STC and the acoustic expectations

The partition is installed by a manufacturer trained distributor and the world is good. Then the phone rings and the site engineer says «I can hear right through your wall. The acoustics suck and I bought the best STC on the market!» This is a common scenario in the partition industry and deserves some exploration, explanations and suggestions especially as it relates to STC and the acoustic expectations of the user.

The False Expectations resulting from a high STC.

Most users have an expectation of a «soundproof» operable partition and that they can do whatever they wants sound wise on one side and it will not interfere with others on the opposite side because they purchased a partition with a high STC. As I said in the first blog in this series there is no such thing as a «soundproof» operable partition. Consider the following chart as reasonable & generally acceptable real world benchmarks.

STC Privacy Afforded Familiar Construction
Less than 30 Normal Speech easily understood HM door without seals
30-35 Normal speech audible, loud speech understood ½” plate glass
36-40 Loud speech audible but probably unintelligible ½” drywall on studs
41-50 Loud speech barely audible 8” block wall
51-55 Shouting barely audible 12” poured conc. wall

Therefore the best practical expectation that one can hope for, assuming that there aren’t any flanking paths (see previous blog), is a “shouting barely audible” result if you have a partition ranging from STC 51 to 55. And since “shouting barely audible” occurs in the primary hearing range of 400 Hz to 5000 Hz then the logical question must be what is happening when sounds below 400 Hz and above 5000 Hz are present?  

STC by itself only gives one a rough idea of how much sound a wall will stop and is the most commonly used but also imperfect way to quantify sound reduction.  But STC is not perfect and should not be relied upon totally in real world expectations.

Very simply STC is calculated by taking the laboratory tested transmission loss values of the partition specimen at 16 standard frequencies over the range of 125 HZ to 4000 Hz (see first blog), plotting results on a graph and comparing the result to a standard STC reference curve (another topic down the road). From this calculation an STC value is determined. The main problem is that the resulting STC only considers frequencies down to 125 Hz. When you realize though that common interior sound sources such as an average home theatre system (amplified noise) or musical instruments such as guitars, bass fiddles and drums have a frequency at 125 Hz or less, then the ability for  STC to predict all possible acoustic expectations are diminished. Even with an STC 55 “shouting is barely audible” number you may hear a drum or guitar or significant parts of amplified noise quite clearly. And what happens above 4000 Hz since we can hear up to 5000 Hz & beyond?

ASTM E413 which explains how to calculate STC describes the limitation. It says (my underlines):

“These single-number STC ratings calculate 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 is not appropriate for sound sources with spectra (range) significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling alleys, power transformers, musical instruments, 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 authors recognized the limitations and applications of STC and cautioned that STC should not be used as a measure of acoustical integrity under all circumstances.
You have to anticipate the frequency of the sounds, especially if low frequency noises will be present, that you wish to control and look at the graph that plots the test results at those frequencies. Choose the wall that best controls sound at those frequencies rather than a wall that simply has the higher STC. It is entirely possible that a wall with a lesser STC is better at controlling particular low frequencies than one with a higher STC.

Speech such as “shouting barely audible”, all in the 400 Hz to 5000 Hz range – no problem. Guitars, drums, bass sound producing instruments and almost all amplified sound, all below 125 Hz – probably lots of problems even with a high STC.

In conclusion we think that STC will always be a good reference in the Operable Wall Industry but it should not be the only factor to look when Operable Wall is selected.