The Sound Transmission Class (STC) is a numerical rating (STC Ratings) that measures the effectiveness of a building partition in reducing airborne sound. Widely used in the US to assess the performance of interior partitions, ceilings, floors, doors, windows, and exterior wall configurations, it is equivalent to the ISO Sound Reduction Index (SRI) which is used internationally. The STC rating provides a rough indication of the decibel reduction of noise achievable by a partition, particularly in relation to speech sounds.
Enhancing the sound transmission class of a partition can be achieved through various methods, including the addition of mass and an increase in overall thickness. This article will discuss methods used in commercial and residential construction here in the US to achieve your desired STC Ratings when building with steel framing.
At US Frame Factory, our expertise is in steel framing for residential, commercial, and multi-family projects. This article is intended to advise builders, developers, and architects on how to build to the proper STC ratings. The examples below show the engineering/architectural details of typical wall assemblies found in commercial construction.
Audio Distinguishability by STC Rating
|Normal speech is understandable
|Loud speech is understandable
|Loud speech is audible but not intelligible
|Loud speech is audible as a murmur
|Loud speech is heard but not audible
|Loud sound and speech are faintly heard
|Most sounds do not disturb adjacent residents
Methodology of Sound Absorption
There are several methods to diminish sound transmission between walls. One effective approach is the addition of dense insulation materials, like mineral wool or fiberglass within the walls, absorbing sound waves and preventing transmission. Another widely used method involves installing acoustic panels which are designed to absorb and dampen sound. They are available in various styles and thicknesses. The use of double drywall, with a damping adhesive between layers, is a common strategy to enhance sound insulation. Additionally, Mass-Loaded Vinyl barriers, resilient channels mounted to studs, and the application of sound-damping compounds like Green Glue between layers of drywall are popular choices for reducing direct sound transmission. Practical measures to further absorb and block sound include filling gaps and cracks in walls with acoustical caulk, furniture placement, and incorporating heavy curtains and soft materials like carpets. Combining these methods often yields the most effective results, tailored to the specific needs and construction of the space.
Wall Assembly Examples for an Apartment Complex (50-55 STC Rating)
Above is an example of a wall with a STC rating of 55. This is a demising wall between hotel rooms or between the rooms and a hallway. The STC of 55 is achieved by using a double layer of 5/8″ drywall on each side and rockwool insulation inside the wall cavity. The other part of the framing assembly are the metal studs.
Above is a lighter, cheaper and more easily assembled wall with a STC of 50. This is achieved by using 5/8″ drywall and resilient channel. A resilient channel is a specialized construction component used to improve sound insulation in walls and ceilings. It is a thin, flexible metal channel that is designed to be mounted to the studs or joists before attaching the drywall. The purpose of resilient channels is to decouple the drywall from the structure, reducing the direct transmission of sound vibrations.
Pictured Above: Images of resilient channel in use.
Wall Assembly Examples for Factories
Above is a UL U493 wall assembly with an STC rating of 62. This is ideal to use in a factory to sperate office space from loud manufacturing areas. It consists of two stud walls with an air gap, 5/8″ drywall on each side, and sound batt insulation in both stud walls.
Pictured above: sound batt insulation being installed.
Above is another example of a factory wall. This is a UL U419 wall, so its STC rating is 50. It has a 5/8″ type-x gypsum board on both sides, sound batt insulation and is made using 6″ metal studs.
STC Ratings of CMU Wall
The detail above is for CMU walls which are often used in commercial construction, specifically first floor builds of hotel/multifamily buildings. The estimated STC for a 8″ thick concrete masonry unit filled with concrete and rebar is 48.
When Should You Care about Sound Transmission?
Sounds transmission is very important in the following builds
- Between medical office rooms to ensure patient confidentiality – ideally 50 STC.
- Between hotel or multi-family apartment walls (called demising) – ideally 50+ STC.
- Between school class rooms so kids don’t get distracted – ideally 50+ STC.
- Between factory floors and the office space – ideally 60+ STC.