An STC rating of 55 or higher eliminates most ceiling noise.
Sound transmission class is a system used to rate soundproofing for ceilings, walls and floors. A higher STC rating indicates an increased ability to mute sound transmissions, and a lower rating is indicative of less effective soundproofing abilities. STC ratings are generally used to evaluate the effectiveness of sound proofing in such settings as office buildings, conference rooms and courtrooms, as well as residential homes, dormitories and apartments.
Poor to Marginal Ratings
STC noise ratings on the "listener's" side for hearing speech are as follows. A rating of 15 to 25 indicates that speech at a normal level, coming from above, below or from the side is easily understood. This is a poor noise control rating. An STC rating of 25 to 35 indicates that loud speech can be clearly understood and normal speech can be understood at a 50-percent rate. This is considered a marginal noise control rating.
Good to Excellent Ratings
An STC rating of 35 to 45 indicates that loud speech can be understood at a 50 percent level. Normal speech can barely be heard, and is not clearly understood. This is considered a good noise control rating. An STC rating of 45 to 55 indicates that loud speech can barely be heard but not clearly understood. Normal speech is very low and usually cannot be heard at all. This is a good noise control rating. An STC rating of 55 or up indicates that loud speech is muted and this is an excellent rating.
Rating Discrepancies
Although STC is the standard system used for determining soundproofing effectiveness, there can be discrepancies with the ratings. STC ratings are established through laboratory testing in controlled settings, which create nearly perfect environments. Manufacturers acknowledge that soundproofing equipment varies in quality and workmanship, and real-world conditions are distinctly different from laboratories. As a result, STC field ratings may vary by 15 to 20 points from laboratory tests.
Applying Sound Ratings
An uninsulated wall has an STC rating of 15. An insulated wall with wood studs has a 38 STC rating. An insulated wall with metal studs instead of wood studs has an STC rating of 45. According to Home Tips, an STC rating of 53 can be achieved by "mounting 1/2-inch gypsum wallboard to resilient channels fastened to 2-by-10 ceiling joists with 3 1/2-inch-thick batts between the joists. In this scenario, the floor above has a plywood subfloor, particleboard underlayment, carpet pad, and carpet."
Other Considerations
Sound insulation appears to be most effective when it is incorporated during construction. Using blanket or batt insulation between the studs and joists is both an effective and cost-efficient way to reduce ceiling noise. However, if even a small portion of the ceiling is left uninsulated, it diminishes the insulation's ability to effectively absorb sound.
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