How Is Noise Tested? – Let’s take a look at a very popular term called STC (Sound Transmission Class) which is an industry standard to calculate sound transmission through barriers. Many building codes now require condos and townhouses to meet a STC of 50; luxury hotels and condos should be looking to reach 60 and higher.
|20-25||Very Poor||Low speech audible|
|25-30||Poor||Normal speech understood|
|30-35||Fair||Loud speech understood|
|35-40||Good||Loud speech heard, but hardly intelligible|
|40-50||Very Good||Loud speech heard faintly|
|50-60||Excellent||Loud sounds barely heard|
A standard 2×4 wood stud wall with 1/2″ drywall 16″ O.C. has an STC of 32. An important point to keep in mind is that STC testing only tests on frequency ranges from 125 – 4000 Hz. One problem with this type of classification is that a lot of the common noise issues are due to sounds that fall below or above this range. Examples of such sounds are bass drums, deep male voice, trucks, etc. – all of these can start at 50 Hz. High pitched female voices, shrill ringing of phone and auto horns are good examples of sounds that reach above the 4000 Hz range.
This will explain why many big projects, after having paid tens of thousands of dollars for acoustical engineers and getting thousands of pages of tested soundproofing materials with lab testing of STC’s of 50 and above, still receive complaints for poor acoustical performance.
For good measure we will add one more reason for poor performance results: the fact that although material can test to a STC of 25 for instance, does not mean that when added to a wall assembly of 32 (like a standard 2×4 wall) that the end result will be a STC of 57 (25+32=57). In reality, it may only add 2-3 points to the wall bringing it to a STC of 35. A perfect example of this phenomenon is the much touted Mass Loaded Vinyl and all the other brand names that it is sold under.