Sound transmission class rating measures the ability of a structure to resist or reduce sound transmission. Developed during the 1960s by ASTM International, sound transmission class ratings provide one of the most widely used methods of measuring sound transmission in building materials. These ratings may be used to measure sound levels through a wall, ceiling, or even a door or window. The higher the sound transmission class, or STC rating, of a structure, the less sound passes through the structure. Lower STC ratings generally mean that the structure does a poor job of blocking sound transmission.
Official STC performance ratings are determined using industry-developed testing methods and benchmarks. As an example, interior walls built with drywall and insulation generally range from STC-30 to STC-42. At this level, loud speech is audible through the wall, but it can be difficult to distinguish specific words or phrases. If plaster is used in place of drywall, the STC rating may increase by 10 points or more, creating a better sense of privacy within a space.
One of the most important things to understand about sound transmission class ratings is that they are not cumulative. Drywall with a rating of STC-20 installed over insulation measuring STC-10 does not automatically produce a wall with an a sound transmission class of 30. Instead, these ratings measure the performance of the entire structure after it has been fully assembled. For this reason, manufacturers often claim that their products can increase STC rating by a specific range of points. The materials themselves can not provide a specific STC rating on their own.
Contractors and designers have three basic options for increasing STC ratings. Soundproofing may involve adding material, or mass, to a structure. It can also include adding some form of separation or air space between adjacent materials or structures. Finally, using products that absorb sound waves can also serve as an effective form of noise control.
To increase sound transmission class rating, builders may choose sound-resistant drywall, or soundboard to replace standard materials. They may also add insulation to absorb sound waves, or use special clips and hangers to create air cavities within the wall or ceiling. Specialty doors with surrounding sound seals can block noise transmission through an opening. Soundproofing foam may also be used to control noise levels in recording studios or home theaters, where high STC ratings may be desired.