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Sound Transmission Class (STC)

STC – What Is It?

Equalizer display representing sound transmission

Transmission loss (TL) begins when sound waves strike one side of a partition or assembly (for example, a wall, ceiling, or floor), causing it to vibrate. The vibration travels through the partition and produces a sound on the other side. Some of the vibration energy may be lost during the transmission, which results in TL. High TL means that less sound is produced on the other side of the partition.

The STC rating is a single number rating of how well a building partition or assembly reduces airborne sounds transmission from one space to another, thus increasing TL. In short, STC measures how much sound is held inside the room, rather than being transferred to other rooms.

How STC Is Determined

A loudspeaker is used to fill a room with sound frequencies within the range of speech from 125 Hz –4000 Hz. The sound is measured in the room that it originated in and then again in the room on the opposite side of the structure in question (wall, floor, etc.). The difference between the two measurements results in the calculation of the STC rating, which determines how well transmission is blocked by the material(s) in question.

STC ratings are given to individual materials as well as entire assemblies. When materials are installed in an assembly, the resulting number is not the addition of all the ratings of the individual materials. Instead, the assembly should be tested in the field after installation in what is called a Field Sound Transmission Class (FSTC) rating. This is the only way to really determine the true STC rating of a complete structure. This way, all materials are taken into account for and given a total STC value. Also, any air vents or other obstacles that sound can travel through are also accounted for with this method. This method of rating in the field is also known as the Field Sound Transmission Class (FSTC) or Apparent Sound Transmission Class (ASTC).

Other Helpful STC Info

The Uniform Building Code (UBC) requires hotels, apartments, and condominiums to have an STC rating of 50 if tested in the laboratory and 45 if tested in the field. Though this is a common building standard, an STC rating of 50 can still transmit loud speech to other rooms, though it is not usually understandable. A single family residential home may only have an STC rating of 25–35, often making airborne sounds more audible throughout the house. A rating of 61 or above is considered an adequate rating for areas that need extra sound insulation.

Because flooring does not have a significant effect on the transmission of airborne sounds traveling from one room to another, STC ratings are more commonly used in reference to wall and other partitions. You can increase the STC of a wall, for example, by adding more mass to it, increasing the air space inside, or adding sound absorbing material to the wall.

For more on reducing sound transmission by using sound absorbing floor/ceiling materials, visit FindAnyFloor's® section on Sound Controlling Floor/Ceiling Materials.