Reading Transmission Loss Tests - a measure of how walls perform
Everyone who has looked into sound isolation has heard of STC, but many people do not understand the data from which STC is derived. In lab tests of wall performance, one uses a setup not unlike the one shown in the simple schematic below.
A typical transmission loss test lab. In this case, transmission loss would be 60 decibels.
Essentially, you make noise in one room, measure how loud it is, and then measure how much quieter it is in the other room. After adjustments for things like absorption and the size of the test wall, the results are given as “transmission loss.”
Literally, transmission loss means “how many decibels of sound a wall can stop at some given frequency.” Now, for EVERY POSSIBLE WALL, transmission loss varies with frequency. Because of this, ratings like STC cannot be used to say “STC=50 means the wall stops 50 decibels of sound.” 50 decibels of sound at what frequency? An STC=50 wall might stop 70 at some frequency, and 10 at another.
For this reason, as a designer of rooms in which sound isolation is an important consideration, understanding transmission loss can help give your customers higher performance rooms.
This chart shows the transmission loss for different walls with STC=53 to 54
We can immediately see the value of understanding transmission loss -vs- simply looking at STC values.
All of these walls have very good STC’s, but the blue and red walls have low frequency performance that is considerably worse than the green wall.
How can this be? Well, STC ignores frequencies below 125 Hz, but more on that in our next tip, which talks about the STC rating in depth.
While the STC’s of these walls are basically the same, at different frequencies transmission loss varies a lot. Note that at 63 Hz transmission loss is 22 dB for green wall, but only 13dB for the another.
We can see, therefore that looking at transmission loss is a lot more valuable that just looking at STC. Before we rush out into the world to compare transmission loss data, however, there are some things we should understand.
Data varies from lab to lab
One cannot blindly compare transmission loss data from different labs, as the data can vary. Variations of STC of about +/- 3 are typical from lab to lab (and variations can also exist from test to test).
These variations become even larger when we look at lower frequencies - at frequencies below the 125 Hz STC cutoff. From lab to lab we might see +/- 6 or even more dB at lower frequencies. If one really wants to compare things, therefore, you have to compare things tested in the same lab.
Furthermore, you have to make sure that even in the same lab the samples were tested in the same manner. At times, for example, manufacturers of acoustical products will send in very small samples to be tested. These small samples do not produce official results. The ASTM standard that governs the testing of transmission loss - ASTM E90 - mandates that samples be at least 8’ x 8’ to be considered a valid test. There are good reasons for this.
Ideally we would compare data taken in the same lab on the same size of wall. A prudent manufacturer will have such data available for you to scrutinize, and they should be able/willing to offer you data extending over the whole relevant frequency range.
Summary
Looking at transmission loss is far more relevant, useful, and powerful than simply looking at STC figures. Ideally one would make comparisons of walls from the same laboratory because lab-to-lab variations exist.
If you can attain transmission loss data over the entire frequency range from the same laboratory, then you can make extremely well informed sound isolation choices.