WHAT IS SOUND TRANSMISSION CLASS?
When considering noiseproofing materials, you’ll likely come across a rating system called STC, which stands for Sound Transmission Class. The STC rating system is the most commonly recognized sound isolation standard in North America, and is heavily used by marketers for promoting soundproofing products. The problem is that the standard is outdated and not always very useful, and the ratings don’t tell you much about how effectively a material works.
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The STC standard dates back to 1961 and ranks walls over the frequency range of 125 to 4,000 Hz. And that’s the main problem—the standard doesn’t consider noise below 125 Hz. If you’re looking for a sound isolation solution, chances are it involves low frequency noise. Most of the sonic energy generated by the average home theater, for example, falls below 125 Hz. So does a lot of noise from traffic, airplanes, music, and machinery. The STC standard is not useful for calculating noise isolation for applications at these frequencies.
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The STC calculations also assume that noise generally has an even energy dispersion. But this is not how noise behaves in most real-world situations. For these reasons consumers are often better off relying on more modern rating systems such as the Outdoor–Indoor Transmission Class (OITC), which considers frequencies down to 80 Hz.
Customers question how just one layer of Green Glue Noiseproofing Compound dissipates sound so effectively. To properly comprehend the science behind the compound you need to understand a little about the energy of sound waves and how they travel.
Sound travels through the air like a wave or an airborne vibration. When these soundwaves hit a ceiling or wall, the vibrations will pass through the structure and exit through the other side as sound again. If the structure is damped (with GreenGlue Comound) this will not happen.
Green Glue Noiseproofing Compound has a unique polymeric formula that converts the mechanical energy from sound waves into small amounts of heat. When the compound is sandwiched between two rigid layers of material (like drywall), it forms a damping system. So, when sound waves pass through the structure, the energy is dissipated in the form of heat.
The result is dramatic! Just one layer of Green Glue Noiseproofing Compound in between two sheets of drywall can eliminate up to 90 percent of noise, even at low frequencies. The diagram above illustrates how this damping system works to convert sound energy into heat and provide efficient noise-proofing.
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