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Understanding STC Ratings
What Is STC?
The STC, or Sound Transmission Class, is a recognized standard and is, by far, the most common sound isolation standard in use in North America today. Virtually every commentary that one reads focuses on STC, yet STC is not without significant limitations, and for a great many applications it is not a good measure of sound isolation at all. Before we talk about the limitations of STC as a rating, let’s take a look at what STC does not accomplish. The STC rating does not measure how many decibels of sound a wall can stop and the STC rating often is not a ranking of how good a wall is in terms of sound isolation overall.
So what is it? It is a very old (1961) method for ranking walls over the frequency range of 125 - 4000 Hz, assuming that the noise the wall is trying to stop is generally even across the frequency spectrum. It is calculated by using a “contour” that is shifted up and down to the highest point where two conditions are satisfied, like the chart below.
The blue line in that graph is the STC contour. The green line is the performance of a wall, in this case a 2x4 wall with Green Glue Compound. To calculate STC, one has to attain this performance data, which is called transmission loss, from a certified laboratory. Once the transmission loss (TL) data is attained, STC is calculated.
Why Is the STC System Used At All?
There are some very good reasons why the STC system is in use.
- It’s been around for so long that essentially every law, regulation, and piece of legislation relating to sound control is based on it. Old habits are hard to break.
- As frequency falls, the ability of the different labs to get consistent results also falters. +/- 3 STC points from lab to lab is typical, but if the STC system were extended down to, say, 40 Hz, this might increase to +/- 10 STC points or more, making the results basically meaningless.
- Its easier for companies marketing commercial products to attain a huge STC increase than it is to attain a huge increase across the full frequency range. This leads to a lot of focus on STC, and less discussion of critical things like low frequency performance.
The Problems With the STC System
The three basic limitations of STC are apparent from the description of the system above.
- It only considers frequencies down to 125 Hz. The first, and most severe, problem with the STC system is that it only considers frequencies down to 125 Hz. What noise exists below 125 Hz?
- Most of the sonic energy generated by the average home theater or recording studio
- A large percentage of the sonic energy generated by traffic, airplanes, and music
- Much of machinery noise
If you have sound isolation problems, there is a very good chance that it is low frequency noise you are having trouble with, so one could say that the STC calculation completely ignores the frequencies that are most problematic. That’s not helpful to you.
- It assumes even energy dispersion. It is accurate within its frequency range only for noise sources that have approximately even energy levels across the frequency band. Most noise sources do not meet this criterion and some (like sound generated from movies and music) are worlds away from this criterion.
- The STC calculation system is archaic. STC dates back to 1961, a time before computers made complex calculations easy, and the method of determining STC reflects this. In today’s world more complex, vastly superior calculations can easily be done.
Problem 1 - STC does not correlate at all to low frequency performance.
The graph below shows two walls, one of STC 47, one of STC 48. Note that in the low frequency ranges (63Hz to 100Hz) that are important for music, theaters, traffic, aircraft, and most other real-world noise sources, the lower STC wall is literally 30 decibels better, yet has a lower STC rating.
Data in chart copyright NRC Canada and used with permission. Documents available at www.nrc.ca. In this graph, higher is better, and the lines reflect how many decibels (dB) of sound a given wall stops at a given frequency. This is called “transmission loss”. However, as you can see, the concrete does not perform all that well in part of the important vocal region. Nonetheless, the vastly superior low frequency behavior of the concrete partition would be preferable in most circumstances.
Problem 2 - Misleading results due to frequency cutoff.
The 125 Hz cutoff also leads to some very misleading results. Take the two hypothetical walls below. They are both poor walls, with very bad low frequency performance, but one is STC 32, the other is STC 42. Two very bad walls, one gets a bad STC score, the other gets a reasonably respectable STC score. Why? Because with one wall, the big problem occurs at 125 Hz, inside the STC frequency range, but in the other wall the big problem occurs just below the STC frequency range.
This creates misleading situations where some construction change or product simply causes a huge problem to shift ever so slightly down in frequency and yields a huge gain in STC. Impressive marketing value, but in reality it doesn’t make the wall better at all. So, yes, problems really do occur. The standard itself openly recognizes exactly the limitations that we discussed above. Why these limitations have been so consistently overlooked by novice and expert alike is another problem altogether.
The STC score has only very limited relevance for most sound isolation applications because it is calculated in an archaic fashion, it assumes noise sources that are unrealistic for most situations, and it is calculated over a very limited frequency range, ignoring perhaps the most important of the frequency band. Therefore you should strive to avoid buying products because of high STC, and strive to find products and designs that yield actual real-world reductions in noise level.