A variety of methods can be used to reduce sound transmission in interior spaces in your home, but one method of soundproofing that is often overlooked is also one of the simplest: replacing standard hollow-core interior passage doors with solid wood doors.
A lot of sound transmission occurs because the drywall on walls and ceilings is too thin, but interior doors comprise a far greater amount of the wall space than you might first think. If the wall adjoining a noisy area comprises 80 square feet, then the door represents close to 20 percent of that area. The door itself often represents the weak spot in the wall when it comes to sound transmission, especially if it is a hollow-core door as is standard in most new home construction.
The Basics of Sound Transmission
There are a variety of ways to measure the capacity of a wall or other surface to resist the transmission of sound. One such method is the Sound Transmission Class (STC) ratings, which assign an acoustical performance of a door or other material. Higher STC values indicate improved ability to resist the transmission of sound.
Here is how the STC relates to sound transmission of a surface:
- 25: Normal speech can be clearly understood
- 30: Loud speech can be understood; normal speech heard but not understood
- 35: Loud speech audible but not understandable
- 41: Loud speech sounds like just a murmur
- 45: Loud speech is barely audible
- 50: Loud musical instruments barely heard
As a means of understanding how a door can be the weak link in a wall, soundproofing-wise, consider the STC class of various construction elements:
- Typical hollow core door: STC of 20 to 25
- Solid door, particleboard core: STC 30
- Standard wall with 1/2" wallboard on both sides and 3.5 inches of airspace: STC 33
- Standard wall with 1/2" wallboard on both sides, wall cavity insulated: STC 39
- Double layer of 1/2″ drywall on each side, wood studs, wall cavity insulated: STC 45
- Solid wood slab door: STC 55 to 60
Door Replacement as a Means of Soundproofing
As the STC numbers show, you can greatly improve the soundproofing of any wall simply by replacing a standard hollow-core door, which is usually filled with a cardboard honeycomb materials, with a solid wood slab door. The door alone will greatly reduce sound transmission from one room to the other, and if you combine this project with adding a double layer of wallboard attached both sides of an insulated shared wall, you can eliminate virtually all sound transmission through the wall.
Door replacement is usually a fairly easy matter since the factory stock doors found in most home construction use standard dimensions and often have the same positions for hinges and lockset. In many cases, it can be as simple as popping the old door off the hinges and putting the new solid-core door onto the same hinge plate already attached to the door jamb. Be aware that solid doors are considerably heavier than hollow-core doors, and you may need to reinforce the hinges with longer, sturdier screws to support the weight. If you cannot find a solid wood door of exactly the same dimensions or with the same hinge and lockset placements, you may then have to cut new hinge and strike-plate mortises into the door jambs in order to hang the new door. It's possible some trimming of the door will be necessary to fit an odd-sized door opening, as is sometimes the case in older homes. This can take some precision work and may be a job for a professional craftsman who is familiar with such work.
Because custom-trimming a replacement door to fit can be an exasperating task, some people choose to remove all the door trim, remove the entire door and frame down to the studs, then install a new prehung solid-core door. Prehung doors come already installed on hinges and fitted into a frame. Installation is as simple as sliding the door-and-frame unit into the rough opening, shimming and nailing it in place, then replacing the door trim moldings. It's a fairly easy job, even for casual DIYers.
The hollow-core doors used for interior passage doors in many homes are the weak link when it comes to sound transmission. You can go a long way toward soundproofing a wall simply by replacing the door with a sturdy solid-wood door.