Why a closed door still lets sound through
Sound travels through the door leaf, but it travels much more easily through any air gap. A standard interior door has a gap under it for air flow and small gaps along the sides and top so it can open. Even a heavy door with open gaps sounds only a little better than a light one.
So there are two jobs: make the leaf heavy and dense enough to block sound, and close the gaps around it when the door is shut.
What STC means
Sound Transmission Class (STC) is a single number that describes how well a building element blocks airborne sound such as speech. Higher is better. Published STC numbers for doors are measured on complete tested assemblies with their seals, so a door leaf by itself in an ordinary frame performs below its rating. Use STC to compare products, not as a promise for your room.
| Setup | Rough result |
|---|---|
| Hollow-core door, no seals | Normal speech is easy to understand through it |
| Solid or engineered core, no seals | Quieter, but gaps still let speech through |
| Solid core with perimeter seals and a door bottom | Speech becomes muffled; a clear improvement |
| Tested acoustic door assembly | Designed for recording rooms, theaters and offices |
The upgrades that make the biggest difference
In order of value for most homes:
- Perimeter seals in the frame, so the door closes against a soft gasket on the sides and top.
- An automatic door bottom: a seal hidden in the bottom of the door that drops to the floor when the door closes and lifts when it opens.
- A solid or engineered-core leaf instead of a hollow one.
- A latch that pulls the door tight against the seals; magnetic latches help the door close quietly.
- Sealing the frame to the wall, so sound does not pass around the frame itself.
Bedrooms
For a bedroom, the goal is usually to block TV, conversation and household noise at night. A solid-core door with seals on the sides and top and an automatic door bottom handles most of it. If the bedroom has its own air return, the door can be sealed without starving the room of air; if it does not, ask your HVAC contractor before closing the gap under the door, because the system may rely on it.
Home offices
For video calls, the problem runs both ways: you do not want the household on your calls, and you do not want your calls in the living room. A swinging door with seals works far better than a sliding door, which is hard to seal. If the office is next to a bedroom, the shared wall may let through more sound than the door, so look at both.
Hidden doors and sound
Hidden doors can be built with seals as well, but the clean look relies on small, even gaps around the leaf. Tell us early if a hidden door also needs to be quiet, so the frame, the seals and the hinges are chosen together.
A FEW USEFUL ANSWERS
Before you begin.
Is a solid-core door soundproof?
Not on its own. It blocks more sound than a hollow door, but without seals around it the gaps still let speech through.
Can I make my existing door quieter?
Often, yes. Adding seals to the frame and a door bottom can help an existing solid door. A hollow door usually benefits from being replaced.
Do sliding doors work for a quiet office?
Not well. Sliding doors are difficult to seal, so a swinging door with seals is the better choice where quiet matters.