Technically speaking, STC stands for Sound Transmission Class, which is the measurement used to
calculate the effectiveness of soundproofing materials in reducing sound transmission between rooms.
Simply put, it measures how much sound a wall, for instance, will block from getting through to the other
side. Generally speaking,
the higher the STC rating, the more effective a material is at blocking sound at the most common frequencies. However, one limitation of STC ratings is that they only measure sounds between 125Hz-4000Hz. For example, traffic noise from airplanes, trucks and heavy equipment operations all have frequencies lower than 125Hz, thus cannot be measured using STC calculations. This presents a situation that can sometimes be misleading due to the fact that most noise complaints come from sources that produce sound lower than 125 Hz.
Unfortunately, explaining STC in exact terms is nearly impossible, unless you flash back to high school
calculus and read up on logarithms. That’s because, technically, measuring sound on an STC scale
requires not only the number (in decibels) of how much sound is lost after applying the soundproofing
material, but also takes into account things called “deficiencies” and “contour adjustments.” After plugging
all of these numbers into an equation, you basically have to test different numbers until one of two limiting
conditions are met. STC tests involve analyzing 18 of the most common frequencies.
The higher the STC rating, the more effective that material is at reducing sound transmission of the most common frequencies. Once this value is determined, the STC rating can be found using the highest number that satisfies both of these limiting conditions.
Whew! That’s a mouthful. Now, to put the idea of calculating an STC rating into layman’s terms, you
should consider a few things.
What is Frequency?
What are Decibels?
What is Transmission Loss?
How are STC Ratings calculated?