a fantastic job of blocking sound.
If you are looking to save yourself a considerable amount of time (and headaches), you might consider simply specifying some sound-rated doors right into your studio. While they are expensive, sound-rated doors give you far superior performance to anything you could do with a single door on your own. Manufacturers of high-quality acoustical doors include:
At most, you can expect an STC-30 to 32 from even the best solid-core door. The best double-frame, back-to-back solid-core door arrangement rarely yields better than STC-50. By contrast, typical single-leaf doors from the manufacturers above can yield ratings of STC-55 and higher. Worth considering if maximum sound isolation is your goal.