The Benefits of STC Rated Doors

When Acoustics are a Priority

Published in Door Safety & Security magazine

In his informative article Understanding Acoustics in Architectural Design, James D. Janning, Architectural Systems Manager for the USG Corporation points out that: 

“Good acoustical design is no longer a luxury but a necessity”

Today, understanding the fundamentals of sound transmission and how to utilize building materials for effective sound isolation is crucial for commercial building designs across the board. Comfort, productivity, privacy, and property value are just a few of the benefits of buildings that are designed to keep unwanted noise to a minimum. 

STC rated doors play a major role in achieving sound absorption and isolation parameters but there is no “universal cure” acoustic product. While an acoustic door is an important element, the walls and ceiling should have noise-resistant properties too. There was an instance where an STC rated door was installed into an existing opening but the occupants still complained about noise. After further investigation, we learned that the room had a drop ceiling and drywall walls that sound could easily pass through. Of course the doors didn’t solve the problem!

For new construction and acoustic remodeling projects alike, appropriate STC rated doors are just one important element in the comprehensive acoustical design for each individual space. 

While an STC rated door is a single crucial element, ceiling and wall materials must also be factored into the acoustic design equation.

Sound Transmission Classification (STC) Ratings Explained

In the article above, architect James Janning points out that acoustical design goals vary widely, using the example of the contrasting sound resistant requirements between an executive conference room and a kindergarten classroom. Understanding the full range of STC ratings is important for selecting the most appropriate acoustic door based on sound parameters specified for each built environment. 

When enhanced acoustic door performance is the goal, the STC rating should take priority rather than selecting by core materials since different manufacturers use a variety of proprietary cores to achieve the STC ratings listed below. It’s important to note that the STC rating scale is a logarithmic progression where an increase of 3 points doubles the level of sound transmission reduction. 

STC Ratings Scale

25-29 Normal conversation can be understood quite easily and distinctly. Doors and materials in this low STC range wouldn’t be categorized as acoustic doors. Many commercial steel doors are frequently rated in the 25-35 range. 

30-34 Resounding loud speech can be understood while normal speech may be audible but difficult to understand. Door assemblies in this range are usually specified as standard assemblies requiring no special attention. 

35-39 Loud speech is still audible but not intelligible. Door assemblies in this range or higher should be specified as special assemblies not included in Spec Sections for standard or custom frames. 

40-41 This is the minimum STC range or “privacy threshold” in acoustical designs where sound control is critical. Normal speech is inaudible, loud speech is indecipherable. This range is adequate in most situations where room-to-room sound control is the goal. It may be advisable to work with an acoustic consultant for sound-sensitive projects at this range or above.  

42-44 Loud speech is still audible but is suppressed to a low murmur.

45-49 Even boisterous loud speech is completely blocked.

50-54 Very loud sounds such as operating machinery, musical instruments, and high-volume stereos are reduced to tolerable background sounds. Many steel doors in this range are the standard 1-3/4″ thickness. 

55-60+ Superior soundproofing makes the majority of sounds inaudible. This level of acoustical enhancement is achievable only with 2¼” or thicker doors and is the most expensive option. 

While a 55+ STC-rated door may be the perfect fit for an office located just off of the loud production line in a manufacturing facility it would be a clear case of overkill at a law firm where privacy could be achieved with a more economical 40 or 42 STC rating. 

Specifying STC Rated Doors

The STC rating is a sound control measure of the entire door assembly including the door, frame, and glass kits. Any acoustic door assembly with a 35 or higher STC rating should be specified as a special assembly requiring rigorous attention to assembly details such as hardware, seals, and installation. To ensure that the STC goal for the built space is achieved it’s important to consider the entire acoustic door assembly with attention to details including:

  • The quality of the seal and threshold assembly can affect performance by 1-5 STC points.  
  • Visibility requirements and sound control priorities for each acoustic door assembly should be clearly established since increases in the size of glass kits decrease the STC rating proportionally. 
  • When glass kits are required laminated glass with an air pocket can reduce sound wave transmission. 
  • Avoid over-specifying. Door assemblies with STC ratings up to 54 can generally be of the standard 1-3/4″ thickness. Higher than that and the door often has to be thicker and/or heavier.  

OITC: Outdoor-Indoor Transmission Class Acoustic Doors

OITC testing measures the sound transmitted from the external environment into the building envelope. OITC uses ASTM E1332 Standard Classification to include a sound frequency range lower than those used for STC testing to simulate sounds generated by rail and vehicular traffic. Acoustic doors assemblies for entry and exit should include both STC and OITC values in the specifications. 

Steel Door Assemblies: Acoustic Solutions and More

While attention to acoustical characteristics has become imperative in today’s architectural environment design professionals know that door assemblies must also satisfy a wide range of requirements in other important performance categories. While solid core wood, aluminum, or fiberglass doors might fulfill acoustical design requirements they may fall short in other performance categories. Steel doors are often the best solution for enhanced acoustical designs but they also satisfy other high-performance specifications such as:

  • Bullet-resistance for government buildings and high-crime environments
  • Steel doors are the only door assemblies to meet FEMA P-361 tornado shelter standards. 
  • Hurricane rated steel door assemblies are specially tested to withstand extremely high wind and storm debris impact.
  • Steel outperforms wood, aluminum, and fiberglass for 3-hour fire ratings. 

The Steel Door Institute does not make or sell any products. If you’re interested in learning more or want to discuss which products make sense for your project, contact an SDI Certified manufacturer of STC rated doors at the bottom of this page