If you’re building a soundproof room, it’s often in the basement. In that instance, more times than not the stairwell becomes an issue. Many people want to use the stairwell wall as the back wall of their new room. Others want to use the space under the stairs for an equipment rack (that’s what I did). That all sounds great, however the stairs are uniquely bad to incorporate into a room.
The framing of the stairwell wall is rigidly connected to the joists overhead. If you have a two story house, the same stairwell framing is attached to the second floor and walls as well. If you were ever trapped in your basement, the best way to communicate upstairs is to knock on your stairwell wall. It will conduct that vibration throughout the house, and someone will hear you. Try it. Because of the intimate framing, the stairwell is uniquely conductive.
Many people have the stairwell with plans to put a heavy door on the top of the stairs. Seems logical, but by that time the stairwell framing is saturated in vibration. Makes the heavy door almost a moot point.