Basic Overview of STC Ratings
Acoustical consultants are able to provide developers with valuable information regarding a structure’s noise reduction abilities, but have you ever wondered what rating systems they use or how those systems work?
Keep reading for a basic run-down on the workings of the currently used STC system for the next time one of your buildings requires a test.
Need-to-Know Abbreviations:
- STC – Sound Transmission Class
- Hz – Hertz
- dB – Decibel
- TL – Transmission Loss
For more advanced concepts, consider looking at our Soundproofing Case Studies or Soundproofing 101.
What is STC?
The current STC system was developed in the early 1960s as a replacement for the previously used nine-frequency average, which had proved to be unsatisfactory. The nine-frequency average ran on the false belief that a strong sound isolation at one frequency would make up for a weak isolation at another frequency. As engineers continued to conduct research and find advancements in the field, they invalidated the nine-frequency average system and replaced it with the new and improved STC system.
The STC system provides a numerical rating for the air-borne sound TL of a given structure, measured at 16, individual 1/3 octave band frequencies. Structures that receive higher STC ratings will provide a greater reduction of sound transmission within the range of the tested frequencies.
How to Determine STC:
- Measure TL values at 16 different 1/3 octave bands with center frequencies ranging from 125-4000 Hz
- Plot TL values against frequency
- Compare resulting figure to the standard STC contour (transparent overlays are often used to do this)
- Find the intersection of the TL value at 500 Hz and the standard contour at the final rating position – this value is the final STC
Note: Maximum deviation of the test curve below the contour can be no greater than 8 dB. Additionally, the sum of the deviations below the contour for all 16 frequencies of the test curve can be no greater than 32 dB
For more information regarding the STC Rating System, visit the American Society for Testing and Materials annual book of standards, or leave a question in the comments below!
STC – Sound Transmission Class – Why does it Matter?
The STC ratings of your walls matter for a number of reasons. First and foremost, it is critical to achieve an STC of 50 or higher for multi-family dwellings to be compliant with the building code. This can lead to a number of legal issues if acoustic field testing or drawing plan reviews reveal that the STC of your partitions does not meet the code.
More indirectly, STC matters because you want to minimize noise complaints. Most hotels aim for an STC of 55+ so that customers in adjacent rooms can listen to the TV or music without disrupting other patrons. Noise complaints are a common detriment to many hotels, but aiming for, and achieving, higher STCs is the most surefire way to prevent these.
Ways to Improve STC
There are a number of common approaches to improving STC ratings in your walls. The cheapest is to move to thinner gauge studs and space them out further. If you do not have wall height constraints, then simply moving to 25-gauge studs at 24 inches on center will improve the STC of your wall by 5-7 points or more! Furthermore, this reduces material cost by using thinner metal studs and using fewer of them (50% fewer if moving from 16 inches on center).
Other options include installing sound membranes on the studs underneath drywall. Quality sound membranes are designed for delta-STC performance of 7-10 points.
A combination of stud changes and inclusion of sound membranes are sure-fire ways to improve the STC of your wall beyond the 50-55 range often needed.
Understanding STC in Review
STC values are broadly misunderstood, but they’ve never been more important. STC is an average soundproofing performance across a broad range of frequencies, and is our most common soundproofing criteria.
Remember to rely on valid lab test data, or require an acoustic field test when in doubt. Apply rules of thumb, such as using thinner gauges and spacing them out more, or implementing sound membranes to ensure a higher STC performance on your wall.
