Understanding STC, what it means and doesn’t mean
The STC - or Sound Transmission Class - is a recognized standard and is, by far, the 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 is.
What is STC?
Is STC a measure of how many decibels of sound a wall can stop? - No, it is not.
Is STC a ranking of how good a wall is? - No, for most applications it is not.
So what the heck 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 to the right.
The blue line in that graph is the STC contour. The Green line is the performance of a wall, in this case a Green Glue 2x4 wall. To calculate STC, one has to attain this performance data - called transmission loss - from a certified laboratory.
One the transmission loss (TL) data is attained, STC is calculated as shown in the appendix.
The problems with the STC system
The three basic limitations of STC are apparent from the description of the system above.
1. 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
- 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 good.
2. 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 the average home theater) are worlds away from this criterion.
3. Its 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. OK, that’s great, but do any real problems actually occur?
Problem 1 - STC does not correlate at all to low frequency performance.
The graph to the right shows two walls, one of STC 47, one of STC 48. Note that in the low frequency range – important for music, theaters, traffic, aircraft, and most other real-world noise sources – the lower STC wall is literally 30 decibels better, yet lower STC
.
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.
Why are the folks at The Green Glue Company the only people that talk about this?
We aren’t! In fact, the very recognized ASTM standard which explains how to calculate STC - ASTM E413 - directly warns us that STC is not suitable for most situations.
“These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television, and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling allies, power transformers, musical instruments, many music systems and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.”
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 something we don’t understand.
So why is the STC system used at all?
- 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.
Well, there are some very good reasons why the STC system is in use.
Are there better rating systems?
Yes, the best standardized rating system in North America is called OITC, and is typically used for exterior wall elements. OITC features a modern calculation system and considers frequencies down to 80 Hz.
In Europe at times full-range standards are applied. An assessment of existing standards is given in the appendices of this document.
The Green Glue Company presents, in several of our technical documents, full frequency range calculations based on advanced techniques. We took a different path than some of our competitors in this market in that, instead of attaining high STC’s and marketing them, we have paid to have many walls (with many more to come) tested down to very low frequencies all in the same lab - to avoid the lab-to-lab problems that can cloud low frequency data. We hope our efforts serve you well. As the ASTM standard that shows us how to calculate STC teaches, in our technical documents we assess the performance of walls considering the entire relevant frequency range, and performance in individual bands.
Summary:
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.
Appendix – calculating STC.
Calculating STC involves adding something called “deficiencies” and utilizing the STC contour. This table will get you started nicely.
Into the table you have to enter transmission loss data, attained from a laboratory. Then you add the STC contour adjustment to attain the adjusted transmission loss. Then you basically play with numbers in the “STC you wish to test column” until one of the 2 limiting conditions listed below are met. The highest number that satisfies both of those conditions is the STC.
The STC is defined as the highest number that meets both of the following conditions after calculations like the above:
The two STC rules:
1. No single frequency band may have more than 8 deficiencies
2. The total deficiencies may not exceed 32
In the case above, the highest # of deficiencies in any frequency band is 4, and the total deficiencies are 21. So both condition 1 and condition 2 are passed. The appropriate thing to do in this situation would then be to raise the STC you are testing to 56. In this case, the wall would pass again at 56, as shown in this table here:
Now the total deficiencies are 30, and it is clear that if we raised the number we wish to test to 57 that the number of deficiencies would exceed 32, and thus the wall could not pass at 57.
Therefore, the STC of this wall is 56. The table below contains some STC values for different walls, along with the answer to their STC. Use the table on the next page to see if you can calculate the STC for those data sets if you like.
And there you have it, that is how the famous STC rating is calculated.