What is an STC rating?
STC (Sound Transmission Class) is an integer rating that indicates how effectively a door or other building element, such as a wall or window, insulates a room from airborne sound. Simply put, it helps you understand how much noise is blocked, i.e., remains on the other side of the door.
The calculation system is based on ASTM E413, which converts complex acoustic data into an understandable STC value. Typically, this rating ranges from 20 to 60, and the higher the number, the quieter the building will be.
The STC rating scale: What do the numbers mean?
The table below contains data to help you quickly understand how effectively the structure blocks sound by STC rating.
STC rating chart
|
STC rating |
Performance |
Description |
|
<20 |
Minimal |
Quiet speech is clearly audible |
|
20-25 |
Very poor |
Quiet speech is audible but not intelligible |
|
25-30 |
Poor |
Normal speech is audible |
|
30-35 |
Fair |
Loud speech is clearly audible |
|
35-40 |
Goog |
Loud speech is audible but not intelligible |
|
40-50 |
Very good |
Loud speech is not audible |
|
50-60 |
Excellent |
Loud sounds (machinery, music) are faintly audible |
|
>60 |
Maximum |
Most loud sounds are not audible |
STC rating for common door types
Different types of doors—wooden, metal, glass—have varying levels of sound insulation, and their STC ratings clearly demonstrate how effectively each protects against noise.
Hollow-core doors
Lightweight and affordable, these hollow interior doors have an STC rating of 25 or less. They are most commonly found in apartments and office spaces, but due to their design, these models provide virtually no sound insulation, which is important in bathrooms and bedrooms, for example.
Solid-core doors (STC 30-35)
Solid-core doors provide significantly better noise insulation, with an STC of 30-35. They can be installed in any room, especially where quiet is critical.
Many Belldinni interior doors use engineered solid core, an innovative, eco-friendly wood fiber material with a density of 12.5 lb/ft³.
Steel doors (STC 25-35 for standard, higher for acoustic models)
Steel doors provide good sound insulation (STC 25-35 for standard models and higher for acoustic ones), especially when sealed around the perimeter of the slab or frame.
Our product range includes the Diamond collection of exterior doors, which have a sound insulation rating of 36 dB—above average.
Wooden doors
The STC rating of wooden doors depends on the slab thickness, the presence of seals, and the type of filler. The type of door also matters: in any case, an exterior door provides greater protection against outside noise than an interior door.
Fiberglass doors (STC 22-38 depending on core)
Energy-efficient fiberglass doors can also have different STC ratings (22-38), which is influenced by the filler. On average, such models are similar in technical characteristics to steel doors.
Factors that influence a door's STC rating
Many factors influence a door's sound insulation characteristics, from slab thickness and filler type to seal quality and installation technology.
Door material and filling
The sound insulation characteristics of a door depend directly on its material and internal filling. Heavy, dense materials such as solid wood or metal with mineral wool inside are more effective at dampening sound waves than hollow or lightweight constructions.
Door slab thickness
The thicker the slab, the better it blocks sound. Increasing the thickness by even a few inches significantly reduces airborne noise transmission.
Door seals
High-quality seals around the perimeter of the door frame or door guarantee protection not only from unwanted odors and dust, but also from sounds on the other side of the door. In addition, some models are available with additional features, such as automatic thresholds that perform a similar function.
Professional installation
Even the most expensive door will not provide the desired STC rating if it is not mounted professionally. Airtightness—the tight fit of the door slab to the frame and the absence of gaps, distortions, and other defects—is critical to ensuring full sound insulation.
Therefore, we recommend that you follow the step-by-step instructions carefully or delegate the task of installing front or interior doors to professionals.
What STC rating do you need for your door?
The choice of doors with a suitable STC rating depends on the room's purpose. Accordingly, the higher and stricter the sound insulation requirements, the higher the rating should be.
Residential applications (bedrooms, home offices)
For bedrooms and home offices, an STC rating of 30-40 is recommended. This is sufficient to reduce household noise and provide a comfortable acoustic environment for sleeping or working.
Commercial applications (conference rooms, private offices)
Conference rooms and private offices require doors with an STC rating of 40-50. This level effectively isolates conversations and sensitive information from prying ears.
Industrial and specialized uses
Recording studios, laboratories, or server rooms typically require an STC of around 55. It reduces noise perfectly and meets acoustic or technical insulation requirements.
FAQ
What is a good STC rating for a bedroom door?
For bedrooms, a solid-core door with an STC rating of at least 30 is recommended—this is sufficient to muffle speech and provide basic sound insulation.
Can a door with a high STC rating still leak sound?
Yes, if the installation is done incorrectly, even a door with an STC 50 will lose its effectiveness if there are gaps or the doorway is poorly insulated.
What is the difference between STC and OITC?
STC (Sound Transmission Class) measures how well a building partition (e.g., a door, wall, or window) blocks airborne sound, primarily in the mid- to high-frequency range (e.g., speech, TV).
OITC (Outdoor-Indoor Transmission Class) is similar but focuses on blocking lower-frequency sounds, such as traffic and aircraft noise, making it better for assessing exterior noise insulation.