The STC score of a door is 30 to 33, that of a window is about 18-20, and an exterior wall of a new home is 55.
There’s an easy temptation to select the highest STC score presented and then move on. But just because a wall has one score on paper, doesn’t mean that’s how it will perform in real-world conditions. Before making promises to your clients based on STC, it’s vital to know what those numbers do and don’t really mean in the field.
1. STC Scores Reflect A Lab Setting that Doesn’t Exist in the Real World.
High STC scores sound great on paper, but they aren’t meant to be predictive of real-world performance. The STC ratings you get quoted are from lab tests where a partition is placed between two carefully controlled rooms built for the specific purpose of testing for sound transmission loss. As a result, STC scores are not great predictors of real-world sound mitigation. In the field, sound transmission and overall noise levels will be determined by many factors for which STC scores do not account.
2. Sound Is Like Water, It Finds a Way Out.
If your bucket has holes in it, it doesn’t matter what it’s made of, it’s still not holding water. The same is true of walls and sound. A room is made up of an office front, as well as 3 other walls, a ceiling, and a floor. The office front can have an excellent STC score, but sound can still get in through the rest of the room. Outlets, windows, doors, gaps in ceiling tiles, floors, joints, joists, plumbing, and any other openings between two rooms will allow for sound transmission, regardless of the STC score of an office front wall. This is known as Flanking Transmission, and it can undermine all the effort you went through to choose top-quality products. Your design and construction choices can nullify the sound transmission properties of your walls. To learn more about Flanking, download our whitepaper on the sound here.
3. Sound Control Comes from Within.
The next issue with STC scores is that noise control is not just about transmission from one room to another, it’s about acoustics, or how sound behaves within a room. Many people expect that installing partitions with high STC ratings will make their offices quieter. Unfortunately, the STC rating of a wall only measures how well it keeps sound from traveling between rooms. A wall can have an STC rating of 45 and a room could still be quite loud if that room has poor Noise Reduction Coefficient (NRC) scores. NRC, the measure of how well a room absorbs sound, reveals a great deal of information for which STC scores do not account. For a more complete discussion of NRC and sound mitigation strategies, click here.
4. Size Doesn’t Matter.
Even in lab settings, there’s no practical difference between slightly different STC scores. These numbers are generated by machines and though it may seem like 37 is better than 36 or 35, to humans, differences that small are barely perceptible and it would take STC score differences up to 5 to be “clearly noticeable,” according to the MSHI. Minute differences in STC scores may matter to machines in a lab, but real offices are staffed by humans who won’t be able to notice the difference.
5. It’s Not the Car, It’s the Driver.
Yet another reason lab-tested STC scores don’t translate into the real world is human assembly. The most expensive partitions on the market, with the highest STC scores, can still perform poorly if they aren’t properly installed. Though it’s easy to compare STC scores and move on, in the real world, it’s usually more accurate to compare the quality, service, reputation, and installation of your partition provider. Working with quality companies that you trust is more critical to a final product than lab-tested STC scores.
6. The Truth Hertz
STC scores measure sound transmissions down to a frequency of 125 Hertz (Hz). Unfortunately, the human ear can hear sounds as low as 20Hz. So, although STC can offer some, not much, predictive ability when it comes to mitigating higher-pitched sounds, there are a host of common noises that STC, even in lab settings, doesn’t test for. Industrial equipment, office equipment, photocopiers, footsteps, traffic, bass sounds, and even many male voices operate well below the 125Hz threshold tested for by STC. To learn more, our whitepaper offers a detailed discussion of the many surprising ways frequency affects sound transmission.
7. 2+2 Does Not Equal 4!
STC scores are logarithmic, not linear, so you can’t just combine a 20 STC wall with a 30 STC wall and expect an STC score of 50. You’d actually end up with an STC score closer to 32. For those interested in math, our whitepaper goes into more detail about the logarithmic nature of STC scores.
8. You Have to Seal the Deal.
Proper sealing is critical to noise management. Buying walls with the highest STC scores but not having full-door gaskets is like buying a yacht with a hole in the bottom. It’ll look nice, but it won’t perform terribly well. Door gaskets, drop seals, and other methods of sealing the space between doors and walls are critical to reducing sound transmission through your walls.
It’s understandable to want the best. But it’s just as important to recognize that the nature of STC scores means that higher scores don’t directly translate into better real-world performance. Looking at STC ratings is a good starting point when comparing two products, but it isn’t an effective way to make real-world decisions. In the field, noise control requires a complex balance of many factors for which STC does not account.
If you have any questions or just want a more detailed look at standard STC scores for construction and in-depth explanations of each of the above points, you can download our whitepaper here to empower yourself with the knowledge you need to make the best possible decisions.