Understanding Transmission Loss Plots

If you are interested in sound isolation, you are probably familiar with the ever-present STC system. A single number – such as 52 – is given as the STC rating for an assembly.

While very convenient, this doesn’t really tell the whole story. The STC figure (which has a series of problems, as outlined in our “Understanding STC” document) is derived from something called “transmission loss”.

Simply put, transmission loss is how many decibels of sound a given assembly stops at a given frequency, as shown in the graph below:

To read transmission loss data, just find the frequency in question, and draw a line up, then over to the left to see how many decibels the construction stops at that frequency.

For example, as shown, this wall stops 41 decibels of sound at 315Hz, but just 14 decibels at 125Hz. The STC of this theoretical assembly is 38.

It is almost always far more valuable to compare transmission loss of different walls than to simply compare STC. One of the most important reasons that this is true is that STC completely ignores low-frequencies. All information below 125Hz is ignored by the STC system.

Low frequency information is often the most important – but its also the most difficult to assess in lab tests.

A variety of factors make it difficult to compare low frequency transmission loss data attained at different labs.

Therefore, if you want to make the best possible comparison, you should always look for data taken at the same lab, and attained in the same manner (same size wall, installation practices, etc.). A prudent manufacturer will be able to provide this information to you.